Plate Tectonics: A Unifying Theory Geology 101 Chapter 2.

Plate Tectonics: Plate Tectonics: A Unifying A Unifying

TheoryTheory

Geology 101Geology 101Chapter 2Chapter 2

A A unifying theoryunifying theory is one that helps is one that helps explain a broad range of diverse observations explain a broad range of diverse observations interpret many aspects of a science on a grand interpret many aspects of a science on a grand

scalescale and relate many seemingly unrelated and relate many seemingly unrelated

phenomenaphenomena Plate tectonics is a unifying theory for Plate tectonics is a unifying theory for

geology.geology.

Unifying TheoryUnifying Theory

Plate tectonics helps to explain Plate tectonics helps to explain earthquakesearthquakes volcanic eruptionsvolcanic eruptions formation of formation of

mountainsmountains location of location of

continents continents location of ocean location of ocean

basinsbasins

Plate TectonicsPlate Tectonics

Tectonic Tectonic interactions affectinteractions affect atmospheric and atmospheric and

oceanic circulation oceanic circulation and climateand climate

geographic geographic distribution, distribution,

evolution and evolution and extinction of extinction of organismsorganisms

distribution and distribution and formation of formation of resourcesresources

Edward SuessEdward Suess Austrian, late 1800sAustrian, late 1800s

noted similarities between noted similarities between the Late Paleozoic plant fossils the Late Paleozoic plant fossils

GlossopterisGlossopteris floraflora

Early Ideas Early Ideas about Continental Driftabout Continental Drift

and evidence and evidence for glaciation for glaciation

in rock in rock sequences of sequences of IndiaIndia AustraliaAustralia South Africa South Africa South AmericaSouth America

He proposed the He proposed the name name GondwanalandGondwanaland (or (or GondwanaGondwana) ) for a for a

supercontinent supercontinent composed of these composed of these

continentscontinents

Frank TaylorFrank Taylor (American, 1910)(American, 1910) presented a hypothesis of continental presented a hypothesis of continental

drift with these features:drift with these features: lateral movement of continents formed lateral movement of continents formed

mountain rangesmountain ranges a continent broke apart at the Mid-a continent broke apart at the Mid-

Atlantic Ridge to form the Atlantic OceanAtlantic Ridge to form the Atlantic Ocean supposedly, tidal forces pulled formerly supposedly, tidal forces pulled formerly

polar continents toward the equator, polar continents toward the equator, when Earth captured the Moon about 100 when Earth captured the Moon about 100

million years agomillion years ago

Early Ideas Early Ideas about Continental Driftabout Continental Drift

German German meteorologistmeteorologist

Credited with Credited with hypothesis of hypothesis of continental continental drift-1912 in a drift-1912 in a scientific scientific presentation – presentation – published a published a book in 1915.book in 1915.

Alfred WegenerAlfred Wegener and the and the Continental Drift Continental Drift

HypothesisHypothesis

He proposed that all landmasses He proposed that all landmasses were originally united into a supercontinent were originally united into a supercontinent he named he named PangaeaPangaea from the Greek meaning from the Greek meaning

“all land”“all land” He presented a series of maps He presented a series of maps

showing the breakup of Pangaeashowing the breakup of Pangaea He amassed a tremendous amount of He amassed a tremendous amount of

geologic, paleontologic, and climatologic geologic, paleontologic, and climatologic evidenceevidence

Alfred Wegener and the Alfred Wegener and the Continental Drift Continental Drift

HypothesisHypothesis

Shorelines of continents fit togetherShorelines of continents fit together matching marine, nonmarine matching marine, nonmarine and glacial rock sequences and glacial rock sequences from Pennsylvanian to Jurassic age from Pennsylvanian to Jurassic age for all five Gondwana continents for all five Gondwana continents

including Antarcticaincluding Antarctica Mountain ranges and glacial deposits Mountain ranges and glacial deposits

match up when continents are united match up when continents are united into a single landmassinto a single landmass

Wegener’s EvidenceWegener’s Evidence

Jigsaw-Puzzle Fit of Jigsaw-Puzzle Fit of ContinentsContinents

Continental FitContinental Fit

Fig. 3-4, p. 39

Jigsaw-Puzzle Fit of Jigsaw-Puzzle Fit of ContinentsContinents

Matching Matching mountain rangesmountain ranges

Matching glacial Matching glacial evidenceevidence

Matching FossilsMatching Fossils

The Perceived Problem The Perceived Problem with Continental Driftwith Continental Drift

