Why do the plates move?

Why do the plates move?

African

Indo-Australian Plate

North American

South American

Eurasian

Pacific

Nazca

Antarctic

Pacific

Plate names

Plate names

Continental drift

Divergent (extensional) plate boundary

(you might know it as a constructive boundary – don’t use this term)

How do we know the sea floor is spreading?

Spreading on land - Rift valleys

• When a plate is splitting apart, forced by the convection currents below.

• Caused when two faults ‘slip’ downwards, creating a trough – in reality a number of troughs are usually formed as in the diagram to the right.

East Africa: The Great Rift Valley

East Africa: The Great Rift Valley

Convergent (destructive) boundaries

• Convergent Boundary 1 - Oceanic vs. Continental

• Convergent Boundary 2 - Continental vs. Continental

• Convergent Boundary 3 - Oceanic vs. Oceanic

Convergent (destructive) plate boundary (1)

Subduction zone

Convergent (destructive) plate boundary (2)

Convergent (destructive) plate boundary (3)

So, how come we have volcanoes away from plate boundaries?

• Within the mantle, there are convectional plumes of extremely hot magma that burn through the crust like a blowtorch through metal – these are known as Hot Spots.

• They are stationary, it is the tectonic plates above that are moving to create island arcs, a perfect example of this is the Hawaiian islands.

Mercalli Scale

This measures how much damage is caused by the earthquake based on observations. It is measured on a scale between I and XII.

Mercalli Scale

• The surface waves are the slowest of the three earthquake wave types. P and S waves take the path of least time (not always a straight line, an application of the Fermat Principle) whilst the L waves take the longer surface route around the circumference of the earth. L waves build up further away from the epicentre and can cause considerable damage at great distances. In a great earthquake (magnitude 8 and above) it may even be possible to see the rolling motion of the surface waves. Buildings and structures weakened by the severe shocks of the S waves can be flattened and completely destroyed by the arrival of the L wave. The L wave behaves most like waves in a pond or on the sea - the surface, with only the atmosphere above, is relatively unconstrained and easier to move.

• In the Kobe earthquake of 1995, observers in the city noted the start of the P phase at 05:47 and 15 seconds. The P waves lasted until 05:47:21 and were followed at 05:47:22 by the start of the S wave shaking. The severe sideways shaking lasted for just 6 seconds registering 7.2 on the Richter scale, dying away after 05:47:28. As a result of those 13 seconds, over 5,000 people died........

Weathering

What is the difference between weathering & erosion?

Weathering is the breakdown of material caused by climactic conditions and weather action upon the rock.

Erosion is the wearing away by constant forces related to, but not directly caused by temperature change or climactic conditions.

Weathering creates the process we know of as Denudation.

There are 3 types of weathering

• They happen ‘in situ’!! Unlike erosion.

• Mechanical (physical)• Biological (physical)• Chemical (chemical)

Mechanical weathering

Sometimes known as Frost shattering or freeze-thaw action

Mechanical weathering

Factors that influence rates of weathering• Microclimate – an area being shaded from the sun can

experience different rates of weathering in comparison to those insolated for longer periods.

• Parent material – some materials are more resistant to weathering than others. Quartz, for example, is resistant – rocks which actively include joints and bedding planes are not.

• Soil & vegetation cover – plant roots speeds up weathering, whereas the decomposition of plants can also add vital chemicals to the soil which could increase rates of chemical weathering.

• Relief – steepness of slopes can of course affect the speed at which material loosens itself and falls off. Aspect can play a vital part – those areas in permanent shadow (north to north-east in the northern hemisphere) will experience greater rates of frost shattering.

• Climate – Heat generally speeds up chemical reactions. In cooler climates carbonation is more rapid due to CO solubility. Fluctuations in climate affect weathering.

Pothole

Grike

Clint

Sinkhole

Limestone pavement

Chalk scenery• Dry valleys• The size of dry valleys suggests different climatic conditions in earlier times.

They may have been formed in several ways:

• Higher sea levels at the end of the last Ice Age created higher water-table levels. Later, as sea level fell the water-table dropped allowing the valleys to dry out.

• Freezing of water in rock pores during the Ice Age caused the rock to become impermeable as the ice melted the rock became permeable again allowing the valleys to dry out.

