Earth and EnvironmEntal SciEncE 2 · Earth and Environmental Science is one of the most dynamic and...

Earth and EnvironmEntal SciEncE 2

David Heffernan • Rob Mahon • John McDougall • Kylie Gillies

Spotlight Earth Environment HSC.indb 1 14/01/2011 1:34:10 PM

AcknowledgementsWe would like to thank the following people and organisations for their help in the preparation of this text: the Baulkham Hills Shire Council; Newports Nursery, Winmalee; Scripps Institute of Oceanography, California; Reg Morrison; Sydney Water; and the US National Oceanic and Atmospheric Administration. However, in acknowledging such contribution, the authors prepared the final text and must take responsibility for any errors that may appear.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Science Press. ABN 98 000 073 861

© Science Press 2009First published 2002 Reprinted 2004, 2007 Second Edition 2009 Reprinted 2011, 2013, 2016

Science PressPrivate Bag 7023 Marrickville NSW 1475 Australia Tel: (02) 9516 1122 Fax: (02) 9550 [email protected] www.sciencepress.com.au


Spotlight: Earth and EnvironmEntal SciEncE 2

iiiScience press

ContentsTo the Student/To the Teacher vVerbs to Watch viSyllabus Cross-reference vii

Chapter 1 Tectonic Impacts 1

1.1 Movement of crustal plates 3

1.2 Mountain building 11

1.3 How Australia has changed 17

1.4 Natural disasters – volcanic eruptions 25

1.5 Natural disasters – earthquakes 35

1.6 Plate tectonics and climate 47

1.7 Summary 55

1.8 Exam-style questions 57

Chapter 2 Environments 61 through Time 2.1 The first life on Earth 63

2.2 Conditions for life on Earth 71

2.3 Dating the Cambrian explosion 75

2.4 Evolution and the invasion of the land 83

2.5 Extinctions and radiations 93

2.6 Summary 101


Chapter 3 Caring for the Country 107

3.1 The development of Australian soils 109

3.2 Soil erosion and its control 113

3.3 The soil salinity problem 119

3.4 The pesticides problem 127

3.5 Managing the supply of water 137

3.6 Global warming 145

3.7 Ozone depletion 155

3.8 Waste management 161

3.9 Summary 171


Chapter 4 Introduced Species 177 and the Australian Environment 4.1 Introduced species in Australia 179

4.2 The vulnerable Australian environment 189

4.3 Case studies: Becoming a pest 197

4.4 Biological control 211

4.5 Quarantine methods – Keeping them out 219

4.6 Summary 225


Chapter 5 Organic Geology: 231 A Non-renewable Resource5.1 Making use of resources 233

5.2 Fossil fuels 239

5.3 Formation of fossil fuels 245

5.4 Coal 253

5.5 Petroleum 261

5.6 Fossil fuels and the greenhouse effect 273

5.7 Alternative energy sources 283

5.8 Summary 297


Chapter 6 Oceanography 303

6.1 The oceans through time 305

6.2 Oceans and salinity 315

6.3 Oceans and climate 323

6.4 The ocean community 333

6.5 Deep ocean vents 339

6.6 Deep ocean sediments 345

6.7 Studying the oceans 351

6.8 Summary 357


Chapter 7 Mining and the 361 Australian Environment7.1 Formation of ores 363

7.2 Mining and the law 373

7.3 Mining and processing ore 381

7.4 Economics of mining 397

7.5 Case study: Northparkes copper-gold mine 405

7.6 Summary 421


Answers 425

Glossary 473

Index 481



vScience press

To the Student Earth and Environmental Science is one of the most dynamic and interesting science courses in the world today. Everything you learn will have relevance to your future roles in life – at work, at leisure and in the community.

In this course we will explore contemporary science and its applications, and study how geological and biological forces have helped shape the environment in which we live. We examine the positive and negative impacts of humans on the biological and geological environment. This course introduces you to a wide variety of topics which may help you in deciding on a future career.

The text examines:

• whytherearemanyextinctvolcanoesinAustralia

• wheretogotoavoidearthquakesandvolcanoes

• whywedonothave toworryabout ‘killer kangaroos’when we go bushwalking

• how we are dealing with ozone depletion and theenhanced greenhouse effect

• methodsbeingusedtodisposeofrubbish

• whywearesoconcernedaboutcanetoads

• methodsforfindingandextractingfossilfuels

• alternativesourcesofenergytoreplacefossilfuels

• howtorestoreaminesiteafterminingiscomplete.

This book uses many hands-on activities that include open-ended investigations. Other activities involve:

• undertakinglaboratoryexperiments,includingtheappropriate use of electronic sensors and computers

• fieldworkwhenonexcursions

• researchusingthelibrary,theinternetandCDencyclopedias

• usingcomputersimulationsformodellingorprocessingdata

• usingandreorganisingdatafromawidevarietyofsources

• extractingandreorganisinginformationintheformofflow charts, tables, graphs, diagrams, prose and keys.

• usinganimation,videoandfilmresourcestoobtaininformation not available in other forms.

To the Teacher This text provides students with a broad and contemporary understanding of geology and environmental science and their application. It emphasises:

• scienceasacontinuallydevelopingbodyofknowledge

• theroleofexperimentationindecidingbetweencompeting theories

• theprovisionalnatureofscientificexplanations.

The study of this course involves students working:

• bothindividuallyandwithothersinthelaboratory

• inthefield

• withinteractivemultimedia

• inapplyinginvestigativeandproblem-solvingskills.

Students learn to effectively communicate scientific information and to appreciate the contribution that a study of science makes to our understanding of the world.

The textbook contains:

• allthecontentneededforthecoresyllabusandallofthe options

• allfirst-handinvestigations

• allthespecifiedactivities

• homeworkquestions

• exam-stylequestions

• scaffoldstoassiststudentswithexaminationterminology

• answerstoallToThinkAboutandExam-stylequestions.



viScience press

Verbs to Watchaccount, account for

State reasons for, report on, give an account of, narrate a

series of events or transactions.

analyse

Identify components and the relationships among them,

draw out and relate implications.

apply

Use, utilise, employ in a particular situation.

appreciate

Make a judgement about the value of something.

assess

Make a judgement of value, quality, outcomes, results or

size.

calculate

Determine from given facts, figures or information.

clarify

Make clear or plain.

classify

Arrange into classes, groups or categories.

compare

Show how things are similar or different.

construct

Make, build, put together items or arguments.

contrast

Show how things are different or opposite.

critically (analyse/evaluate)

Add a degree or level of accuracy, depth, knowledge and

understanding, logic, questioning, reflection and quality to

an analysis or evaluation.

deduce

Draw conclusions.

define

State the meaning of and identify essential qualities.

demonstrate

Show by example.

describe

Provide characteristics and features.

discuss

Identify issues and provide points for and against.

distinguish

Recognise or note/indicate as being distinct or different

from, note difference between things.

evaluate

Make a judgement based on criteria.

examine

Inquire into.

explain

Relate cause and effect, make the relationship between

things evident, provide why and/or how.

extract

Choose relevant and/or appropriate details.

extrapolate

Infer from what is known.

identify

Recognise and name.

interpret

Draw meaning from.

investigate

Plan, inquire into and draw conclusions about.

justify

Support an argument or conclusion.

outline

Sketch in general terms; indicate the main features.

predict

Suggest what may happen based on available data.

propose

Put forward (a point of view, idea, argument, suggestion

etc) for consideration or action.

recall

Present remembered ideas, facts or experiences.

recommend

Provide reasons in favour.

recount

Retell a series of events.

summarise

Express concisely the relevant details.

synthesise

Put together various elements to make a whole.



viiScience press

Syllabus Cross-referenceChapter Unit Syllabus

Chapter 1 Tectonic Impacts

1.1 Movement of crustal plates 1. Lithospheric plates and their motion.

4. Natural disasters are often associated with tectonic activity and environmental conditions caused by this activity may contribute to the problems experienced by people.

1.2 Mountain building 2. The movement of plates results in mountain building.

1.3 How Australia has changed 3. Continents evolve as plate boundaries move and change.

1.4 Natural disasters – volcanic eruptions


1.5 Natural disasters – earthquakes


1.6 Plate tectonics and climate 5. Plate tectonics and climate.

Chapter 2 Environments through Time

2.1 The first life on Earth 1. Evidence from early Earth indicates the first life forms survived in changing habitats during the Archaean and Proterozoic eons.

2.2 Conditions for life on Earth 2. The environment of the Phanerozoic eon.

2.3 Dating the Cambrian explosion

3. The Cambrian event.

4. Exploiting new environments.

2.4 Evolution and the invasion of the land

4. Exploiting new environments.

2.5 Extinctions and radiations 5. Past extinction and mass extinction events.

Chapter 3 Caring for the Country

3.1 The development of Australian soils

1. Australia’s land surfaces exhibit the effects of long periods of weathering and erosion.

3.2 Soil erosion and its control 2. Soil as a resource that requires careful management.

3.3 The soil salinity problem 3. Salinity of soils and water.

3.4 The pesticides problem 4. The effect of excessive use and long-term consequences of using some pesticides.

3.5 Managing the supply of water 5. Maintenance of environmental flows and natural processes in water.

3.6 Global warming 6. The results of the Industrial Revolution on the atmosphere and hydrosphere.

3.7 Ozone depletion 6. The results of the Industrial Revolution on the atmosphere and hydrosphere.

3.8 Waste management 7. Rehabilitation and safe use of previously contaminated sites.

Chapter 4 Introduced Species and the Australian Environment

4.1 Introduced species in Australia

1. Survey of introduced species in Australia.

4.2 The vulnerable Australian environment

2. An analysis of introduced species indicates that they may impact on either the biological and/or the abiotic aspects of the environments.

4.3 Case study: Becoming a pest 3. Identification of the conditions leading to introduced species becoming pests.

4. Development of a case study on an introduced species that has had an impact on the physical and/or biological environment.

