Plate Tectonics

Plate TectonicsPlate Tectonics

Wegener, Continental Drift and Wegener, Continental Drift and PangaeaPangaea

PangaeaPangaea

PangaeaPangaea

Evidence for Continental DriftEvidence for Continental Drift

Jigsaw Puzzle fit of Jigsaw Puzzle fit of continentscontinents

Alfred Wegener during Greenland expedition

More evidenceMore evidence

Matching fossils on Matching fossils on continents now continents now located thousands located thousands of miles apart. of miles apart. Example = Example = Mesosaurus, a Mesosaurus, a freshwater reptilefreshwater reptile

Many othersMany others

Matching Matching mountain mountain rangesranges

More evidenceMore evidence

Matching geologic Matching geologic structures structures including:including:– Mountain chainsMountain chains– Ore depositsOre deposits– Same rocks of Same rocks of

same agesame age

More evidenceMore evidence

Climate change Climate change evidenceevidence– Glacial deposits at Glacial deposits at

current equatorcurrent equator– Fossilized palm Fossilized palm

trees in Greenlandtrees in Greenland Map shows why Map shows why

according to the according to the placements of placements of current continents current continents within Pangaeawithin Pangaea

Wegener not believedWegener not believed

Why? -Why? -– What could possibly force the What could possibly force the

continents to move across the continents to move across the ocean floor in this way. They ocean floor in this way. They would be crushed.would be crushed.

– He was a meteorologist, He was a meteorologist,

not a geologistnot a geologist

Developments 1950s and 1960sDevelopments 1950s and 1960s World War II submarines using sonar World War II submarines using sonar

found mountains under the oceans – found mountains under the oceans – the mid-ocean ridgesthe mid-ocean ridges

Sea floor drilling showed rocks Sea floor drilling showed rocks younger than expected and youngest younger than expected and youngest towards the center of the mid-ocean towards the center of the mid-ocean ridgeridge

Theory of seafloor spreading Theory of seafloor spreading suggested by Princeton professor Dr. suggested by Princeton professor Dr. Harry HessHarry Hess

Seafloor spreadingSeafloor spreading

First look at the earth’s layers as shown here.

Structure of the EarthStructure of the Earth

The Earth is The Earth is made up of 3 made up of 3 main layers:main layers:– CoreCore– MantleMantle– CrustCrust

Inner core

Outer core

Mantle

Crust

The CrustThe Crust This is where we live!This is where we live!

The Earth’s crust is The Earth’s crust is made of:made of:

Continental Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- young

Evidence for sea floor spreadingEvidence for sea floor spreading Alvin and other Alvin and other

submersibles found cracks submersibles found cracks and lava showing and lava showing spreading and volcanism spreading and volcanism at mid ocean ridges and at mid ocean ridges and odd life formsodd life forms

Matthews and Vine’s Matthews and Vine’s survey of the Indian survey of the Indian Ocean sea floor showed Ocean sea floor showed matching stripes of matching stripes of reversing polarities on reversing polarities on either side of ridge – what either side of ridge – what caused these?caused these?– Lava spreading during Lava spreading during

alternating magnetic periodsalternating magnetic periods

The Earth’s magnetic fieldThe Earth’s magnetic field It is produced by the It is produced by the

outer core of the earth outer core of the earth which is made of liquid which is made of liquid iron and nickeliron and nickel

This moving magnetic This moving magnetic material produces a material produces a moving magnetic field, moving magnetic field, which in turn produces which in turn produces a moving electric field. a moving electric field. It is a dynamo!It is a dynamo!

Earth’s magnetic field Earth’s magnetic field varies over time and it varies over time and it protects us from protects us from cosmic radiationcosmic radiation

Seafloor spreading and Seafloor spreading and paleomagnetismpaleomagnetism

Pangaea revisitedPangaea revisited

By piecing together By piecing together this information, this information, we can see how we can see how the continents the continents have moved over have moved over the past 200 the past 200 million years, due million years, due to seafloor to seafloor spreadingspreading

Plate tectonicsPlate tectonics Sea floor spreading provides the Sea floor spreading provides the

driving mechanism for movementdriving mechanism for movement However, it is not the continents that However, it is not the continents that

are moving, but the “plates” of are moving, but the “plates” of lithosphere “floating” in effect on the lithosphere “floating” in effect on the asthenosphereasthenosphere

The lithosphere is made up of about The lithosphere is made up of about 20 plates which move relative to 20 plates which move relative to each other in several wayseach other in several ways

Let’s look at a generalized sketchLet’s look at a generalized sketch

Evidence supporting the theory of plate tectonics:

Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted.

The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.

Confirmation of the Theory of Confirmation of the Theory of Plate TectonicsPlate Tectonics

Paleomagnetism: strips of alternating magnetic polarity at spreading regions.

Confirmation of Plate Tectonics TheoryConfirmation of Plate Tectonics Theory

The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

The PlatesThe Plates

““Plates” of lithosphere are moved around by the underlying hot mantle Plates” of lithosphere are moved around by the underlying hot mantle convection cellsconvection cells

Plate TectonicsAn overview of the tectonic system.