• Streams/valleys developed on an overlying layer of impermeable rock – as this was removed by weathering and erosion the valley was superimposed on the rock below

• Surface drainage developed during high rainfall periods, when the rainfall decreased the water table dropped, leaving behind the dry valleys

Granite scenery• Igneous rock formed when magma intruded into the earth’s crust.

• Tors• Bare stacks of rounded rocks 5-10 metres in height.• Joints were weathered chemically in a warm interglacial period,

underneath a layer of regolith.• The regolith was then removed by solifluction during periglacial times,

leaving behind the tors.

• The impermeability of granite creates complex drainage patterns.

Granite intrusion

Types of mass movement• Ordered by speed:

• Heave (slowest)

• Flow

• Slide

• Fall (fastest)

• It is important to note that it is difficult to categorise types of movement as they overlap so much in terms of processes involved

Heaves

• The slowest but most widespread type of movement. It can take place on slopes as low as 5 degrees.

Flows

• There are 4 types of flow:

• Soil creep• Solifluction• Earthflow (debris

flow)• Mud flow (mudslide)

Soil creep

• Usually on slopes >5 degrees

• Rates of less than 1cm per year

• Can be caused by:

• Rain lubrication• Freeze-thaw• Animals burrowing• Thermal

expansion/contraction

Examples of soil creep

Solifluction

• Can be on very gentle slopes

• Rates of between 10cm (at top) and 10mm (at bottom) per year

• Caused by saturation of the regolith. In periglacial areas the seasonal melting can cause flow over the frozen subsoil, this is known as gelifluction.

Solifluction lobes

Earthflow (debris flow)• Usually on slopes between 5 - 15

degrees

• Rates of between 1mm per day and 1m per second

• Can be caused by:

• Spells of heavy rain• Snowmelt• Landslides• Earthquakes• Basal undercutting by streams

• Can produce flow tracks over 100m in length, but may be too slow to break vegetation

• Earthflows are thicker than mudflows as they have lower water content

Aberfan

• 21 October, 1966

• 144 people killed • 116 of them children

Mudflow (mudslide)• Similar to earthflows but thinner in consistency

• Rates of over 1km per hour

• Can be found further down the slope of an earthflow where the more saturated material has travelled

• Can be caused by:

• Spells of heavy rain in mountainous areas

• Periglacial summer thaw• Volcanoes where on the slopes

snow melts and mixes with ash

• May 1998 – 350 people killed in Naples, Italy (Mt Sarno)

• Lahars – rivers of mud can be formed (such as Mt St Helens)

Slides/Falls

• There are several types of slide & fall:

• Landslide• Rotational slump/slip• Debris/rock

avalanche• Rockfall

Landslide

• Normally in areas of pronounced relief

• Rates of between 1mm/day – 10m/second

• Rapid movement under the influence of gravity, with little or no associated flow

Rotational slump (slip)

• Normally in areas of pronounced relief

• Rates of between 1mm/day – 10m/second

• Rapid movement under the influence of gravity, with little or no associated flow

Debris/rock avalanche

• Caused by:

• Slope undercutting• Snow/sediment

accumulation• Increased pore water

pressure• Growth of ice crystals• Trigger events (such as

an earthquake)

• Speeds of 1-100cm/sec

• Speeds can reach 400km/hour, due to:

• Air trapped beneath rock fragments increasing movement like a hovercraft cushion

• Acceleration increased as energy from falling debris increases

Huascaran avalanche – Peru, 1970

• Triggered by an earthquake of magnitude 7.7

• Buried the towns of Yungay & Ranrahirca

• 50 million cubic metres of ice and rock moving at 160 km/hour. A front of 1000m travelled for 17km.

• A magnitude 7.7 earthquake triggered a huge avalanche from the summit of Nevado Huascaran, the highest peak in Peru. Part of the landslide jumped a 200-metre ridge, wiping out the town of Yungay and killing all but about 100 of its 20,000 inhabitants.

• This is the worst avalanche disaster in history. In addition, the earthquake killed another 50,000 people elsewhere in Peru. The total death toll of about 70,000 makes this the worst natural disaster ever in the Southern Hemisphere.