4.4 Biological control 5. Rehabilitation programs for ecosystems damaged by introduced species.

4.5 Quarantine methods – Keeping them out

6. Modern quarantine methods continue to restrict the introduction of new species to Australia.



viiiScience press

Chapter Unit Syllabus

Chapter 5 Organic Geology: A Non-renewable Resource

5.1 Making use of resources 1. The properties of economically important Earth materials formed from organic material.

5.2 Fossil fuels 1. The properties of economically important Earth materials formed from organic material.

5.3 Formation of fossil fuels 2. The environment, and process of coal and petroleum formation.

5.4 Coal 3. Searching for coal and oil.

4. The uses of coal and oil.

5.5 Petroleum 3. Searching for coal and oil.

4. The uses of coal and oil.

5.6 Fossil fuels and the greenhouse effect

5. The environmental impacts of fossil fuel use – complete versus incomplete combustion.

5.7 Alternative energy sources 6. The search for alternative sources of fuels.

Chapter 6 Oceanography

6.1 The oceans through time 1. The oceans have evolved over the history of Earth.

2. The shape, distribution and age of the current oceans has been determined by plate tectonics.

6.2 Oceans and salinity 3. There are differences in physical, chemical and biological environments between past and present-day oceans.

6.3 Oceans and climate 4. The mass motion of oceans influences terrestrial climates.

6.4 The ocean community 5. The physical conditions at different depths in the oceans constitute different environments and can support different communities of organisms.

6.5 Deep ocean vents 6. Hydrothermal vents support unusual communities.

6.6 Deep ocean sediments 7. The type of sediment that accumulates on the floor of the deep oceans varies according to water depth, supply of nutrients to surface waters, and distance to land masses.

6.7 Studying the oceans 8. Oceanographers have a range of technology available to assist the collection of data about the oceans.

Chapter 7 Mining and the Australian Environment

7.1 Formation of ores 1. The relationship between minerals and geological formations indicates where to search for the ore.

7.2 Mining and the law 2. The laws related to mining leases, rights of the land-holder and the role of governments in granting leases.

4. Ore deposits need to be evaluated before they can be mined.

7.3 Mining and processing the ore 3. There is a range of conditions under which mining an ore deposit becomes economically viable.

4. Ore deposits need to be evaluated before they can be mined.

7.4 Economics of mining 3. There is a range of conditions under which mining an ore deposit becomes economically viable.

7.5 Case study: Northparkes Copper-Gold Mine

4. The exploration and evaluation of an ore deposit.

5. Environmental issues need to be considered and dealt with during the exploration, extraction and processing of the ore.



1Science press

chapter 1TecToNIc ImpAcTs

By the end of this chapter you should be able to:

• describe the crustal plates and their motion

• explain how the movement of plates results in mountain building

• explain how continents grow and change shape as plate boundaries move and change

• describe how natural disasters are often associated with tectonic activity, such as earthquakes and volcanoes

• explain how environmental conditions caused by tectonic activity may contribute to the problems experienced by people

• explain the link between plate tectonics and climate, both in the short term and through geological history.

Earthquakes and volcanic eruptions cause a great deal of human misery in the short term (Figure 1.1), but they help produce abundant food, geothermal energy and many important minerals in the longer term. We also use the location of earthquakes and volcanic eruptions to map the edges of crustal plates. Earthquake waves are used to investigate the interior structure of the Earth.

Volcanic eruptions can alter the Earth’s climate for briefperiods of time. For example, they appear to have played a role in some of the major extinctions of life forms during geological history. However, bigger modifications to the Earth and its environment have resulted from changes in climate, which are in turn primarily caused by the movement of crustal platesovertheEarth’ssurface.Thismovementsometimesproduces either greenhouse or icehouse conditions.

Figure 1.1 Pyroclastic flow Volcanic ash races down the slope of Mount St Helens in the United States.



3Science press

chaptEr 1 tEctonic impactS

Another way to study oceanic crust is to find examples on

the surface of the Earth. For instance, Iceland sits on both

sides of a mid-ocean ridge – a long submarine mountain

range with a deep valley down the middle. This has allowed

geologists to examine the lavas produced (Figure 1.3).

There are also a few locations in other parts of the world

where oceanic crust has been pushed onto the surface of

the Earth, enabling us to examine its structure.

Figure 1.3 Studying oceanic crust Iceland sits astride a mid-ocean ridge and has allowed geologists to examine the lavas produced.

Oceanic crust is produced at mid-ocean ridges. Molten magma rises to the surface and solidifies to form dark mafic rocks on contact with cold ocean water. While the upper layers of the oceanic crust are fine-grained basalt, further down larger-grained gabbro can be found. The oldest rocks ever discovered on the ocean floor are around 175 million years old. This is the maximum time between the production of new ocean floor at the mid-ocean ridge, its movement away from the ridge to its destruction at a subduction zone, where it is forced down into the mantle.

The detailed shape of the ocean floor has taken some time to measure. Surface vessels can map the topography of the ocean bottom using sonar but this is quite slow. In recent years data has come from satellites which use satellite radar to determine the topography of the ocean floor by measuring the shape of the surface of the ocean (Figure 1.4). This is because the force of gravity due to the rocks on the ocean floor alters the depth of water by a few centimetres.

Continental crustContinental crust is thicker and more complex than oceanic crust and is mainly composed of light-coloured felsic rocks, such as granite. Beneath the granitic rocks is a layer of darker mafic rocks, mostly gabbro. Continental crust is about 40 kilometres thick, but it can be 65 kilometres thick under mountain ranges.

Unit 1.1 Movement of crustal platesA knowledge of crustal plates, their movement and their boundaries is vital in a number of ways. At a practical level it helps predict the locations of not only potentially destructive earthquakes and volcanoes but also valuable minerals. As well, the movement of crustal plates helps us to explain the current distribution of plants and animals and to predict the possible effects of future climate change.

Crustal plates TheEarth’scrustis not a single solid layer like the shell of an egg – rather, it is broken into several dozen pieces similar to a cracked eggshell or a jigsaw puzzle. However, it is more complex than this. Unlike the pieces of a jigsaw puzzle, the pieces of crust are moving. Each of the large pieces of crust is called a plate.AlloftheEarth’splates,sevenofwhichareverylarge,moveatafairlycommon‘speed’of5centimetresper year, although this varies a lot.

Oceanic crustOceanic crust is much thinner and simpler than continental crust. It ranges from 3 to 15 kilometres thick – however the average 3.7-kilometre depth of the oceans makes its study difficult. The composition and vertical structure of the ocean floor can be studied in a number of ways. Seismic waves, which are generated by earthquakes or artificially produced, allow the layered structure to be determined. Some boreholes have been drilled into the ocean floor but only through the sedimentary layers so that the basaltic rock beneath could be recovered and analysed.

Ships have long been able to dredge the ocean floor and return samples to the surface, but they could not see what they were collecting. In recent years deep-sea submersibles have been able to visit the deep ocean floor, allowing geologists to carefully select samples for study. They have also been able to observe the formation of pillow lavas at mid-ocean ridges (Figure 1.2).

Figure 1.2 Forming new crust Pillow lava forming along a mid-ocean ridge.



4Science press

chaptEr 1 tEctonic impactS Unit 1.1 movement of crustal plates

Australia

New Zealand

New Guinea

Trench

Trench

Trench

Trench

LordHoweRise

NorfolkRidge

Fiji

New Caledonia

Vanuatu

Solomon Is

Figure 1.4 The ocean floor Modern satellites can measure variations in sea level so accurately that maps of the ocean floor like this one can be produced.

The formation of felsic rocks is complex. As magma rises, crystallisation of minerals results in mafic rocks. The remaining magma is high in light-coloured minerals such as feldspar and quartz. As well, the invading magma may melt some of the surrounding rocks and they become incorporated into the magma. If the rock slowly cools underground then granite with large crystals results. If the rock cools more quickly then andesite with smaller crystals may result. If the magma reaches the surface, gases are lost and the lava formed crystallises very quickly to form rhyolite.

The granitic material is less dense than the gabbro beneath – about 2.7 grams per cubic centimetre for the granite and around 3 grams per cubic centimetre for the gabbro. In a sense the graniticmaterial ‘floats’ on the gabbro and thefluid mantle beneath. Just as a ship sinks further into the water when carrying a cargo, the continents sink further into the mantle to carry the weight of the mountains above. This principle is called isostasy.

Speed of movementHow fast do plates move? The first method used to measure their speed made use of the oldest rocks on the ocean floor. If the oldest rocks are about 175 million years old, we can measure the distance to an appropriate mid-ocean ridge and calculate the speed.

The magnetic patterns left on the ocean floor can also be used to measure this speed. For reasons not yet fully understood, the polarity of the Earth’s magnetic fieldreverses every few million years.

In other words, the magnetic north pole becomes the magnetic south pole and vice versa. As magma rises from the mid-ocean ridges, a banded pattern of magnetism is produced, thus allowing geologists to determine the movements of the ocean floor away from the mid-ocean ridges and toward subduction zones. This method produces results ranging from 2 centimetres per year at the Mid-Atlantic Ridge and 12 centimetres per year in the eastern Pacific.

The speed of movement is not our only concern – direction is important as well. Crustal plates are rigid bodies and move across the asthenosphere as such. By locating both the mid-ocean ridge where crustal plates are created and the subduction zone where they are destroyed, we can infer their direction of movement.

The pattern of magnetism left on the ocean floor can indicate not only speed but also if the direction of movement has changed. In a few cases, hot spots on the middle of a plate will show the direction of movement from the line of volcanic islands they produce.

You may be familiar with the use of the global positioning system(GPS)tofindaperson’slocationinacityoraremoteplace. Originally developed for military use, it can be used both to measure the movements of the continents and to show the speed and direction of the plates.

GPS produces results similar to those from the magnetic patterns – speeds range from around 1.8 centimetres per year in the north Atlantic to more than 24 centimetres per year near Samoa (Figure 1.5). The fastest plates are quite small as most have been subducted; the slowest plates have no subduction zones, or carry large continents.

