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Earth‟s crust is composed of numerous plates, which move due to
convection processes in the mantle.
Theory of plate tectonics
1
The lithosphere is broken up into interconnected slabs that geologists call
plates. Plate tectonics is the theory that describes how these plates move
about and interact with each other at their boundaries.
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What are Tectonic Plates?
The Earth’s crust consists of about a dozen large slabs of rock, or PLATES, that the continents
and oceans rest on. These tectonic plates can move centimeters per year (1-15 cm per year)
Tectonic plates are also called lithospheric plates because the crust and the upper-most
mantle make up a sub-layer of the earth called the lithosphere. The plates can move about
because the uppermost mantle, or the asthenosphere, is partially molten and possesses a physical
property called plasticity, allowing the strong, rigid plates of the crust to move over the weaker,
softer asthenosphere.
The word TECTONICS is of Greek origin and it means “to build.” The word “tectonism” refers
to the deformation of the lithosphere. This deformation most notably includes mountain building.
Plates and relative plate motion.
Modified after NOAA
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Earth‟s Sublayers
Lithosphere: This layer combines the rigid crust plus the upper-most mantle. (Greek: Rock)
Asthenosphere: Partially molten part of upper mantle (Greek: weak). Tectonic plates are able
to move about on top of the softer, partially molten asthenosphere.
The outer-most layers of the earth.
McGraw Hill/ Glencoe, 1st ed., pg. 142.
What are Tectonic Plates? (continued)
5
There are 8 primary plates on the planet (some time Indo-Australian
Plate is considered as a single plate), and they comprise of the
majority of the World's continents' landmass, along with most of the
surface area of the World's Ocean's.
The secondary plates (7) are smaller in size than the primary plates,
and they do not cover any substantial landmass, apart from the Arabian
Plate.
There are a further group of smaller plates, often called tertiary plates,
which are the disappearing remains of much larger ancient plates that
are now on the edges of our major plates, plus some micro-plates,
many of whom will be widely-considered as a part of a primary or
secondary plate on maps and in scientific publications.
Major Tectonic Plates in the world:
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7
Major Tectonic Plates in the world:
8
Primary Tectonic Plates Secondary Tectonic Plates
1. African Plate 1. Arabian Plate
2. Antarctic Plate 2. Caribbean Plate
3. Australian Plate 3. Cocos Plate
4. Eurasian Plate 4. Juan de Fuca Plate
5. Indian Plate 5. Nazca Plate
6. North American Plate 6. Philippine Sea Plate
7. Pacific Plate 7. Scotia Plate
8. South American Plate
List of Major Tectonic Plates :
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Plate Tectonics: The Beginning
At the beginning of the 20th Century, scientists realized that that they could not explain many ofthe Earth’s structures and processes with a single theory. Many scientific hypotheses weredeveloped to try and support the conflicting observations. One hypotheses was continental drift,
which was proposed by Alfred Wegener in a series of papers from 1910 to 1928.
The principal thought of continental drift theory is that the continents are situated on slabs ofrock, or plates, and they have drifted across the surface of the Earth over time; however, originally,they were all joined together as a huge super-continent at one time.
In the 1960’s, the theory ofcontinental drift was combined withthe theory of sea-floor spreading tocreate the theory of plate tectonics.
Alfred Lothar Wegener (1880-1930)
Background
310
Additional evidence supporting the continental drift theory:
Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and
South America.
Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal
could not have swum across the existing Atlantic Ocean
Glacial deposits found in current warm climates and warm climate plant fossils found
in what is now the Arctic (Paleoclimate Indicators).
Nearly identical rock formations found on the east coast of U.S. and the west coast of
Europe and eastern South America and western Africa.
Coal reserve found under ice cap of Antarctica.
The idea for Wegener's theory was sparked by
his observation of the nearly perfect “fit” of the
South American and African continents.
The “fit” of two continents.
Plate Tectonics: The
Beginning
412
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Continents FIT together
like the pieces of a puzzle
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Fossil evidence- Glossopteris & Mesosaurus
Paleoclimate Indicators
Glacial till of the same age is found in southern Africa, South America, India and
Australia-areas that it would be very difficult to explain the occurrence of glaciation.
At the same time, large coal deposits were formed from tropical swamps in
N.America and Europe.
