plate tectonics

(c) Vicki Drake, 2011(c) Vicki Drake, 2011 11

PLATE TECTONICSPLATE TECTONICSTheory of Lithospheric Plate Theory of Lithospheric Plate

MotionMotion

(c) Vicki Drake, 2011 2

Continental Drift TheoryContinental Drift Theory►Proposed by Alfred Wegener, early Proposed by Alfred Wegener, early

1900s, Germany1900s, Germany►Suggested that Earth’s continents have Suggested that Earth’s continents have

split and rejoined many times during split and rejoined many times during Earth’s history.Earth’s history.

►Last joining was approximately 200 Last joining was approximately 200 million years ago with the formation of million years ago with the formation of “Pangaea” (“Pangaea” (all landall land) with Europe, North ) with Europe, North America, Africa and South America America, Africa and South America joined together.joined together.


PANGAEA

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Evidence for Plate TectonicsEvidence for Plate Tectonics► ““Fit” of continents, especially along edges of continental shelvesFit” of continents, especially along edges of continental shelves

► Similar fossils found on continents separated by oceansSimilar fossils found on continents separated by oceans Dinosaurs of similar species found widely separated continentsDinosaurs of similar species found widely separated continents

► Indicates that continents were joinedIndicates that continents were joined Mammals differentiated on separate continentsMammals differentiated on separate continents

► Indicates that continents were separatedIndicates that continents were separated

► ““Belts” of mountain ranges with similar geology and agesBelts” of mountain ranges with similar geology and ages Mountains in North and South America connect to mountains in AfricaMountains in North and South America connect to mountains in Africa

► Age of ocean floor and magnetic reversalsAge of ocean floor and magnetic reversals Moving from mid-ocean ridges on the sea floor, ocean sediments grow Moving from mid-ocean ridges on the sea floor, ocean sediments grow

older (youngest closest to the ridge)older (youngest closest to the ridge) Oldest age of any ocean floor dates only to approximately 180 million Oldest age of any ocean floor dates only to approximately 180 million

years ago years ago Moving from mid-ocean ridges on sea floor, the magnetism of specific Moving from mid-ocean ridges on sea floor, the magnetism of specific

rocks making up the deep ocean floor, indicate reversals of normal rocks making up the deep ocean floor, indicate reversals of normal magnetism (i.e, north and south magnetic poles ‘flip’ positions)magnetism (i.e, north and south magnetic poles ‘flip’ positions)

► Glacial ‘scour’ as large glaciers moved across continents, Glacial ‘scour’ as large glaciers moved across continents, ‘scratching’ the surface.‘scratching’ the surface. Pattern discernible when continents pushed back to Pangaea formationPattern discernible when continents pushed back to Pangaea formation


Magnetic Reversals and Age of Magnetic Reversals and Age of Ocean Crust Ocean Crust

Mid-Ocean Ridge

Edges of Continents


What is the Theory of Plate What is the Theory of Plate Tectonics?Tectonics?

► The Earth’s crust and upper mantle are The Earth’s crust and upper mantle are bonded together into the Lithospherebonded together into the Lithosphere Lithosphere composed of Earth’s crust and upper Lithosphere composed of Earth’s crust and upper

rigid Mantle bonded togetherrigid Mantle bonded together► Lithosphere is broken into many moving Lithosphere is broken into many moving

slabs/sections (‘plates’).slabs/sections (‘plates’).► Plates are sliding on softer, hotter layer of Plates are sliding on softer, hotter layer of

mantle just below Lithosphere: mantle just below Lithosphere: AsthenosphereAsthenosphere

► Theory of Plate Tectonics explains the Theory of Plate Tectonics explains the motion and interaction of lithospheric platesmotion and interaction of lithospheric plates


Map of tectonic plates and relative motions


What’s Causing the Plates to What’s Causing the Plates to Move?Move?

