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Plate TectonicsPlate Tectonics
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Continental Drift
Continental Drift
Researchers noted geographic fit of
continents, e.g. Africa and S. America
Seuss, 1885, proposed super continent by
studying fossils, rocks, mountains
Alfred Wegener - First proposed his
continental drift hypothesis in 1915
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Continental Drift
Continental Drift
Hypothesis:
Supercontinent called
Pangaea began
breaking apart about
200 million years ago
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Break up of PangaeaB
reak up of Pangaea
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Wegener¶s Evidence
Wegener¶s Evidence
Fit of the continents
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Wegener¶s Evidence
Wegener¶s Evidence
Fit of the continents
Rock type & structural
similarities
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Wegener¶s Evidence
Wegener¶s Evidence
Fit of the continents
Rock type & structural
similarities
Fossil evidence
Fossils of Mesosaurus have been found on both
sides of the South Atlantic and nowhere else in the
world. Fossil remains of this and other organisms
on the continents of Africa and South America
appear to link these landmasses during the late
Paleozoic and early Mesozoic eras.
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Wegener¶s Evidence
Wegener¶s Evidence
Fit of the continents
Rock type & structural
similarities
Fossil evidence
Paleoclimatic evidence
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Sea Floor SpreadingSea Floor Spreading
HypothesisHypothesis Proposed by Harry Hess in the late 1950s /
early 1960s
± New crust forms at ocean ridges
± Old crust consumed at trenches
± Developed theory of sea floor spreading driven
by convection cells
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Sea Floor SpreadingSea Floor Spreading
HypothesisHypothesis Evidence
Earthquakes at ridges and subduction zones
Heat flow highest at ridges, decreases awayfrom ridges
Radiometric dating shows oldest rocks on seafloor of 200 m.y.
Sediments older away from ridges
Earth¶s magnetic field reverses, this changerecorded by iron minerals in rocks
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SeaSea--Floor Spreading and PlateFloor Spreading and Plate
BoundariesBoundaries
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The Earth¶s Magnetic FieldThe Earth¶s Magnetic Field Earth's magnetic
field resembles that
produced by a large
bar magnet.
Magnetic north is
not in the samelocation as true
north
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PaleomagnetismPaleomagnetism Magnetic field of the Earth
is imprinted upon iron-
bearing rocks when theyform
Provides a record of magnetic reversals andoriginal latitude
Seafloor mapping in1950¶s and 1960¶srevealed magnetic stripingon the ocean floor.
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The ocean floor as a magneticThe ocean floor as a magnetic
tape recorder.tape recorder.
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MagneticMagnetic
patterns off patterns off
the Eastthe Eastcoast of coast of
NorthNorth
America.America.
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PolarPolar
WanderWanderA. The more westerly
path determined from
North American data
is thought to have been caused by the
westward drift of
North America by
about 24 degrees
from Eurasia.
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PolarPolar
WanderWander
B. The positions of
the wandering paths
when the landmasses
are reassembled in
their predrift
locations.
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Putting it all together: ThePutting it all together: The
Theory of Plate TectonicsTheory of Plate Tectonics
Continental Drift
+ Sea-Floor Spreading
+ Paleomagnetism
= Plate Tectonics
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Plate TectonicsPlate Tectonics Earth¶s lithosphere is broken up into plates
Hot, weak asthenosphere allows for plates
to move
Plates are in motion and continually
changing in shape and size
± Move very slowly ± 5 cm/yr
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Because Earth¶s size is constant, expansionB
ecause Earth¶s size is constant, expansionof the crust in one area requiresof the crust in one area requires
destruction of the crust elsewhere.destruction of the crust elsewhere.
Currently, the Pacific Ocean basin is shrinking as other
ocean basins expand.
Destruction of sea floor occurs in subduction zones.
Seismicity is the frequency, magnitude and distribution
of earthquakes. Earthquakes are concentrated alongoceanic ridges, transform faults, trenches and island arcs.
Tectonism refers to the deformation of Earth¶s crust.
3-3Global Plate Tectonics
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Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench
downward in the direction of the island arc.
Subduction is the process at a trench wherebyone part of the sea floor plunges below another
and down into the asthenosphere.
3-3 Global Plate Tectonics
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Earth¶s surface is composed of a series of Earth¶s surface is composed of a series of
lithospheric plates. Plate edges extend throughlithospheric plates. Plate edges extend throughthe lithosphere and are defined by seismicity.the lithosphere and are defined by seismicity.
Plate edges are trenches, oceanic ridges and
transform faults. Seismicity and volcanism are concentrated
along plate boundaries.
Movement of plates is caused by thermalconvection of the ³plastic´ rocks of the
asthenosphere which drag along the
overlying lithospheric plates.