Sea-Floor Spreading and Plate Sea-Floor Spreading and Plate BoundariesBoundaries

3 types of boundaries3 types of boundaries

Convergent – Convergent – where plates come where plates come together. See together. See example next slideexample next slide

Age of Oceanic CrustAge of Oceanic Crust

Courtesy of www.ngdc.noaa.gov

Mid Atlantic Ridge

Mid Atlantic Ridge

Spreading ridgesSpreading ridges– As plates move apart new material is As plates move apart new material is

erupted to fill the gaperupted to fill the gap

Divergent BoundariesDivergent Boundaries

Iceland has a divergent Iceland has a divergent plate boundary running plate boundary running through its middlethrough its middle

North American Plate and North American Plate and Eurasia Plate divergingEurasia Plate diverging

Iceland: Iceland: An example of continental riftingAn example of continental rifting

East Africa Rift ValleyEast Africa Rift Valley

East Africa Rift ValleyEast Africa Rift Valley

Oceanic lithosphere Oceanic lithosphere subducts underneath the subducts underneath the continental lithospherecontinental lithosphere

Oceanic lithosphere Oceanic lithosphere heats and dehydrates as heats and dehydrates as it subsides it subsides

The melt rises forming The melt rises forming volcanismvolcanism

E.g. The AndesE.g. The Andes

SubductionSubduction

Called SUBDUCTIONCalled SUBDUCTION

Continent-Oceanic Crust CollisionContinent-Oceanic Crust Collision

Deep Ocean Trench

Andes Mountains in South AmericaAndes Mountains in South America

Andes Mountains Andes Mountains formed along the coast formed along the coast

of South Americaof South America

Nazca Plate dives Nazca Plate dives under the South under the South American PlateAmerican Plate

SubductionSubduction

Island FormationIsland Formation

JapanJapan

Volcanoes in Japan

Japan formed as an island chain as Japan formed as an island chain as the Pacific Plate dives beneath the the Pacific Plate dives beneath the

Eurasian PlateEurasian Plate

Mt Fuji

Aleutian Islands Chain Aleutian Islands Chain off Alaskaoff Alaska

Aleutian Islands formed Aleutian Islands formed as an island chain as as an island chain as

Pacific Plate dives Pacific Plate dives beneath North beneath North American PlateAmerican Plate

Forms mountains,Forms mountains, e.g. European Alps, e.g. European Alps, HimalayasHimalayas

Continent-Continent CollisionContinent-Continent Collision

HimalayasHimalayas

Converging Margins: India-Asia Converging Margins: India-Asia CollisionCollision

Alps in Alps in EuropeEurope

Collision of Africa and EurasiaCollision of Africa and Eurasia

Interesting plate collisionInteresting plate collision This picture shows a This picture shows a

place in place in Newfoundland where Newfoundland where a massive collision a massive collision actually forced actually forced mantle rock on top of mantle rock on top of the crust, during the the crust, during the collision that formed collision that formed Pangaea and the Pangaea and the Appalachian Appalachian mountains. This mountains. This looks down the old looks down the old plate boundary.plate boundary.

Mantle rocks are toxicMantle rocks are toxic These rocks have very These rocks have very

different compositions different compositions than crustal rocks.than crustal rocks.

They contain heavy They contain heavy metals, which do not metals, which do not support life forms on support life forms on the earth’s surface, so the earth’s surface, so few organisms live few organisms live there.there.

However, in some However, in some places their heavy places their heavy metal concentrations metal concentrations produce rich metal produce rich metal deposits and are mineddeposits and are mined

Divergent BoundariesDivergent Boundaries Found at spreading Found at spreading

centers – either mid centers – either mid ocean ridges or mid ocean ridges or mid continental rift zonescontinental rift zones

Transform fault boundaryTransform fault boundary

This shows the San This shows the San Andreas Fault.Andreas Fault.

It is a transform fault It is a transform fault boundary, where the boundary, where the plates move sideways plates move sideways past each other, rather past each other, rather than away from each than away from each other (at divergent other (at divergent boundaries), or towards boundaries), or towards each other (at each other (at convergent boundaries)convergent boundaries)

Transform Fault BoundaryTransform Fault Boundary

Transform Faults and Seafloor Transform Faults and Seafloor SpreadingSpreading

Review of different boundariesReview of different boundaries Divergent –mid ocean ridge like Iceland or Divergent –mid ocean ridge like Iceland or

continental rift zone like the African Rift continental rift zone like the African Rift ValleyValley

ConvergentConvergent– Ocean/ocean like Japanese IslandsOcean/ocean like Japanese Islands– Continent/ocean like Andes and CascadesContinent/ocean like Andes and Cascades– Continent/continent like the HimalayasContinent/continent like the Himalayas

Transform fault like the San Andreas faultTransform fault like the San Andreas fault Hot spots are not at plate boundaries, but Hot spots are not at plate boundaries, but

give us information about plate motiongive us information about plate motion Activity Activity

What causes plate tectonics?What causes plate tectonics? Convection in the mantle, as Convection in the mantle, as

the plastic asthenosphere the plastic asthenosphere flows, carrying the plates with flows, carrying the plates with it.it.