200 km

X

Y

Daly WatersWyndham

DerbyCairns

TownsvilleDampier

Alice SpringsRockhampton

Brisbane

Broken Hill

GeraldtonKalgoorlie

Perth

EsperanceAlbany

Sydney

CanberraAdelaide

Melbourne

AUSTRALIA

Hobart

Figure 1.5 Satellites measure movement The distance to three satellitesiscalculated,allowingone’spositiontobedetermined.



5Science press

chaptEr 1 tEctonic impactSUnit 1.1 movement of crustal plates

Plate edges – where the action isCrust is produced at divergent zones, where two plates separate. Such zones occur either at mid-ocean ridges or on land. The best studied mid-ocean ridge is in the Atlantic Ocean, and includes the island of Iceland. The most famous example on land is the Great Rift Valley (Figure 1.7). As the continent moves apart, a valley forms. As a result, the area has numerous volcanoes that produce basaltic lava and experiences earthquakes whose foci are near the surface. For the Great Rift Valley, the sea will eventually invade the area as the floor sinks. This has already happened in two other rift valleys – the Red Sea and the Gulf of California.

If new crust is created, an equal amount must be destroyed somewhere else. If this did not happen the overall diameter of the Earth would increase. This destruction occurs at convergent zones when crustal plates meet, one of which may be subducted. The details depend on how the collision occurs (Table 1.1).

When two oceanic plates meet, one plate may be subducted and a chain of islands such as Japan may result. If an oceanic plate meets a continental plate then the oceanic plate may be subducted and a chain of mountains such as the Andes may result. If two continental plates meet the crust may crumple without subduction to form a mountain chain such as the Himalayas.

ACTIVITY 1.1 MEASURING PLATE MOVEMENTS

1. Some rocks dredged from the ocean floor in the western Pacific are found to be 105 million years old. They are located 2100 kilometres from their most likely source at a mid-ocean ridge. Calculate their average speed of travel.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

2. Figure 1.5 shows how the position of a GPS device located at X can be determined from a satellite. Many years later the three satellites located the GPS device at point Y. In which direction has the area moved?

..............................................................................................................................................................................................................................................

3. Figure 1.6 shows a pattern of magnetic anomalies on the ocean floor. Also marked are the distances and ages of rock samples obtained from boreholes drilled down to the magnetised rocks.

(a) Find the average speed for the rocks to reach points X and Y.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

(b) Is the speed at which magma is produced constant? Justify your answer.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

ACTIVITY 1.2 SEA FLOOR SPREADING

There are a number of animations that show the formation of magnetic patterns on the ocean floor as it moves away from a mid-ocean ridge. You can find the animations by usingasearchenginetolookfor‘animationmagneticmid-oceanridge’.Atthetimeofwritinganimationscouldbefound at:

• www2.nature.nps.gov/geology/usgsnps/animate/pltecan.html

• www.classzone.com/books/earth_science/ terc/navigation/visualization.cfm

125 km Y

X

5.0 million

years old

2.8

2.8 3.3

3.3

0.7 0 0.7

5.0

N

N

NN

N

N N

N

NS

S

S

S

S

S

S

S

Mid-ocean ridge

Direction ofmovement

Direction ofmovement

Figure 1.6 Magnetic anomalies What is the average speed to reach points X and Y?



6Science press


Table 1.1 Magma and igneous rocks.

Location Type of movement Magma formation Lavas produced

Divergent boundaries

Mid-ocean ridges Partial melting of the asthenosphere produces mafic magma. Magma rises and collects in a magma chamber in the lower oceanic crust.

Mafic lavas erupt to form pillow basalts on the ocean floor. Magma also cools at depth to form multitudes of basalt dykes and lower in the oceanic crust layers of gabbros are produced.

Continental rifts Same as mid-ocean ridge. Two types of eruption. Mafic lavas erupt to form basalts, but rising mafic magmas also melt continental crust causing felsic lavas to erupt forming rhyolites.

Convergent boundaries

Oceanic-oceanic subduction zones

The mixing of water with the surrounding asthenosphere lowers the melting point to produce mafic rich magmas.

Eruptions range from mafic to intermediate. Often early eruptions are mostly mafic becoming more intermediate as the island arc grows.

Oceanic-continental subduction zones

Same as oceanic-oceanic. Eruptions are predominantly intermediate with some mafic magmas. Rising magmas are altered by melting and then mixing with molten continental crust. Heating and hydration of the continental crust also causes felsic magmas to form which usually solidify at depth forming large masses of granite called plutons.

Continental collisions

Small amounts of felsic magmas formed as wet sedimentary material is compressed due to crustal thickening.

None. These felsic magmas solidify before reaching the surface forming large masses of granite called plutons.

Hot spots Under oceanic crust Partial melting in the asthenosphere produces mafic magmas.

Two types of eruption. Mafic lavas erupt to form basalts, but rising mafic magmas also melt continental crust causing felsic lavas to erupt forming rhyolites.

Under continental crust

Same as under oceanic crust. Mafic lavas erupt.

Red Sea

Persian Gulf

ARABIANPLATE

EURASIANPLATE

AFRICANPLATE

Equator

NileRiver

IndianOcean

Plate movement

Plate boundaries

Rift Valley

Volcano

(a)

(b) Cross-section Rift Valley

Magma

AFRICAN PLATE SOMALI SUBPLATE

(b)

Figure 1.7 Great Rift Valley (a) As the continent moves apart, the crust sinks and ocean water begins to intrude. (b) Cross-section.



7Science press


The crustal plates may pass next to each other in opposite directions such as the San Andreas fault. These transform or conservative boundaries often show little volcanic activity.

Divergent and convergent zones produce magmas that sometimes solidify under ground to produce large granitic masses called plutons. Locally, examples are found under both the Blue Mountains, west of Sydney (visible at Hartley), and the New England Plateau. When magmas reach the surface with little mixing with surrounding rocks, the lavas solidify as lava flows composed of mafic rocks, such as basalt. The lava flows generated by such eruptions can be seen at Mount Tomah, west of Sydney.

However, if the hot rising magmas melt continental crust, the felsic lavas formed solidify to produce rocks such as rhyolite and andesite. These can be found around Eden and Mount Warning, in southern and northern New South Wales respectively. The association between plate tectonics – the rearrangement of the Earth’s continentsthrough time – and lava types is summarised in Table 1.1.

An analysis of igneous rocks can tell us a lot about the previous geology of an area. (In unit 1.3 we will see how the igneous rocks in an area can determine if a divergent or convergent zone once existed.) They also assist with the location of mineral-rich zones. For example, massive sulfide ore bodies that are rich in iron, copper and other minerals are found along mid-ocean ridges, near to which are large nodules of manganese ores that cover the sea floor.

Why plates moveThe theory of plate tectonics describes how the plates move but does not explain why. What do we know about the Earth’smantleandcrustthatmighthelpusdeterminethemechanism involved?

• Seismic data tells us that the mantle is very viscous –transverse seismic secondary waves will not pass through it.

• ThecoreoftheEarthisquitehot–theheatwasleftoverfromtheEarth’sformationandisproducedbythedecayof radioactive isotopes.

• Themantlenearthesurfaceiscoolerowingtothelossof heat through the thin crust.

• Hotmaterialrisesandcoldmaterialsinks.

• Theslabofcrustatsubductionzonesextendsalongwayinto the mantle.

• Mountainsofdensevolcanicrocksexistalongmid-oceanridges.

• Astherocksmigratefrommid-oceanridgestosubductionzones, they cool and become even more dense.

• Densermaterialssinkwhenplacedintolessdensefluidssuch as those in the mantle.

• Plate movements are at constant speed, neitheraccelerating nor decelerating. This implies the forces are nearly in balance.

• Plateswithlargesubductionedgesrelativetotheirsizemovemore quickly than those with small subduction edges.

ACTIVITY 1.3 MODELLING THE MOVEMENT OF CRUSTAL PLATES

Two models of crustal movement are shown in Figure 1.8.

1. What is the main driving force in the convection current model?

..............................................................................................................................................................................................................................................

Figure 1.8 Two models of plate tectonics (a) In the convection current model the plates are moved by convection currents intheEarth’smantle.(b)Inthepush-pullmodelthecrustalplatesmovebecauseofforcesintheplatesthemselves.Thepushresults from the weight of crust high on mid-ocean ridges; the pull, from the weight of denser crust as it sinks into the mantle.

Subductionzone

Mantle

Outercore

Innercore

Convectioncell

Mid-oceanridge Lithosphere

Subductionzone

Mantle Mantle

Outercore

Innercore

Denserocksinks Dense

rocksinks

Plates slide off

mid-ocean ridge(a) (b)



8Science press


2. What evidence listed above supports the convection current model? What contradicts it?

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

3. What is the main driving force in the push-pull model?

..............................................................................................................................................................................................................................................

4. What evidence listed above supports the push-pull model? What contradicts it?

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

5. Compare the convection current and push-pull models.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

6. Assess which of the models is more likely. If neither seems reasonable, propose an alternative model.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

ACTIVITY 1.4 MOVEMENT OF CRUSTAL PLATES

ThereareanumberofanimationsthatshowtheconvectioncurrentsintheEarth’smantle.Youcanfindtheanimations byusingasearchenginetolookfor‘animationmantleconvection’.Atthetimeofwritinganimationscouldbefoundat:

• http://homepages.see.leeds.ac.uk/~eargah/Conv.html

• www.edumedia-sciences.com/a399_l2-mantle-convection.html

Identify the main driving force being pictured in each animation.

1. Describe means provide characteristics and features. Be as thorough as the word limit will allow, making sure you concentrate on the most important points. You do not have to explain or interpret.

Describe the characteristics of lithospheric plates.

Points you may wish to include are:

• ThelithosphereisthesolidmantleandcrustoftheEarth.

• Thecrustisthethin,hardoutersurfaceoftheEarth.

• Thecrustisdividedintoanumberoflargemovablesegmentscalledplates.

• Thecrustalplates‘float’onthemantlebeneath.