► Many hypotheses have been presented to explain the Many hypotheses have been presented to explain the motion of the lithosphere/tectonic platesmotion of the lithosphere/tectonic plates

► Wegener thought some kind of centrifugal forces broke Wegener thought some kind of centrifugal forces broke up Pangaea and drove the continents apartup Pangaea and drove the continents apart

► Scientists today postulate that strong convection Scientists today postulate that strong convection currents within the mantle are moving the lithosphere currents within the mantle are moving the lithosphere aroundaround Shallow Cell Convection: only the upper mantle (the Shallow Cell Convection: only the upper mantle (the

asthenosphere) is fluid enough to move the platesasthenosphere) is fluid enough to move the plates Deep Cell Convection: The entire mantle (from the outer core Deep Cell Convection: The entire mantle (from the outer core

boundary to the asthenosphere) is hot enough to produce strong boundary to the asthenosphere) is hot enough to produce strong convection cellsconvection cells

Two-tiered Convection: The asthenosphere has its own Two-tiered Convection: The asthenosphere has its own convection cells, driven in part by larger convection cells in the convection cells, driven in part by larger convection cells in the lower mantlelower mantle


Deep Convection cell hypothesis to explain movement of lithospheric plates


Three Basic Types of Plate Three Basic Types of Plate BoundariesBoundaries

►Convergent BoundariesConvergent Boundaries:: two lithospheric/tectonic plates are on a two lithospheric/tectonic plates are on a

collision coursecollision course►Divergent BoundariesDivergent Boundaries::

two lithospheric/tectonic plates are splitting two lithospheric/tectonic plates are splitting into two or more pieces and are moving away into two or more pieces and are moving away from each other in opposite directionsfrom each other in opposite directions

►Transform BoundariesTransform Boundaries:: two lithospheric/tectonic plates are sliding two lithospheric/tectonic plates are sliding

past each other, horizontally, in opposite past each other, horizontally, in opposite directions (neither colliding nor splitting apart)directions (neither colliding nor splitting apart)


TYPES OF PLATE TYPES OF PLATE BOUNDARIESBOUNDARIES


CONVERGENT BOUNDARIESCONVERGENT BOUNDARIES►When two lithospheric plates are When two lithospheric plates are

moving towards one another – they moving towards one another – they form a “Convergent Boundary”form a “Convergent Boundary”

►One of two interactions are the result One of two interactions are the result of a Convergent Boundaryof a Convergent Boundary Subduction ZoneSubduction Zone Collision ZoneCollision Zone


Subduction Zone: Convergent Subduction Zone: Convergent BoundaryBoundary

► In a subduction zone, one lithospheric plate In a subduction zone, one lithospheric plate will be pushed beneath another (‘subducted’). will be pushed beneath another (‘subducted’).

► The non-subducted plate will ride over the The non-subducted plate will ride over the subducted plate.subducted plate. This is a ‘destructive’ boundary, as lithosphere is This is a ‘destructive’ boundary, as lithosphere is

destroyed and consumed back into the mantledestroyed and consumed back into the mantle► Subducted plate subjected to extreme heat Subducted plate subjected to extreme heat

with melting and formation of magma with melting and formation of magma occurringoccurring Volcanic arcs (either on land or the ocean floor)Volcanic arcs (either on land or the ocean floor) EarthquakesEarthquakes Deep trenches on ocean floorDeep trenches on ocean floor


Subduction Zone: Oceanic to Subduction Zone: Oceanic to OceanicOceanic

►When two oceanic plates enter a When two oceanic plates enter a subduction zone, the older, colder and subduction zone, the older, colder and more dense of the two will be subducted.more dense of the two will be subducted.

►A chain of volcanic islands on the ocean A chain of volcanic islands on the ocean floor will develop.floor will develop. PhilippinesPhilippines JapanJapan IndonesiaIndonesia MarianasMarianas

(c) Vicki Drake, 2011 18Convergent Plate Boundaries: oceanic-oceanic subduction zone


Subduction Zone: Oceanic to Subduction Zone: Oceanic to ContinentalContinental

►When an oceanic plate and continental When an oceanic plate and continental plate enter a subduction zone, the plate enter a subduction zone, the oceanic plate will be subducted.oceanic plate will be subducted.