Rock Type and Structural Similarities
We find similar rock types on continents on opposite sides of the Atlantic
Ocean.
Similar, age, structure and rock types are found in the Appalachian Mtns.
(N.A.) and mountains in Scotland and Scandinavia.
FT 18
FT 01
FT 02
FT 03
FT 04
FT 05
FT 06
FT 07
FT 08
FT 09
FT 10
FT 11
FT 12
FT 13
FT 14
FT 15
There are three basic ways that plates interact with one another. Each of these plate
boundaries has the potential to create different geological features.
Plate Boundaries
1. When plates collide with each other = Convergent boundary
2. When plates separate from each other = Divergent boundary
3. When plates slide along side each other = Transform boundary
934
The tectonic plates and plate boundaries.
McGraw Hill/Glencoe, 1st ed., pg 143
35
1. Convergent Boundary:
Ocean-Continent Collision Because the oceanic crust is more dense than continental crust, when these two
collide, the continental crust rides up over the oceanic crust and the oceanic crust is
bent down and sub ducted beneath the continental crust. This is called a subduction
zone, where the old oceanic crust is dragged downward and “recycled.”
Deep-sea trenches are created at subduction zones. Trenches are narrow, deep
troughs parallel to the edge of a continent or island arc. They typically have slopes
of 4-5 degrees, and they are often 8-10 km deep. The deepest spots on earth are
found in oceanic trenches. The Mariana Trench is the deepest ocean depth at 11 km
(35,798 ft) below sea level.
Figure depicting oceanic crust
subducting beneath continental
crust, creating volcanoes on the
land surface above, and a deep-
sea trench off of the coast.
Credit: U.S. Geological Survey
Department of the Interior/USGS36
If two continental plates collide, mountain building usually takes place because they
are both relatively low in density.
Earthquake activity at these boundaries is common; however, because igneous
activity is different from ocean-continent collisions, volcanoes are rare.
Convergent Boundary:
Continent-Continent Collision
The Himalaya mountains are still forming today as the Ind-Australian Plate collides with the Eurasian Plate
Examples: The Himalayan and the Appalachian mountain chains.
1137
Convergent Boundary:
Ocean-Ocean Collision If 2 oceanic plates collide, the older, denser one is subducted downward into the
mantle and a chain of volcanic islands can form, called a volcanic arc.
Example: Mariana Islands (Mariana Trench). It is deeper than the earth’s tallest
mountain is tall. Mariana Trench: 11,000 meters deep. Mt. Everest: 8850 meters
high.
The interaction of the descending oceanic plate causes incredible amounts of
stress between the plates. This usually causes frequent earthquakes along the
top of the descending plate known as the “Benioff Zone.” The focii of Benioff
earthquakes can be as deep as 700 km below sea level.
Oceanic/oceanic collision
resulting in a chain of island
arcs.
Credit: U.S. Geological Survey
Department of the Interior/USGSBenioff Zone
1238
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At a divergent boundary, two oceanic plates pull apart from each other through a
process called sea-floor spreading.
Sea-floor spreading was proposed by Harry Hess in the early 1960’s.
Hess proposed that hot magma rises from the asthenosphere and up into existing
ocean crust through fractures. The crust spreads apart making room for new magma
to flow up through it. The magma cools, forming new sea floor and resulting in a
build-up of basaltic rock around the crack, which is called a mid-ocean ridge.
2. Divergent Boundary: Sea-floor Spreading
New material is constantly being created. This is the opposite of a convergent
boundary, where material is constantly being destroyed.
Sea-floor spreading at an oceanic divergent boundary.
Modified after McGraw Hill/ Glencoe, 1st ed., pg. 138 (with permission)
1440
3. Transform Boundary When two plates slide past each other moving in different directions or the same
direction, it is termed a transform boundary and is characterized by a transform
fault and earthquake activity.
An example of a transform fault is the San Andreas Fault in California. Here the
North American Plate joins the Pacific Plate. The difference in plate motion along
the contact (fault) leads to a buildup of strain energy that sometimes slips releasing
a huge amount of energy and causing an earthquake.
An aerial photo of the San Andreas
fault line. McGraw Hill/Glencoe, 1st ed., pg.