• ThecrustalplatesaremovingoverthesurfaceoftheEarth.

• Crustalplatesarecreatedatmid-oceanridges.

• Crustalplatesaredestroyedatsubductionzones.

• Theedgesof crustal plates canbedetermined from theearthquakeand volcanic activity thatoccurswhen crust is createdordestroyed.

SCIENCE SKILLS



9Science press


TO THINK ABOUT

Set 1

1.1.1 Define the term crustal plate.

........................................................................................................................................................................................................................................................

1.1.2 Contrast the chemical properties and modes of formation of mafic and felsic igneous rocks.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.3 Distinguish between convergent and divergent plate boundaries.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.4 Describe what happens during subduction.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

2. Distinguish means recognise or note/indicate as being distinct or different from; note differences between.

Distinguish between mafic and felsic rocks.


• Maficrocksaredarkcolouredigneousrocksduetohighconcentrationsofferromagnesianminerals.

• Maficrocksareproducedatmid-oceanridgesandotherplaceswheremagmarisestothesurface.

• Felsicrocksarelightcolouredigneousrock,withrelativelylargeamountsoffeldsparsandquartz.

• Felsicrocksareproducedfromtheremainingmagmaaftermineralsthatformmaficrockshavecrystallisedout.Theyarealsoformedwhen magma melts surrounding rocks.

3. Contrast means show how things are different or opposite.

Contrast the convection model and push-pull model to explain plate movements.


In the convection model:

• Unequalheatdistributioninthemantlemayproduceconvectioncellsbelowthelithosphere.

• Hotmaterialrises(correlatestospreadingcentre),spreadslaterally,coolsandsinksdeeperintothemantletobereheated.

In the push-pull model:

• Lithosphericplatesarepushedapartathotspreadingcentressuchasmid-oceanridges.

• Coldlithosphericplatesaredenseandwhentheyreachsubductionzonestendtosinkintothemantle,pullingtherestoftheplatewithit. Each part of the model can operate independently and are gravity driven.

• Frictionbetweenthemovingmantleandcrustalplatesresultsintheplatesmoving.



10Science press


1.1.5 Describe what happens at mid-ocean ridges.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

Set 2

1.1.6 Contrast oceanic and continental crust.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.7 Identify the relationship between the general composition of igneous rocks and plate boundary type.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.8 Describe the motion of crustal plates.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.9 Distinguish between the three types of plate boundaries: convergent, divergent and conservative.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.1.10 Describe current hypotheses used to explain plate motion.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


11Science press



Unit 1.2 Mountain building Mountain ranges provide some of the most spectacular scenery in the world. However, Australia has such an ancient landscape that extensive erosion means its mountains are quite small. For example, Mount Kosciuszko is only 2228 metres in height compared with Mount Everest at 8848 metres.

Figure 1.9 Mountain escarpment The Great Dividing Range is the result of the warping of crust during the formation of the Tasman Sea. Erosion has produced the escarpment.

ThemountainsoftheGreatDividingRangealongAustralia’seast coast are some of our more recent mountains, forming around 80 million years ago by the rifting of the ocean floor as Australia and New Zealand separated (Figure 1.9). In contrast, the Himalayas of northern India resulted from the huge forces produced during the collision of two continental plates around 25 million years ago.

Mountains and divergent plate boundaries When plates move apart from each other, or diverge, there is a weakness in the crust. Magma is then able to come to the surface from the mantle below. When it reaches the surface it loses gases and forms lava. As it cools down, this lava creates new crust.

The mountains that result form the mid-ocean ridges (Figure 1.10). These ridges are chains of mountains running along the ocean floor. Sometimes the tops of the mountains rise above the surface of the ocean water to form an island such as Iceland.

Figure 1.10 Mid-ocean ridges A chain of mountains formed mostly under the oceans at divergent plate boundaries such as the East-Pacific Rise.

Many volcanoes are found along the mid-ocean ridges. These ocean floor volcanoes produce mafic rocks such as basalt from the mantle. Because the lavas are formed in water, they cool quickly while flowing down the side of the underwater volcano. They are called pillow lavas because the cooling basalt forms pillow-shaped lumps.

Mountains and convergent plate boundaries When crustal plates collide, the enormous compression forces involved help create mountain ranges. This is why the mountains of New Zealand and New Guinea are so much more spectacular than those of Australia. They are both located where crustal plates meet.

Australia has been located toward the centre of a crustal plate for such a long time that the effects of weathering and erosion have worn down what mountains have formed. For this reason we will need to look at mountains in other parts of the world to see the effects of plate collisions.

Figure 1.11 shows a simplified picture of New Zealand geology. You will notice that a transform plate boundary connects two subduction zones. The alpine fault has moved some 400 kilometres at a rate of around 1 centimetre per year. This movement still occurs and results in extensive earthquakes.

On a cross-section of South Island shown in Figure 1.12, you can see a number of major geological structures produced by the intense forces of crustal plates moving relative to each other. As a result, many of the rocks are metamorphosed. Associated volcanic activity can result in even more complex geological structures.


12Science press

chaptEr 1 tEctonic impactS Unit 1.2 mountain building


Alpine fault

37 mm/yr

38 mm/yr

Nelson

47 mm/yr

0 400

km

Tonga-Kermadec Tr

ench

Metamorphic rocks

Old oceanic crust (Ophiolites)

Schist and gneiss

Volcanic and ultramafic rocks

Mac

quar

ie T

renc

h

Figure 1.11 Southern Alps, New Zealand Deformations along the fault have created the very high mountains of the South Island.

Conglomerates

Volcanics

Mixed sedimentaryrocks

Volcanic intrusionsSchists

Semischists

Quartzite

Faults FaultFault

0

–3000 m

NW SE

0 5 10 20 km15

Figure 1.12 South Island, New Zealand This geological section is through the north of South Island, near Nelson.

Compression forces can produce folds and faults. The geological structures that result depend on the brittleness of the rock and the time over which the forces are exerted. Rocks deep in the crust are much hotter and may fold more readily than those on the surface. Rocks that bend or fold can produce arches or upfolds called anticlines and troughs or downfolds called synclines (Figure 1.13).

If the compression forces are very large or occur over a short time or the rocks are brittle, they may snap to produce faults. If the rock above the fault line slides over the other, it is called a reverse fault. Some of the largest known faults result from compression forces at subduction zones.

Tension forces can also produce folds and faults. Monoclines result when one end of a horizontal bed is raised, producing a gentle incline. Often this is associated with faulting, as has occurred on the Lapstone Monocline near Sydney (Figure 1.20). Normal faults produced by tension forces show evidence of the side above the fault line moving down relative to the other. When on a massive scale, such faults include those that occur in rift valleys (Figure 1.7).

Compressionforces

Tensionforces

Shearforces

Folding

Faulting Faulting Faulting

StretchingTension

Uplift

Shearing

(a) (b) (c)

Figure 1.13 Folding and faulting (a) Compression forces result in folding or reverse faulting. (b) Tension forces can result in monoclines or normal faulting. (c) Shear forces produce folds and faults in the horizontal plane.

The forces that produce geological structures can also result from shear forces. These produce movements in horizontal directions. Both faulting and folding can occur in the horizontal plane. On a small scale, they are called lateral faults. Transform faults, where crustal plates slide past one another, are an example of shear forces generating faulting on a massive scale.

Many of these geological structures occur together on a massive scale where crustal plates meet. South Island, New Zealand, for example, shows massive folding and faulting. In the distant past there must have been similar forces in Australia – the McDonnell Ranges, near Alice Springs, were brought about by immense faulting and folding (Figure 1.14).

Waterhouse Range

SouthNorth

Alice Springs

Carboniferous sediments

Devonian sandstone

Ordovician sandstone

Cambrian sediments Quartzite

LimestoneProterozoicsediments

Granite

Gneiss

0 5 10 km

Faults

Erosion surfaces

Erosion surfaces

}

Figure 1.14 McDonnell Ranges (a) Erosion has cut through the folded ranges. (b) Mountain ranges produced by the collisions of crustal plates will show folding and faulting.

(a) (b)


13Science press

chaptEr 1 tEctonic impactSUnit 1.2 mountain building


Mountains and riftingBoth tension and compression forces can work together to form mountains. We have already seen how tension forces produce rift valleys, such as the Great Rift Valley. The Great Dividing Range along eastern Australia has also been produced by rifting. Starting around 95 million years ago, tension forces caused the Australian continent to separate from what was to become New Zealand. The Tasman Sea formed as the crust separated along a rift zone (Figure 1.15).

Australia

Antarctica

South Pole

60 mya

Continentalmargin

Sea floorspreading

Elevatedterrain

NewZealand

Rifting

TasmanSea

Figure 1.15 Tasman Sea Rifting along the sea floor saw the Australian continent separate from what was to become New Zealand, causing the formation of the Great Dividing Range.

As crust moved toward the west, the inertia of the rest of the Australian continent produced compression forces that resulted in the warping of crust along what is now the Great Dividing Range. On the western side, the land dips gradually toward Central Australia. Erosion along the seaward side has led to the formation of a coastal plane and an escarpment.

Most of the Great Dividing Range is 700-1000 metres high. In the Snowy Mountains region it rises to the 2228 metres of Mount Kosciuszko. Here major faulting with block uplift, involving a kilometre of vertical movement, occurred in Miocene times.

Bobbing continents Many parts of the world have horizontal strata that are a kilometre or more above sea level. However such strata contain marine fossils that prove they were once below sea level – either the sea rose, the land sank or both. The Australian continent had a related problem – it gradually sank during the Cretaceous without disturbing the continental crust significantly and with large parts being covered by ocean. Plate tectonics cannot explain these gentle up and down movements.

Seismic evidence and studies of gravity have shown that huge masses of molten rock rise from the outer surface of the core – a process that might be called thermal uplift (Figure 1.16a). One of these superplumes is under southernAfrica.Ithasresultedinoneoftheworld’slargestand highest plateaus whose relatively horizontal rocks are more than 1.5 kilometres high and 1500 kilometres across. This process also explains the similar-sized plateaus in the western United States.