►Chains of volcanic mountains will form Chains of volcanic mountains will form on the continental plate.on the continental plate. Cascade Range (CA to WA), North AmericaCascade Range (CA to WA), North America Andes, S. AmericaAndes, S. America


Pacific Plate subducting beneath North American Plate; producing Cascade Volcanoes extending between northern California through Washington

Pacific Plate subducting beneath South American Plate; producing volcanoes in the Andes Mountains


Pacific Ring of Fire: volcanoes and earthquakes found along the subducting edges of the Pacific Plate


Collision Zone: Continental to Collision Zone: Continental to ContinentalContinental

► When two continental plates collide, neither When two continental plates collide, neither will be subducted.will be subducted. Continental plates are too thick and buoyant for Continental plates are too thick and buoyant for

subduction.subduction.► Great folded and faulted mountains will result Great folded and faulted mountains will result

from the collision, along with earthquakesfrom the collision, along with earthquakes Himalaya Mtns, AsiaHimalaya Mtns, Asia Swiss Alps, EuropeSwiss Alps, Europe Appalachians, North AmericaAppalachians, North America Atlas Mtns, NW AfricaAtlas Mtns, NW Africa


COLLISION ZONE: EUROPECOLLISION ZONE: EUROPE

INDIA COLLIDING WITH ASIAINDIA COLLIDING WITH ASIA


India’s collision with Asia pushed up the Himalaya Mountains and the Tibetan Plateau

India is still pushing up the Himalaya Mountains even today!


Earthquakes and Collision Earthquakes and Collision ZonesZones


DIVERGENT BOUNDARYDIVERGENT BOUNDARY► When a lithospheric plate is split into two or more When a lithospheric plate is split into two or more

pieces, a Divergent Boundary is formedpieces, a Divergent Boundary is formed► ““Sea-floor spreading” is a type of divergent Sea-floor spreading” is a type of divergent

boundary in which new ocean floor is formedboundary in which new ocean floor is formed This is a ‘constructive’ boundary where new lithosphere is This is a ‘constructive’ boundary where new lithosphere is

createdcreated First identified by Dr. Harry Hess, PrincetonFirst identified by Dr. Harry Hess, Princeton

► Rift Valleys are another type of divergent boundary Rift Valleys are another type of divergent boundary where the plate has not completely split apart.where the plate has not completely split apart. East African Rift ValleyEast African Rift Valley

► Region of Eastern Africa extending from Mozambique to Region of Eastern Africa extending from Mozambique to Ethiopia is being separated from the rest of continental AfricaEthiopia is being separated from the rest of continental Africa


MID-ATLANTIC RIDGE: Sea floor spreading zone

South America Africa

Atlantic Ocean Basin


East African Rift Valley


TRANSFORM BOUNDARYTRANSFORM BOUNDARY► When two lithospheric plates slide past each When two lithospheric plates slide past each

other in opposite directions, this is a Transform other in opposite directions, this is a Transform Boundary (also known as a Strike-slip Boundary (also known as a Strike-slip boundary)boundary) This is a ‘conservative’ boundary – crust is neither This is a ‘conservative’ boundary – crust is neither

created or destroyedcreated or destroyed San Andreas Fault, North AmericaSan Andreas Fault, North America

► Plate boundary between the Pacific Plate and North Plate boundary between the Pacific Plate and North American plateAmerican plate

► There will be no subduction, no volcanoesThere will be no subduction, no volcanoes► There will be earthquakes and horizontal There will be earthquakes and horizontal

displacement of surface featuresdisplacement of surface features


Transform Boundary – two tectonic plates slip past each other, parallel, but in opposite relative directions


San Andreas Fault, western San Andreas Fault, western North AmericaNorth America

San Andreas Fault through Carrizo Plain (west of Bakersfield)

plate tectonics

Documents

pangaeac vicki drake

pangaea formationc vicki

relative motionsc vicki

lower mantlec vicki

earths continents

deep ocean floor

africaage of ocean floor

ocean floor dates