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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
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