ACTIVITY 1.5 CONVERGENT PLATE BOUNDARIES

There are a number of animations that show the formation of mountains at convergent plate boundaries. You can find theanimationsbyusingasearchenginetolookfor‘animationsubduction’or‘animationHimalayas’.Atthetimeofwriting animations could be found at:

• http://www.pbs.org/wnet/savageearth/animations/index.html

• http://serc.carleton.edu/NAGTWorkshops/visualization/collections/orogeny.html

• www.wwnorton.com/college/geo/egeo/animations/ch2.htm

• www.classzone.com/books/earth_science/terc/navigation/visualization.cfm

ACTIVITY 1.6 RIFTING

There are a number of animations that show the formation of rift valleys. You can find the animations by using a search enginetolookfor‘animationrifting’.Atthetimeofwritinganimationscouldbefoundat:

• www.wwnorton.com/college/geo/egeo/animations/ch2.htm

• www.school-portal.co.uk/GroupDownloadFile.asp?file=21402


14Science press



Superplume

Africarises

Drags Australiadownward

Descending crustfrom old subduction

(b)

(a) Australia has been subjected to the reverse process – a sinking continent (Figure 1.16b). The Gondwana supercontinent was originally surrounded by a subduction zone. When it began to break up, the zone ceased to exist. However, large slabs of oceanic crust were still sinking through the mantle. When the Australian continent moved over these subduction zones during the Cretaceous, it began to sink and large parts were covered by ocean. It rose again once it had moved past the old subduction zone.

ACTIVITY 1.7 RISING AND FALLING CONTINENTS

The rise and fall of Australia has been animated – the CSIRO version includes a commentary. You can find this animationbyusingasearchenginetolookfor‘upsanddownsofAustralia’.Atthetimeofwritinganimationscouldbe found at:

• www.earthbyte.org/Resources/Movies/aad.html

• www.csiro.au/promos/ozadvances/Series5UpsDowns.html

ACTIVITY 1.8 COMPARING HOW MOUNTAINS FORM

Use the data in this unit to complete Table 1.2.

Table 1.2 How mountains form.

Process of formation Example Rock types Structure of mountain

Thermal uplift

Rifting

Collision between two oceanic plates

Collision between oceanic plate and continent

Collision between two continents

Figure 1.16 Bobbing continents (a) A superplume of hot, buoyant magma comes up from the outer surface to cause a continent to rise. (b) A large piece of subducted continental plate sinks through the mantle and drags any continent above it downward.


15Science press

chaptEr 1 tEctonic impactSUnit 1.2 mountain building


1. Explain means relate cause and effect; make the relationships between things evident; provide why and/or how. You need to clarify and interpret the material you present. Where appropriate, give reasons for differences of opinions or results, and try to analyse causes.

Explain why Iceland is made from mafic rocks and has extensive rifting.


• Icelandisapartofamid-oceanridgethatisabovesealevel.

• Sincethemid-oceanridgeisadivergentplateboundary,eachsideismovinginoppositedirectionsresultinginextensiverifting.

• TherocksthatcooltoformIcelandcomefromthemantleandaremaficincomposition.

SCIENCE SKILLS

TO THINK ABOUT

Set 1

1.2.1 Contrast an anticline with a syncline.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.2 Contrast a normal and a reverse fault.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.3 Compare the effects that compression and tension forces have on rocks.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.4 Explain how horizontal sediments can be raised vertically hundreds of metres with little disturbance.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.5 Explain how horizontal sediments can be caused to sink hundreds of metres with little disturbance.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


16Science press



Set 2

1.2.6 A mountain range is highly folded and faulted with extensive lava flows. The rocks are metamorphosed marine sediments crossed by volcanic dykes. Explain how the mountain range might have formed.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.7 A mountain range is highly folded and faulted. The rocks are metamorphosed freshwater sediments. Explain how the mountain range might have formed.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.8 A large plateau is 900 metres above sea level and is composed of horizontal layers of relatively undisturbed sedimentary rock. The plateau is 1000 kilometres from any plate boundary. Explain how it might have formed.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.9 Distinguish between mountain belts formed at divergent and convergent plate boundaries.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.2.10 Explain why Mount Kosciuszko is much higher than the rest of the Great Dividing Range.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


17Science press



Unit 1.3 How Australia has changed The overall history of the Australian continent is relatively easy to determine following the break-up of Pangaea at the end of the Permian. Evidence of plate movements still exists from this time on the ocean floor. Plate movements before the Permian, however, are harder to predict because ocean-floor evidence has long ago been subducted into the mantle. All we have to go on is the evidence preserved in various ancient mountain chains found on the continents.

When the evidence is pieced together we find that at least twice in the Earth’s history the continents have joined toform one body that later broke apart. The process seems to be cyclic and, for this reason, has been called the tectonic supercycle (or the supercontinent cycle). It has helped shape the geology and climate and thereby influence biological evolution.

Plate tectonic supercycleIn Unit 1.1 we looked at why crustal plates move. It is thus easy to imagine that from time to time these movements result in the formation of a large supercontinent. This seems to have happened twice – the supercontinent Rodinia broke up around 750 mya in the late Proterozoic and the resulting crustal plateswandered theEarth’s surfaceuntilthey created a new supercontinent called Pangaea around 280 mya in the early Permian (Figure 1.17).

Pangaea itself began to separate in the late Permian, becoming two large continents called Laurasia and Gondwana. Gradually, the northern continent, Laurasia, divided to form North America and Europe. The southern continent of Gondwana split up to form South America, Africa, India, Antarctica and Australia. This process is continuing today.

You might be wondering why the large supercontinents would break apart. When you have such a huge supercontinent, it acts as an insulating blanket over the mantle. This results in a heat build-up, causing expansion. The continents rise, and eventually cracks appear that begin to rift apart. The upwelling of hot material then provides the new ocean floor.

This understanding of plate tectonics has brought about significant change for the science of geology:

• Economicgeologistsusethepreviouspositionsofcrustalplates to help locate mineral resources.

• Palaeontologistshavehadtorethinkaspectsofevolution.

• Mechanismsnowexisttoprovideexplanationsofgeologicalstructures and landforms.

• Oceanographers are using plate tectonics to better understand the ocean currents.

SouthChina

Kazakhstan

Baltics

North America

NorthAmerica

NorthAmerica

South America

SouthAmerica Africa

Africa

Africa

India

280 myaEarlyPermian

335 myaEarlyCarboniferous

SouthAmerica

Antarctica

SouthAmerica

AntarcticaIndia

535 mya Early CambrianNorthChina

Siberia

460 mya Middle Devonian

400 mya Early Devonian

NorthAmerica

Figure 1.17 First cycle The supercontinent of Rodinia breaks up in the late Proterozoic. The movements from the early Cambrian to the formation of the new supercontinent of Pangaea in the early Permian are shown.


18Science press

chaptEr 1 tEctonic impactS Unit 1.3 how australia has changed


ACTIVITY 1.9 PLATE TECTONIC SUPERCYCLE

There are a number of animations that show the movement of the continents throughout geological history. You can findtheanimationsbyusingasearchenginetolookfor‘animationcontinentaldrift’or‘animationplatetectonics’.Atthe time of writing animations could be found at:

• www.ucmp.berkeley.edu/geology/anim1.html


ACTIVITY 1.10 DESCRIBING THE PLATE TECTONIC SUPERCYCLE CONCEPT

By using Figure 1.17 and other information from this unit and secondary sources, summarise the plate tectonic supercycle by completing Table 1.3.

Table 1.3 Plate tectonic supercycle.

Eon Era PeriodAge

(millions of years ago)

Major continents Major movements of continents in progress

Phanerozoic

Cainozoic

Quaternary

1.8

65.5

145.5

199.6

251

299

359

416

444

488

542

–2500

–4000

–4600

Tertiary

Mesozoic

Cretaceous

Jurassic

Triassic

Palaeozoic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Proterozoic

PrecambrianArchean

Hadean


19Science press

chaptEr 1 tEctonic impactSUnit 1.3 how australia has changed


Growth of AustraliaThe movement of Australia through time can be seen in Figure 1.17. As previously mentioned in this chapter, the passage of Australia over old subduction zones has resulted in the lowering of the continent, its sinking under water and then its rising again. As a result, the overall size and shape of Australia has changed over time and much of the continent was formed in regions of the Earth far removed from where we are today.

Each of the continents has the same basic structure (Figure 1.18):

• largestableareasofPrecambrianrocks

• beltsoffoldedrocksshowingintensedeformation

• broadareasofrelativelyundisturbedsedimentaryrockscalled continental basins lying on top of the stable Precambrian areas and belts of folded rocks.

Somewhere in each continent are large stable areas of complex Precambrian rocks known as cratons. These rocks underwent intense deformation in Precambrian times and are composed of granite and various metamorphics. If cratons are exposed at the surface they can be called shields,

though part of them might be covered by relatively thin layers of undisturbed sedimentary rocks called platforms. In Australia the major cratons are in the western part of the continent and were in place by the beginning of the Proterozoic. How cratons might have formed is the subject of a lot of geological investigation.

The eastern part of Australia is a region of fold belts – also called mobile belts. Even though such folded rocks show they were once along the edge of a crustal plate, they are now stable. In the period between the end of the Proterozoic and the break-up of Gondwana, a subduction zone was present along the eastern edge of the growing continent. Thus for a period of 300 million years the Australian continent grew toward the east. The eastern fold belt shows evidence of a number of volcanic island arcs off the‘coast’ofthecraton.Betweentheislandarcsandthecratonwas a shallow inland sea.

Many of the processes we have already looked at when crustal plates collide helped stabilise the fold belt (Figure 1.19). Between the subduction zone and the island arc is piled-up sediment scraped from the oceanic crust. The water that accompanies the crust as it descends into the mantle helps to lower the melting points of the rock so that any magma that forms rises toward the surface.