146 (with permission)
Movement between the 2 plates at the San Andreas
Transform Fault. McGraw Hill/Glencoe, 1st ed., pg. 146 (with
permission). 2041
Tectonic Plate „s
Boundaries
Earthquake
epicenters
(1963-1998)
43
Earthquakes
44
Introduction
Earthquake is a quaking or shaking of theground caused by the sudden release of energystored in the rocks beneath the earth’s surface
45
What causes earthquakes?Tectonic plates move past each other causing stress.
Stress causes the rock to deform
– Plastic deformation – does not cause earthquakes
– Elastic deformation – rock stretches then reaches a
breaking point, releasing energy.
46
Energy released and propagates in all
directions as seismic waves causing
earthquakes
Focus = location of initial slip on the fault; where the earthquake originates
Epicenter = spot on Earth’s surface directly above the focus 47
Body Waves• Seismic waves that travel through the earth’s
interior, spreading outward from the focus in all
directions
• P (primary) wave: a compressional (or longitudinal)
wave in which rock vibrates back and forth parallel
to the direction of wave propagation; can travel
through rock, gas, or liquid
48
Body Waves (cont.)
• S (secondary/shear) waves: a slower, transverse
wave that travels through near surface rocks at 2 to
5 kilometers per second; the rock vibrates
perpendicular to the direction of wave propagation;
can travel through rock but not gas or liquid
49
Surface Waves
surface waves: seismic waves that travel on the earth’s
surface away from the epicenter
Long waves: waves that have no vertical displacement;
they move side to side in a horizontal plane that is
perpendicular to the direction the wave is traveling or
propagating; do not travel through liquids; because of
horizontal movement the waves tend to knock buildings
off their foundation
50
Seismic Waves
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How do scientists calculate how far a location
is from the epicenter of an earthquake?
• Scientists calculate the difference betweenarrival times of the P waves and S waves
• The further away an earthquake is, thegreater the time between the arrival of the Pwaves and the S waves
52
Measuring Earthquakes
• Seismograph: a recording device that produces apermanent record of earth motion detected by aseismometer, usually in the form of a wiggly line drawnon a moving strip of paper
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Seismogram Printout
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Magnitude -- measure of energy released during earthquake.
There are several different ways to measure magnitude.
Most common magnitude measure is Richter Magnitude, named for the renowned seismologist, Charles Richter.
Richter Magnitude
Measure amplitude of largest S wave on seismograph record.
Take into account distance between seismograph & epicenter.
Richter Scale
Logarithmic numerical (NOT a physical) scale
Increasing one whole unit on Richter Scale represents 10 times greater magnitude.
Going up one whole unit on Richter Scale represents about a 30 times greater release of energy.
Intensity
Intensity refers to the amount of damage done in an earthquake
Mercalli Scale is used to express damage
Determining the magnitude of an earthquake
55
Richter Scale
Measures the energy released by
fault movement
related to the maximum amplitude of
the wave measured from the
seismogram
Logarithmic-scale
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6
31.5 times energy
7
992 times more energy!!
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Effects of Earthquakes
• ground motion
• fire
• landslides
• liquefaction (a special type of ground failure)
• permanent displacement of the land surface
• aftershocks: small earthquakes that follow the main shock
• tsunami
Liquefaction of soil by a 1964 earthquake in Niigata, Japan, causedearthquake-resistant apartment buildings to topple over intact. An example of permanent displacement of the land surface - fence
compressed by ground movement, Gallatin County, Montana, 1959.58
TsunamisTsunamis (seismic sea waves): huge ocean
wave produced by displacement of the sea floor
59
A tsunami is a series of
ocean waves with very long
wavelengths (typically
hundreds of kilometers)
caused by large-scale
disturbances (earthquake) of
the ocean
60
World Distribution of Earthquakes
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Pancake-Style Collapse — 1985 Mexico City Quake
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Timber-Frame Buildings in Turkey
Landslide from Seattle
Earthquake, 1965
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Effects of Soil Liquefaction,
Japanese Quakes (Nigata), 1964
Sand Boils after Loma Prieta
Earthquake
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Damage due to
the Tsunami in
2004, Sri Lanka
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Japan earthquake,
March 2011
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Summary-Earthquake
1. Earthquakes generate waves that travel throughthe earth
2. Earthquakes occur when rocks slip along faults
3. Magnitude and Intensity
4. Seismic waves are used to map the earth’s interior
5. Predicting earthquakes is not yet possible
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