Plate Tectonics
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Transcript of Plate Tectonics
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Plate TectonicsPlate Tectonics
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BIGBIG Idea: Idea: Most geologic activity
occurs at the boundaries between plates.
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I. Drifting ContinentsI. Drifting Continents
• MAIN IDEA: The shape and geology of the continents suggests that they were once joined together.
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Continental DriftContinental Drift
theory proposed by Alfred Wegener (VAY guh nur) in 1912
stated that the continents were once a single landmass
the continents have moved...
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--Pangaea (“all the earth”): Pangaea (“all the earth”): supercontinent; single landmass single landmass --Panthalassa (“all seas”):Panthalassa (“all seas”): huge ancient huge ancient oceanocean
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A. Evidence of Continental A. Evidence of Continental DriftDrift
1. Puzzle-Piece Fit
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2. Rock Formations2. Rock Formations
Ages, types and layers of coastal rocks of widely separated regions matched…
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Western Africa and Eastern Brazil
Appalachians (along eastern U.S.) fit Greenland and Europe
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3. Fossils3. Fossils
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Mesosaurus: freshwater reptile that lived 270 million years ago
• remains (fossils) found in South America and Africa
• impossible for these reptiles to have crossed the Atlantic
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4. Climate4. Climate
(a) Glossopteris: fern that grew in temperate climates
places where fossils had been found were once closer to the Equator
rocks containing these fern fossils had once been joined
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(b) Coal Deposits: Coal forms from ancient
swamp plants Coal beds found in
Antarctica indicated that this frozen land once had a tropical climate
Antarctica must have been closer to the Equator
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(c) Glacier Deposits: found in Africa and South
America climates are much warmer
there today
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A Rejected Notion…A Rejected Notion…
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• Wegener’s Continental Drift Hypothesis was never accepted by the scientific community
• He could not explain what forces could cause such massive movement
• Wegener died in 1930, on expedition in Greenland, while collecting evidence to further support his theory
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II. Seafloor SpreadingII. Seafloor Spreading
• MAIN IDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor.
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1.Mid-Atlantic Ridge: undersea mountain range with a steep narrow valley down the center
• the middle of the ocean floor is very young
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2. Renewal of the Ocean Floor:seafloor spreading - floor moving
away from the centerHarry Hess and Robert Dietz
3. Paleomagnetism: as magma solidifies the iron-rich minerals align with magnetic north like a compass
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reverse polarityreverse polarity: magnetic : magnetic orientation pointing southorientation pointing south
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III. Plate BoundariesIII. Plate Boundaries • MAIN IDEA: Volcanoes,
mountains, and deep-sea trenches form at the boundaries between the plates
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Theory of Plate TectonicsTheory of Plate Tectonics• Plate Tectonics:
theory that the lithosphere is made of rigid plates that ‘float’ on the asthenosphere
• Tectonics: the study of the formation of Earth’s features
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What are the What are the inferredinferred properties of Earth’s properties of Earth’s
interior?interior?
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Zones of EarthZones of Earth1. Crust: thin,
solid outermost layer; surrounds Earth like a shell surrounds an egg.
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Two Types of Crust:Two Types of Crust:
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Continental Crust: makes up the continents (land); thicker, less dense; granitic rock
Oceanic Crust: under the oceans; thinner, MORE dense; basaltic rock
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2. Moho: boundary between the crust and mantle.
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3. Mantle: thickest layer sits below the crust makes up 2/3 of the Earth’s
mass
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•Lithosphere (‘rocky sphere’): upper part of mantle, cool and rigid
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•Asthenosphere (‘weak sphere’): lower (‘plastic’) mantle; flows like hot tar
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•Plasticity: the ability of a solid to flow
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4. Core: center of the Earth; two parts:
• InnerInner Core: ball of hot, solidsolid metals (iron and nickel)
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OuterOuter Core: only true liquidliquid zone
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If we’ve never been If we’ve never been there, how have we there, how have we learned about the learned about the interior of Earth?interior of Earth?
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By studying seismic By studying seismic waves…waves…
Seismic Wave:Seismic Wave: vibration that travels through the Earth; results from earthquakes or huge explosions
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Types of Plate Types of Plate BoundariesBoundaries
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1. 1. Divergent BoundariesDivergent Boundaries: • two plates that are moving
AWAY from each other• as plates move apart, the
asthenosphere flows up to fill the empty space
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What forms when plates What forms when plates DIVERGEDIVERGE??
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a. Mid-Ocean Ridges Mid-Ocean RidgesMid-Atlantic RidgeMid-Atlantic Ridge
b. Rift Valleys:b. Rift Valleys:(1) narrow valleys in the (1) narrow valleys in the middle middle of ocean ridgesof ocean ridges
(2) where continents (2) where continents separateseparate
East African Rift East African Rift ValleyValley
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2. 2. Convergent Convergent BoundariesBoundaries
• direct collision of one plate with another
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Three types of converging Three types of converging boundaries:boundaries:
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• Types of Convergent Boundaries
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OceanicOceanic vs. vs. ContinentalContinental
Subduction Zone:Subduction Zone: – oceanic crust moves (dives) under continental crust
forms an oceanic trench and volcanic mountain range
oceanic crust is more dense than continental, thus it dives down or subducts
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Andes Mountain RangeAndes Mountain Range
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Mount Saint Helens - Mount Saint Helens - Cascade Cascade
Mountain RangeMountain Range
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OceanicOceanic vs. vs. OceanicOceanic
older, more dense oceanic crust subducts
forms trenches and arcs of volcanic islands
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Aleutian IslandsAleutian Islands
Mariana TrenchMariana Trench
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ContinentalContinental vs. vs. ContinentalContinental
no subduction two continental plates collide
pushing up and creating folded mountains
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3. 3. Transform BoundariesTransform Boundaries • Plates slide horizontally
past each other
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IV. Causes of Plate IV. Causes of Plate MotionsMotions•MAIN IDEA: MAIN IDEA: Convection Convection
currents in the mantle currents in the mantle cause plate cause plate motions.motions.
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coolcool material sinkssinks as hothot material risesrises, due to
DENSITY DIFFERENCESDENSITY DIFFERENCES.
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