PilbaraBlock

KimberleyBlock

Mt IsaBlock

YilgarnBlock

Broken Hill

0 800 km

Tibooburra Bourke Brewarrina Walgett Bingara Inverell Glen Innes

ClarenceBasin

Great Artesian Basin

Metamorphics Igneous Sedimentary

Grafton

(a)

(b)

Figure 1.18 The Australian continent (a) The major shields are in the western part of the country, with fold belts along the eastern parts. (b) A cross-section shows how the Great Artesian Basin lies in a depression of the fold belt.


20Science press



Adelaide Broken Hill Bathurst Sydney Tonga TrenchLord Howe Island

Present position:

Craton

Late Cambrian (500 mya)

Early Carboniferous (350 mya)

Upper mantle

OceanOceanic crustContinental crust

Accretion wedge

Lord HoweIsland rise

Sydney Basin

Sydney Basin

Permian-Triassic (250 mya)

Subduction

Subduction

Subduction

Tertiary (60 mya) TasmanSearift

Lord Howe IslandSydney Basin

Subduction

TongaTrenchPresent

Tasman Sea

Rifting occurs from around100 mya to around 50 mya

Figure 1.19 Growing continent Island arcs are stabilised by metamorphism and the intrusion of magma.

Some escapes as lava, but a lot cools beneath the surface to form large batholiths, such as the one under the New England Plateau.

The combined effects of volcanism and the metamorphism due to the pressure of the sliding plates result in the stabilising of the mountains in the island arc. As the island arcs move eastward, they leave behind folded but stable rocks that we see today as fold belts. Currently, the subduction zone has moved hundreds of kilometres and is located along a line running from Tonga to New Zealand.

Large areas of relatively undisturbed sediments called continental basins lie on top of the shields and fold belts, the most famous of which is the Great Artesian Basin. This formed during the Jurassic and Cretaceous behind the island arcs as they moved eastward. Further east lies the Sydney Basin, with its extensive Permian coal deposits overlaid by sandstones and shales of the Triassic (Figure 1.20).


21Science press



ACTIVITY 1.11 GROWTH OF AUSTRALIA

An animation showing the geological history of Australia can be found on the Science Press website.

ACTIVITY 1.12 AUSTRALIA: A GROWING CONTINENT

You will need a copy of a geological map of Australia to answer these questions.

1. Place a ruler so that it runs north-south through the Western Australian town of Broome. What are the ages of the rocks along this part of the continent?

..............................................................................................................................................................................................................................................

2. Repeat by placing the ruler so that it runs north-south through first Alice Springs and then Canberra. What are the ages of the rocks along this part of the continent?

..............................................................................................................................................................................................................................................

3. What general trend do you find in the ages of the rocks as you move from west to east across Australia?

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

4. Outline an explanation for the pattern you have found in the ages of rocks.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

5. Is Australia continuing to grow toward the east? Justify your answer.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

Mt Tomah

Fault LapstoneMonocline Prospect

Lithgow

Bondi

0

1000 m

-1000 m

Shoalhavengroup (Permian)

Illawarra coalmeasures (Permian)

Narrabeengroup (Triassic)

Hawkesburysandstone (Triassic)

Wianamattagroup (Triassic) Basalt (Tertiary)

Alluvium (Quaternary)Granite(Carboniferous)

Metamorphics(Devonian)

GUNNEDAH BASIN

............

...........

.....

NEW ENGLANDFOLD BELT

LACHLANFOLDBELT

SYDNEYBASIN

Ulan Muswellbrook

Thrust

Singleton

Cessnock

Newcastle

Laps

tone

Mon

oclin

e

Antic

line

Antic

line

Antic

line

Sync

line

Lithgow

Wollongong

Sydney

Nowra

Edge

of c

ontin

enta

l she

lf

N

0 20 40 km

(a) (b)

Figure 1.20 Sydney Basin (a) The area covered by the Sydney Basin. (b) A cross-section of the Sydney Basin, showing the Lapstone Monocline.


22Science press



SCIENCE SKILLS1. Compare means show how things are similar and/or different.

Compare a platform with a continental basin.


• Aplatformissimilartoacontinentalbasininthattherearelayersofrelativelyundisturbedsedimentslyingontopofmoreancientrocks.

• Platformsandshieldsdifferinthataplatformisquitethinwhileasedimentarybasinisthickerandhasabasinshapeoftenholdingunderground water.

2. Summarise means express concisely the relevant details.

Summarise the movements of the Australian continent from the early Cambrian to the early Permian.


• IntheearlyCambrianAustraliawaslocatednorthoftheequator.

• DuringtheDevonianAustraliacrossestheequatorheadingtowardsthesouth.

• BytheearlyPermianAustraliaispartofPangaeaandlocatednearwhatisnowtheSouthPole.

TO THINK ABOUTSet 1

1.3.1 Define the term tectonic supercycle.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.3.2 Define the term supercontinent.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.3.3 Compare a craton with a fold belt.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.3.4 Identify the locations of most cratons in Australia.

........................................................................................................................................................................................................................................................

1.3.5 Identify the location of a major fold belt in Australia.

........................................................................................................................................................................................................................................................


23Science press



Set 2

1.3.6 Draw a flow chart, showing the major stages of the tectonic supercycle from the break-up of the supercontinent Rodinia to today.

1.3.7 Summarise the plate tectonic supercycle.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.3.8 Explain how supercontinents form and then break up.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.3.9 Describe how the eastern fold belt of Australia might have formed.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


24Science press



1.3.10 Summarise how the Australian continent has grown since the start of the Proterozoic as a result of plate tectonic processes.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


25Science press



Unit 1.4 Natural disasters – volcanic eruptionsOn 13 November 1985 the volcano Nevado del Ruiz erupted in Colombia, South America. The eruption caused catastrophic mudflows that destroyed a large section of the town of Armero, killing at least 23 000 people. Detailed warnings had been given but were ignored.

Figure 1.21 Mount Pinatubo When Mount Pinatubo erupted in 1991 the region had already been evacuated.

When Mount Pinatubo, in the Philippines, erupted on 15 and 16 June 1991, the lessons from Nevado del Ruiz had been learnt. Steps were taken to educate local government, police and military officials so that they would take the need to evacuate the area seriously (Figure 1.21).

Plate tectonics and volcanoes Both Nevado del Ruiz and Mount Pinatubo are located on subduction zones. Nevado del Ruiz is part of the Andes mountain chain, where the oceanic plate is being subducted beneath the plate carrying South America. In contrast, Mount Pinatubo is part of an island chain where two oceanic plates are colliding.

In view of the forces involved, both locations are strong candidates for earthquake and volcanic activity – certainly both have experienced explosive eruptions that often occurred along the subduction zones. (Eruptions along divergent zones, such as mid-ocean ridges, tend to be more gentle.) Thus the edges of crustal plates are to a certain extent plotted by using the locations of volcanoes.

Volcano

Lava flow

0 500 km

Figure 1.22 Australian volcanoes Australian volcanoes are caused by the northerly drift of the crustal plate carrying Australia over hot spots.

Not all volcanoes are located at the edge of crustal plates – some are found toward the centre. The active volcanoes on the Hawaiian Islands are found in the middle of a crustal plate and are generally more gentle. Those found in Australia are all now extinct but when active were also toward the centre of a crustal plate. As the plate drifted north over a stationary hot spot, it left a trail of volcanoes. These become progressively younger as you follow them south (Figure 1.22). For example, Mount Schanks, in South Australia, erupted most recently around 500 CE (Figure 1.23).


26Science press

chaptEr 1 tEctonic impactS Unit 1.4 natural disasters – volcanic eruptions


Figure 1.23 Mount Schanks This volcano, near Mount Gambier in South Australia, was the last to erupt in Australia.

People and volcanoesVolcanoes both harm and help humanity. Besides Nevado del Ruiz and Mount Pinatubo, many catastrophic volcanic eruptions have occurred through history. For instance, in 1883 Mount Krakatoa, a small volcanic island in Indonesia, erupted and killed more than 36 000 people – it was so loud that it could be heard 3500 kilometres away in Darwin.

(a)

(b)

Figure 1.24 Benefits from volcanoes (a) Fertile soils in Indonesia. (b) Geothermal energy in New Zealand.

Although volcanoes can cause such short-term havoc and devastation, it is balanced by long-term benefits. Volcanic materials ultimately break down to form some of the most fertile soils on Earth (Figure 1.24). The cultivation of rich volcanic soils has sustained civilisations, including some of the earliest in history – Greek, Etruscan and Roman in the Mediterranean-Aegean region. For this reason, people still risk living near volcanoes.

There are a number of other uses for volcanism:

• volcanicproductsforconstructionmaterials

• abrasiveandcleaningagents,suchaspumiceandscoria

• rawmaterials formanychemicaland industrialuses–valuable minerals, including copper, tin, gold and silver occur in deposits mined throughout the world

• geothermalenergy,suchasthesteamusedtogeneratepower in New Zealand.

Volcanoes and volcanic featuresWhen under ground, molten rock is called magma. Upon reaching the surface, magma loses many of its gases and we call it lava. The major factor in determining the nature of a volcano is the viscosity of its magma (Table 1.4). Mafic magmas tend to be dark in colour and have lower viscosity, thus tending to run more freely. Felsic magmas are lighter in colour and much more viscous.

Figure 1.25 Flowing lava Mafic lavas tend to be produced at diverging plate margins and over hot spots.

As a result, volcanoes at diverging plate margins or over hot spots in the centre of crustal plates produce free-flowing mafic lavas that form lava fountains and lava flows (Figure 1.25). In contrast, volcanoes at converging plate margins produce a stickier, more viscous felsic lava likely to be erupted as many fragments, often violently. Any fragments ejected from such volcanoes are called pyroclastics.


27Science press

chaptEr 1 tEctonic impactSUnit 1.4 natural disasters – volcanic eruptions


The explosive nature of felsic magmas can be explained by the water and gases present in the magma. While deep in the Earth, they are chemically part of the magma. As the latter rises toward the surface, the pressure is lowered and the water and gases turn into bubbles.