3-3 Global Plate Tectonics
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Structure of the EarthS
tructure of the Earth Layers ± by composition
± Crust
± Mantle
± Core
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Structure of the EarthS
tructure of the Earth Layers ± by physical properties
± Lithosphere
± Asthenosphere
± Mesosphere
± Outer Core
± Inner Core
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Structure of the EarthS
tructure of the Earth Crust
± Continental crust
Average rock density about
2.7 g/cm3
Composition = the felsic
igneous rock granodiorite
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Structure of the EarthS
tructure of the Earth Crust
± Oceanic crust
Density about 3.0 g/cm3
Composed mainly of the
igneous rock basalt
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Structure of the EarthS
tructure of the Earth Mantle
± Contains 82% of Earth¶s
volume ± Solid, rocky layer
± Upper portion has thecomposition similar to
peridotite ± Two parts
Mesosphere (lower mantle)
Asthenosphere or upper
mantle
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Structure of the EarthS
tructure of the Earth Core
± Larger than the planet
Mars ± Mostly iron with some
nickel
Average density is nearly
11 g/cm3 ± Two parts
Outer core - liquid
Inner core - solid
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Structure of the EarthS
tructure of the Earth Core
± Responsible for
Earth¶s magneticfield
Made of material
that conducts
electricity
Core is mobile
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S
tructure of the EarthS
tructure of the Earth Seismic waves
± P waves
Travels through liquids as
well as solids
In all materials, P waves
travel faster than do S waves
± S waves
Cannot travel through
liquids
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S
tructure of the EarthS
tructure of the Earth Seismic waves
± Seismic waves refract as they pass from one
material to another
± P & S wave shadow zones
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P wave shadow zoneP wave shadow zone
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S
wave shadow zoneS
wave shadow zone
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I
sostacyI
sostacy The balancing of
pressures exerted
by mass of continents andocean crust onmantle
Continents float onmantle likeicebergs in water
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MantleC
onvectionMantleC
onvection
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Distribution of earthquake fociDistribution of earthquake foci
at plate boundariesat plate boundaries
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Lithospheric PlatesLithospheric Plates
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Lithospheric PlatesLithospheric Plates
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Plate TectonicsPlate Tectonics
Plate boundaries
± All major interactions among individual plates
occur along their boundaries
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Plate TectonicsPlate Tectonics
Plate boundaries
± Types of plate boundaries
Divergent plate boundaries
Convergent plate boundaries
Transform fault boundaries
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Plate TectonicsPlate Tectonics
Divergent Plate boundaries
± Most are located along the crests of oceanic
ridges
± Also continental rifts
± A few are on land
Quicktime Movie
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Formation of Formation of Ocean BasinsOcean Basins
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The East African riftThe East African rift
Spreading the continent
apart 100 times as slow as
a typical oceanic rift zone.
The rift forms one arm of a
triple junction, from which
the Red Sea and Gulf of Aden form somewhat more
rapidly spreading rifts.
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ContinentalContinental
RiftingRifting Basin and Range
± Region is slowly
pulling apart
± No active
spreading center
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This Basin and Range terrain is found southwest of Salt Lake City, Utah.
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Plate TectonicsPlate Tectonics
Convergent plate boundaries
± Where two platescollide
± Types
Oceanic-continental
convergence Oceanic-oceanic
convergence
Continental-continentalconvergence
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OceanOcean--Continent ConvergenceContinent Convergence
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Cascade Mountains, Pacific NWCascade Mountains, Pacific NW
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OceanOcean--Ocean ConvergenceOcean Convergence
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ContinentContinent--Continent ConvergenceContinent Convergence
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Converging Margins: IndiaConverging Margins: India--
Asia CollisionAsia Collision
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The collision of India and AsiaThe collision of India and Asia
produced the Himalayas (before)produced the Himalayas (before)
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The collision of India and AsiaThe collision of India and Asia
produced the Himalayas (after)produced the Himalayas (after)
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Plate TectonicsPlate Tectonics
Transform fault boundaries
± Plates slide past one another
± No new lithosphere is created or destroyed
± Most join two segments of amid-ocean ridge as parts of
prominent linear breaks in theoceanic crust known as fracturezones
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Transform FaultsTransform Faults
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Transform FaultsTransform Faults
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Terrane FormationTerrane Formation
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Plate TectonicsPlate Tectonics
Hot spots
± Caused by rising plumes of mantle material
± Volcanoes can form over them (HawaiianIsland chain)
± Most mantle plumes are long-lived structures
and at least some originate at great depth,
perhaps at the mantle-core boundary
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Global Hot SpotsGlobal Hot Spots
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Volcanism on a tectonicVolcanism on a tectonic
plate moving over a hot spotplate moving over a hot spot
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The Hawaiian Islands haveThe Hawaiian Islands have
formed over a stationary hot spotformed over a stationary hot spot
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Hawaiian IslandsHawaiian Islands--EmperorEmperor
Seamount ChainSeamount Chain
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Wilson
Cycle
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