This is probably aided by slab This is probably aided by slab pull at subduction zones and pull at subduction zones and ridge push at mid ocean ridges ridge push at mid ocean ridges and rising plumes in the mantleand rising plumes in the mantle

This diagram shows several This diagram shows several different model hypothesesdifferent model hypotheses

Plate tectonics causes Plate tectonics causes volcanic activityvolcanic activity

and earthquakesand earthquakes

It causes rocks to be tiltedIt causes rocks to be tilted

Or even to fold or breakOr even to fold or break

Volcanoes and Earthquakes and Volcanoes and Earthquakes and Plate Boundaries – GIS activityPlate Boundaries – GIS activity

Structural GeologyStructural Geology

IsostasyIsostasy MountainsMountains Stress and strainStress and strain FoldsFolds FaultsFaults

Formation of mountainsFormation of mountains

Two forces are Two forces are constantly at constantly at work on the work on the earth.earth.– Weathering Weathering

and erosion and erosion tear structures tear structures down whiledown while

– Plate tectonics Plate tectonics builds them upbuilds them up

So we have mountains!So we have mountains!

But they will not last forever.

Mountains form in different waysMountains form in different ways-Volcanic mountains--Volcanic mountains-

Volcanoes form by subduction and mVolcanoes form by subduction and melting of plates elting of plates

Volcanic mountains form over hot spVolcanic mountains form over hot spotsots

Volcanic mountains form at rift zonesVolcanic mountains form at rift zones

Other mountain typesOther mountain types

Folded mountains form Folded mountains form From converging continentsFrom converging continents like the like the HimalayasHimalayas

How do these look?How do these look? Direction?Direction? Fault block mountains form where blocks of Fault block mountains form where blocks of

rock drop at faults – mostly near plate rock drop at faults – mostly near plate boundaries, but not alwaysboundaries, but not always

Uplifted mountains – where large sections Uplifted mountains – where large sections of the crust are pushed up, perhaps by of the crust are pushed up, perhaps by magma, or other forcesmagma, or other forces

IsostasyIsostasy Just as a boat sinks or rises with changes in Just as a boat sinks or rises with changes in

weight, so does the crust sink or rise with weight, so does the crust sink or rise with changes in weight. Plate tectonics builds changes in weight. Plate tectonics builds mountains and the extra weight causes the crust mountains and the extra weight causes the crust to sink. As erosion occurs the weight of the to sink. As erosion occurs the weight of the mountains decreases and the crust rises again. mountains decreases and the crust rises again. This process is called isostasy or isostatic changeThis process is called isostasy or isostatic change

IsostasyIsostasyIsostasy is balance. A floating object is balanced in the water, like an iceberg. If some of the top melts, the iceberg rises in the water to stay in balance. If you get into a boat, the boat sinks to maintain balance. The same thing happens with mountains. As plate motions push it higher the mountain sinks into the mantle to stay in balance. On the other hand, when the mountain erodes, it will rise in the mantle as the top erodes. So, a mountain may lose 1 meter from erosion, but regain 0.8 meters from rising due to isostasy. It maintains isostatic equilibrium.

•This is also a great example of feedback within a system. Is it positive or negative? Figure it out. Remember that maintaining equilibrium is __ feedback because it keeps the system in balance. So, what is it, positive or negative?

•This is a good example of Newton’s 3rd law. For every force, there is an equal and opposing force. In this case the force of gravity is opposed by the buoyant force.

Right! It’s negative.

Stress Stress

Due mostly to plate movements, the earth’s crust is under a lot of stress. There are 3 types, shown at the right

A occurs where plates pull apart, divergent boundaries, and is called tension

B occurs where plates converge, and is called compression

C occurs where plates move past each other, at transform fault boundaries and is called shearing

StrainStrain This stress leads to strain This stress leads to strain

on the crust which bends it. on the crust which bends it. – –

If it is warm, underground, it If it is warm, underground, it can bend. This called ductile can bend. This called ductile deformation. Features are deformation. Features are called folds. Upturned folds called folds. Upturned folds are anticlines while are anticlines while downturned folds are downturned folds are synclines. synclines.

Or, the rock may break, if it Or, the rock may break, if it is brittle. This causes faults is brittle. This causes faults –breaks of the earth.–breaks of the earth.

A fold above and a fault below

anticline syncline

Stress and strain video

Economic value of folds/faults

Different faultsDifferent faults

Faults move in Faults move in different ways, different ways, depending on the depending on the type of stress on type of stress on them. Here are 3 them. Here are 3 types that form. types that form.

1 is a normal fault1 is a normal fault 2 is a strike slip 2 is a strike slip

faultfault 3 is a reverse fault3 is a reverse fault

2

3

1

MatchingMatching

These form at These form at different plate different plate boundaries. Can you boundaries. Can you figure out which figure out which forms where? Match forms where? Match themthem

A. Convergent

C. Transform Fault

B. Divergent

2

3

3

2

1

1

Can you match the stress and strain?Can you match the stress and strain?

A is 1st, B is 3rd, C is 2nd

2

3

1