Mafic magmas have little water and gas and because they are less viscous, the gases can escape freely when the magma reaches the surface. Felsic magmas contain a lot more water and gas and, because they are more viscous, can result in the magma being fragmented on reaching the surface, sometimes explosively. Between these two extremes, some lavas solidify to produce rocks with cavities called vesicles, indicating the presence of gases.

Some lavas change their composition during either an eruption or during the life of a volcano. This is because magmas can alter as they rise through the crust. If lavas reside in a magma chamber for any length of time, they may differentiate as the magma separates into components. Magma can also interact with the rock they pass through, changing their composition.

The size and shape of volcanoes depend on the type of lava produced. When fluid lava flows from a volcano, the resulting mountain has a broad, slightly domed structure and so is called a shield volcano (Figure 1.26). They are primarily built out of mafic lava flows and contain only a small percentage of pyroclastic materials. The most famous shield volcano is Mauna Loa, as well as other volcanoes on Hawaii.

While shield volcanoes are by far the largest volcanoes, cinder cones, which are made almost entirely of loose pyroclastic material, tend to be rather small and have steep sloping sides. Mount Schanks, in South Australia, is an example of this type (Figure 1.23).

Finally, composite volcanoes are symmetrical structures formed from interbedded lava flows and pyroclastic deposits that come from the central vent. They produce the traditional picturesque shape of, for example, Mount Fuji, in Japan.

Composite volcanoes tend to produce the most violent type of eruption. Their vents have been known to become plugged by felsic lava domes that solidify (Figure 1.27).

Magmareservoir

Flankeruption

Central vent Lava flow

120 km

5.8 kmdeep

Magma

Lava flowCrater

Pyroclastic material

Central vent filledwith rock fragments

Crater

(a)

(b)

(c)

Figure 1.26 Types of volcano (a) Shield volcano. (b) Composite volcano. (c) Cinder cone.

The pressure under the domes then builds up until they explode violently. This happened during the eruption of Mount Vesuvius in 79 CE when it destroyed the ancient city of Pompeii; Mounts St Helens, Pinatubo and Krakatoa are more recent examples. In the latter eruption, 21 cubic kilometres of rock was blown into the atmosphere and the remaining material collapsed to produce a submerged crater. When such craters form on land, they can become filled with water and so form a lake.

Table 1.4 Magmas and their properties.

Composition Silica content Viscosity Gas contentTendency to form

pyroclasticsType of volcano

Mafic (basaltic magma)

Least (about 50%) Least Least (about 1-2%) Least Shield and cinder cone

Intermediate

(andesitic magma)

Intermediate (about 60%)

Intermediate Intermediate (3-4%)

Intermediate Composite

Felsic (granitic magma)

Most (about 70%) Greatest Most (4-6%) Greatest Composite


28Science press



The situation is very different with composite volcanoes that erupt explosively. Pyroclastic flow (once called Nuée ardente) is composed of hot gases and volcanic ash that can race down the slopes of a volcano at up to 200 kilometres per hour. Such flows often carry much larger pieces of rocks along with them (Figure 1.1).

It was a pyroclastic flow that buried the ancient city of Pompeii. It also buried the city of Saint-Pierre, on the island of Martinique in the West Indies, when Mount Pelée erupted in 1902. Although many in the city had fled because of the volcano’s activity, around 20 000 remained and perishedwhen the city was buried – except a prisoner held under ground in a jail.

Figure 1.28 Lahars Mudflows can bury a whole town such as Armera near Nevado del Ruiz.

Huge mudflows called lahars can also pose significant danger (Figure 1.28). One way they can occur is when an eruption melts ice and snow on the slopes of the volcano, which is what happened at Nevado del Ruiz. Another way is when the loose pyroclastic material becomes saturated with water during or after an eruption. For example, during the eruption of Mount Pinatubo, a cyclone drenched the surrounding area. Either way, enormous amounts of water and mud race down the valleys, destroying all before it.

There are a number of other volcanic hazards. When a volcano on an island or in the ocean erupts, it can produce a huge wave called a tsunami, such as the one generated by the Krakatoa eruption that was responsible for most of the deaths.

Volcanoes can also produce poisonous gases. Normally these mix with the atmosphere before they can do considerable damage, though an exception was a crater lake in the Cameroon called Lake Nyos. In 1986 a build-up of carbon dioxide in the bottom of the lake bubbled to the surface. Being denser than air, the gas moved down valleys and asphyxiated more than 2000 people and numerous animals while they slept along its path.

Dome

Sea level

SECraterNW

(a)

(b)

Figure 1.27 Changing volcanoes (a) A volcanic dome plugs the volcano and a build-up in pressure can lead to an explosion. (b) The caldera of Krakatoa left after its eruption.

Volcanic hazardsIf humanity is to benefit from volcanoes, we need to find ways to reduce the hazards they pose to life and property. There are 50 to 60 volcanic eruptions around the world each year, most well away from populated areas. Lava flows from shield volcanoes may travel fairly quickly, but they usually follow valleys and their paths can be predicted. While some damage is sustained, there is rarely loss of life.

Cinder cones also form quite slowly. For instance, in the early 1940s the Mexican village of Paricutínoun was buried bypyroclasticsfromacinderconethatformedinafarmer’sfield. Its growth was slow enough for there to be no loss of life.

ACTIVITY 1.13 VOLCANIC ERUPTIONS

There are a number of animations that show volcanic eruptions including the different types and their relationship to plate tectonics. You can find the animations by using a search engine to look for ‘animationvolcaniceruption’.Atthetimeofwritinganimations could be found at:

• http://www.edumedia-sciences.com/en/a485-volcano-diagram

• www.pbs.org/wnet/savageearth/animations/volcanoes/index.html



29Science press



Predicting volcanic eruptions Volcanic eruptions are beyond human ability to control. However, we can attempt to predict when and where they will occur and take steps to protect human life and property. Plate tectonics can assist with predicting where volcanic eruptions will occur – predicting when is much more difficult.

Table 1.5 lists a number of the methods used. Although they have been successful in giving general warnings of impending eruptions, none has been successful in giving reliable forecasts a few days or hours in advance. Short-term predictions are essential to minimise the human costs of false alarms or worse. Failing to successfully anticipate an eruption at all can result in massive devastation and loss of life. In this context there is a need for continued research into the nature of volcanic eruptions and finding methods to make successful short-term predictions.

The human factor is becoming increasingly important in reducing the damaging effects of volcanic eruptions. There will always be political and economic interests wanting to wait to the last moment before acting. These were both factors that contributed to the Mount Pelée and Nevado del Ruiz disasters.

This problem was overcome prior to the Mount Pinatubo eruption by showing films of the eruptions made by French vulcanologists Maurice and Katia Krafft (they were killed during an eruption on Mount Unzen, Japan, in 1991).

A number of steps can be taken to help reduce the impacts of volcanic eruptions:

• Increasepublicandofficialawarenessofthedangersofvolcanoes, particularly ash flows and lahars.

• Constructgeologicalmapssothattownsandcitiesarenot rebuilt on known danger areas. The town of Armero, near Nevado del Ruiz, had been destroyed by a lahar in 1700s but was rebuilt on the same site.

• Usegeologicalmapstoprepareevacuationplans.

• Ensure that dangerous volcanoes are continuouslymonitored.

Unfortunately, due to the inaccuracy of volcanic prediction, there is always the danger of false alarms. This occurred on the island of Guadeloupe in the Caribbean in 1976. Geologists warned of an eruption and 72 000 people were moved from their homes for three months. No eruption occurred, and there have been political and economic consequences ever since.

ACTIVITY 1.14 PREDICTING VOLCANIC HAZARDS

Figure 1.29 is a map of the world, showing the crustal plates and the locations of many of the most active volcanoes.

1. Identify three volcanoes that are likely to erupt gently most of the time.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

2. Identify five volcanoes that are likely to erupt explosively. Justify your answer.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

Table 1.5 Prediciting volcanic eruptions.

Method What it tells us

Seismic activity Earthquakes provide the earliest warning of an impending eruption. The movement of magma produces large numbers of low-intensity earthquakes called earthquake swarms or harmonic tremors.

Geophysical monitoring

The movement of magma closer to the surface prior to an eruption can be monitored:

• Infra-redphotostakenfromaircraftorsatellitescandetectheatingofthecrust.

• Magmaaltersthemagneticpropertiesofthemountain.

• Themagmacanalsochangethegravitationalpropertiesofanarea.

Topographical monitoring

The shape of the volcano changes as magma accumulates under the volcano. As the volcano swells, the ground may tilt, cracks may open and the water level of lakes may change.

Volcanic gases The amount of gas and the relative proportion of gases produced can change. In particular, the rate at which sulfur dioxide is produced can be a useful tool.

Geological history Except for Iceland, few areas have a reliable historical record of eruptions. Geologists map and radiometrically date lava flows to determine the frequency of volcanic eruptions in the area.


30Science press



3. Assess the likelihood of damage due to volcanic eruption for the following cities, justifying your answer for each: Perth, Mexico City, New York, Seattle, Tokyo, Moscow, London, Jakarta, Bombay and Manila.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

4. Table 1.6 lists some of the most famous volcanic eruptions in history.

(a) Using Figure 1.29, explain why these volcanoes were often so destructive.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

(b) Outline how the level of human fatalities has changed over time.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

(c) Explain why Nevado del Ruiz was an exception to this pattern.

..............................................................................................................................................................................................................................................

..............................................................................................................................................................................................................................................

Eurasian plate

O-shimaBandai-san

Unzen

Philippineisland arc

Bezymianny

Bogoslof

Redoubt

Rainer

Mt Helens

Lassen

NorthAmerican

plate

El Chichon

Popocatepetl

Ruiz

Cotopaxi

Misti

Azul

TamboraIndian-Australian

plate

Krakatoa

Mayon

Taal PinatuboPhilippine

plate

Java Trench

JapanTrench

Fujiyama

Myojin-syoMarianas Trench

Tarawera

Heard Island

South-east Indian ridge

Mid-Indian R

idge

Pacific plate

Mauna LoaGordaplate

Kilauea

HawaiianIslands

Antarctic plate

Eurasian plate

Burney

Nazcaplate

SouthAmerican

plate

Partcutin

Cerro Negro

Cocos plate

Coseguina

Irazu

Caribbeanplate

Pelee

Kilimanjaro

Africanplate

CapeVerdeIsland

Fayal

St PaulLa Soufriere

Mid-Atlantic Ridge

Etna

Nyamiagira

EastAfrican

RiftSomaliplate

Grimsvoth..

HelkaSurtsey

Laki

Anatolianplate

VesuviusDemavend

KurilTrench

Aleutian Trench Katmai

Juan deFuca plate

StromboliSantorini(Thera)

AraratArabian

plate

South-west

Indian Ridge

/

Tong

a T

renc

h

Figure 1.29 Active volcanoes Which of the volcanoes are the most dangerous?


31Science press



5. Use the information you have just read, and other sources if needed, to complete Table 1.7.

Table 1.7 Volcanic hazards.

Volcanic hazards Nature of the hazard Effect on the environment Effect on people

Poisonous gases

Ash flows

Lahars

Lava flows

Tsunami

Table 1.6 Famous volcanic eruptions.

Volcano or city Effect

Vesuvius, Italy, 79 CE Destroyed Pompeii and killed 16 000 people. City was buried by volcanic activity and rediscovered in 1595.

Skaptar Jokull, Iceland, 1783 Killed 10 000 people (many died from famine) and most of the island's livestock. Also killed some crops as far away as Scotland.

Tambora, Indonesia, 1815 Global cooling; produced ‘year without a summer.’

Krakatoa, Indonesia, 1883 Tremendous explosion; 36 000 deaths from tsunami.

Mt Pelée, Martinique, 1902 Ash flow (nuée ardente) killed 30 000 people in a matter of minutes.

La Soufrière, St. Vincent, 1902 Killed 2000 people and caused the extinction of the Carib Indians.

Mt Lamington, Papua New Guinea, 1951 Killed 6000 people.

Villarica, Chile, 1963-64 Forced 30 000 people to evacuate their homes.

Mt Helgafell, Heimaey Island, Iceland, 1973 Forced 5200 people to evacuate their homes.

Mt St Helens, Washington State, USA, 1980 Debris avalanche, lateral blast, and mudflows killed 54 people, destroyed over 100 homes.

Nevado del Ruiz, Colombia, 1985 Eruption generated mudflows that killed at least 22 000 people.

Mt Unzen, Japan, 1991 Ash flows and other activity killed 41 people and burned over 125 homes. Over 10 000 people evacuated.

Mt Pinatubo, Philippines, 1991 Tremendous explosions, ash flows, and mudflows combined with typhoon killed over 300 people; several thousand people evacuated.


32Science press



SCIENCE SKILLS1. Predict means suggest what may happen based on available information.

Use plate tectonics to predict where volcanic eruptions are likely to occur near Perth.


• Perthislocatedmidwaybetweenasubductionzoneandamid-oceanridge.

• TothesouthofPerththereisamid-oceanridgewhereunderwatereruptionsarepossible.

• TothenorthofPerththereisasubductionzonenearIndonesia.

2. Justify means support an argument or conclusion. You need to show adequate grounds for your decisions or conclusions. Answer or refute the main objections likely to be made against them.

Justify continued research into reliable prediction of volcanic activity.


• Volcanicactivitycanendangerhumanlife,anddisrupttourismandtrade.

• Reliablepredictionreducesthechanceoffalsealarmsordisastersastoolittlewarningisgiven.

• Falsealarmscanhurtbusiness,tourismandtheabilityofpeopletoearnaliving.

• Falsealarmsalsomeanpeoplemaynotrespondtowarningnexttime.

• Adequatewarningwillallowevacuationsandsavelives.

• Adequatewarningwillalsoallowcitizens,businessesandtouristoperatorstotakeprecautionstoprotecttheirproperties.

TO THINK ABOUT

Set 1

1.4.1 Compare magma, lava and pyroclastics.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.2 Contrast the properties of mafic and felsic lava. Explain the effect these properties have on the nature of volcanic eruptions.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


33Science press



1.4.3 Distinguish between shield, cinder cone and composite volcanoes. Predict the danger each type of volcano poses to nearby human populations.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.4 Contrast the dangers posed to human populations by nuée ardente, lahars and tsunamis.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.5 Explain how volcanic domes form and why they are potentially dangerous.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

Set 2

1.4.6 Use the plate tectonic model to predict where volcanic eruptions are likely to occur near to Sydney.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.7 A number of volcanically active regions close to Australia have a high population density.

(a) Describe the general physical, chemical and biotic characteristics of such an area.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

(b) Explain why people would inhabit such regions despite the risk.

........................................................................................................................................................................................................................................................


34Science press



(c) Justify the development of evacuation plans to deal with volcanic eruptions in such areas.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.8 Describe the following hazards associated with volcanoes and their impact on people and other living things.

(a) Poisonous gas emissions.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

(b) Ash flows.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

(c) Lahars.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

(d) Lava flows.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.9 Describe two methods used for the prediction of volcanic eruptions. Predict how successful they are at making long- and short-term predictions.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

1.4.10 Justify research into new methods for predicting of volcanic activity.

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................

........................................................................................................................................................................................................................................................


425Science press

anSWErS


Answers

Chapter 1 Tectonic Impacts1.1.1 A crustal plate is any of the large movable segments into

whichtheEarth’scrustisdivided.

1.1.2 Mafic rocks are dark coloured igneous rocks due to high concentrations of ferromagnesian minerals. In contrast, felsic rocks are light coloured igneous rocks, with relatively large amounts of feldspars and quartz. Mafic rocks form as magma rises from beneath the crust. Felsic rocks form from the remaining magma, or may form if magma melts and incorporates surrounding rocks.

1.1.3 Convergent boundaries form when two crustal plates move towards each other and collide. Divergent boundaries form where two plates diverge or separate from each other.

1.1.4 Subduction takes place at convergent plate boundaries such as along a deep oceanic trench. One crustal plate descends beneath another and melts within the mantle below.

1.1.5 Amid-oceanridgeisamountainrangeundertheworld’soceans often with a deep valley along its centre. They are often the site of divergent plate boundaries where magma rises to the surface and cools to form oceanic crust. This crust moves away from the ridge in opposite directions. Hydrothermal vents may also be present.

1.1.6 Oceanic crust is the thin crust of the Earth that is under the oceans. Made of basalt and other rocks it is denser than that of the continents. In contrast, the thicker continental crust makes up the continents. Made of granite and other rocks it is less dense than that of oceanic crust which may form a layer under the continental crust.

1.1.7 Felsic igneous rocks rich in quartz and feldspars are often found associated with convergent plate boundaries. Mafic igneous rocks that contain high concentrations of ferromagnesian minerals are often found associated with divergent plate boundaries.

1.1.8 Crustal plates move independently across the surface of the Earth’s mantle at an average speed of around 5cm per year. The movement appears to be produced by movement of magma beneath the plates. They move away from divergent boundaries where new crust is produced. They move towards convergent boundaries where crust is lost. They move past each other at transform boundaries.

1.1.9 Convergent boundaries form when two crustal plates move towards each other and collide. Divergent boundaries form where two plates diverge or separate from each other. Crustal plates move past each other at transform boundaries.

1.1.10 There are two main models used to explain the movement of crustal plates. Neither explains all that is known about plate movements.

• Intheconvectionmodel:

Unequal heat distribution in the mantle may produce convection in the mantle below the lithosphere. Hot material rises (a spreading centre), spreads laterally, cools and sinks deeper into the mantle to be reheated. The moving mantle exerts friction forces on the crust above making it move.

• Inthepush-pullmodel:

The hot magma is less dense than the mantle and rises. Thus the new crustal material is pushed apart at hot spreading centres such as mid-ocean ridges. Cold crustal plates are dense and when they reach subduction zones tend to sink into the mantle, pulling the rest of the plate with it. Each part of the model can operate independently and are gravity driven.

1.2.1 In an anticline, the layers of rock strata curve downward from the centre to form a crest or ridge. In contrast, in a syncline the rock strata curve upwards from the centre to form a trough in a roughly U shape.

1.2.2 In a normal fault the rock above the fault line has moved down relative to the rock below. In contrast, in a reverse fault the rock above the fault line has moved upwards relative to the rock below.

1.2.3 Compression forces can produce both folds and faults. The geological structures that result depend on the brittleness of the rock and the time over which the forces are exerted. Rocks that bend or fold can produce arches or anticlines and troughs called synclines. If the compression forces are very large or occur over a short time or the rocks are brittle, they may snap to produce faults.

Tension forces can also produce folds and faults. Monoclines result when one end of a horizontal bed is raised, producing a gentle incline. The tension is at an angle to the strata that is folded. Faults are produced by tension forces so that the rocks move apart and strata between moves downward. When on a massive scale, such faults include those that occur in rift valleys.

1.2.4 Huge masses of molten rock can rise from the outer surface of the core to form a superplume. If this takes place under a large continent, large sections of the continent can be raised vertically with little disturbance. Large areas of horizontal strata may be raised vertically to produce a plateau.

1.2.5 Old subduction zones can contain large masses of old continent sinking into the mantle. As a continent moves across such an old subduction zone, it may begin to sink with little disturbance and large parts are covered by ocean. The continent may rise again once it had moved past the old subduction zone.

1.2.6 Marine evidence indicates the sediments accumulated at the bottom of the ocean. At subduction zones, such sediments are scrapped from the surface of the oceanic crust as it is subducted. In the process the marine sediments are intensely folded and metamorphosed. As the oceanic crust sinks into the mantle volcanic activity produces magma that rises through the metamorphosed sediments.


Date post:	09-Jun-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

Earth and EnvironmEntal SciEncE 2 · Earth and Environmental Science is one of the most dynamic and...

Documents