Most geologists did not accept the idea Most geologists did not accept the idea of moving continentsof moving continents There was no suitable mechanism to explain There was no suitable mechanism to explain how continents could move over Earth’s how continents could move over Earth’s

surfacesurface Interest in continental drift only revived Interest in continental drift only revived

whenwhen new evidence from studies of Earth’s new evidence from studies of Earth’s

magnetic field magnetic field and oceanographic research and oceanographic research showed that the ocean basins were showed that the ocean basins were

geologically young featuresgeologically young features

Earth as a Earth as a giant dipole giant dipole magnetmagnet magnetic poles magnetic poles

essentially essentially coincide coincide

with the with the geographic geographic polespoles

and may result and may result from different from different rotation rotation speedsspeeds

of outer core of outer core and mantleand mantle

Earth’s Magnetic FieldEarth’s Magnetic Field

Strength and orientation of the Strength and orientation of the magnetic field variesmagnetic field varies weak and horizontal at the equatorweak and horizontal at the equator strong and vertical at the polesstrong and vertical at the poles

Magnetic Field VariesMagnetic Field Varies

PaleomagnetismPaleomagnetism is is a remanent magnetism a remanent magnetism in ancient rocks in ancient rocks recording the direction recording the direction and the strength of Earth’s magnetic and the strength of Earth’s magnetic

field field at the time of the rock’s formationat the time of the rock’s formation

When magma cools When magma cools below the below the Curie pointCurie point temperaturetemperature magnetic iron-bearing minerals align magnetic iron-bearing minerals align with Earth’s magnetic fieldwith Earth’s magnetic field

PaleomagnetismPaleomagnetism

Polar WanderingPolar Wandering

Magnetic poles Magnetic poles apparently moved.apparently moved. The apparent movement The apparent movement

was called was called polar polar wandering.wandering.

Different continents had Different continents had different paths.different paths.

In 1950s, In 1950s, research revealed research revealed that that

paleomagnetism paleomagnetism of ancient rocks of ancient rocks showed showed

orientations orientations different from the different from the present magnetic present magnetic fieldfield

The best The best explanation explanation is stationary poles is stationary poles and moving and moving

continentscontinents

Earth’s present magnetic field is called Earth’s present magnetic field is called normalnormal, , with magnetic north near the north geographic with magnetic north near the north geographic

pole pole and magnetic south near the south geographic and magnetic south near the south geographic

polepole At various times in the past, At various times in the past,

Earth’s magnetic field has completely Earth’s magnetic field has completely reversedreversed, , with magnetic south near the north geographic with magnetic south near the north geographic

pole pole and magnetic north near the south geographic and magnetic north near the south geographic

polepole This is referred to as a This is referred to as a magnetic reversalmagnetic reversal

Magnetic ReversalsMagnetic Reversals

Measuring paleomagnetism Measuring paleomagnetism and dating continental lava and dating continental lava flows led to flows led to the realization that magnetic the realization that magnetic

reversals existedreversals existed the establishment of a the establishment of a

magnetic reversal time scalemagnetic reversal time scale

Magnetic ReversalsMagnetic Reversals

Ocean mapping revealedOcean mapping revealed a ridge systema ridge system more than 65,000 km long,more than 65,000 km long, the most extensive mountain range in the most extensive mountain range in

the worldthe world The Mid-Atlantic RidgeThe Mid-Atlantic Ridge

is the best known part of the systemis the best known part of the system and divides the Atlantic Ocean basin and divides the Atlantic Ocean basin in two nearly equal partsin two nearly equal parts

Mapping Ocean BasinsMapping Ocean Basins

Atlantic Ocean BasinAtlantic Ocean Basin

Mid-Atlantic RidgeMid-Atlantic Ridge

Harry HessHarry Hess, in 1962, proposed the , in 1962, proposed the theory of theory of seafloor spreading:seafloor spreading: Continents and oceanic crust move togetherContinents and oceanic crust move together Seafloor separates at oceanic ridgesSeafloor separates at oceanic ridges

where new crust forms from upwelling and where new crust forms from upwelling and cooling magma, andcooling magma, and

the new crust moves laterally away from the the new crust moves laterally away from the ridgeridge

The mechanism that drives seafloor The mechanism that drives seafloor spreading was spreading was thermal convection cellsthermal convection cells in in the mantlethe mantle

hot magma rises from mantle to form new crusthot magma rises from mantle to form new crust cold crust subducts into the mantle at oceanic cold crust subducts into the mantle at oceanic

trenches, where it is heated and recycledtrenches, where it is heated and recycled

Seafloor SpreadingSeafloor Spreading

In addition to mapping mid-ocean In addition to mapping mid-ocean ridges, ridges, ocean research also revealed ocean research also revealed magnetic anomalies on the sea floormagnetic anomalies on the sea floor

A A magnetic anomalymagnetic anomaly is a deviation is a deviation from the average strength from the average strength of Earth’s Magnetic fieldof Earth’s Magnetic field

Confirmation of Hess’s Confirmation of Hess’s HypothesisHypothesis

The magnetic anomalies were The magnetic anomalies were discovered to be parallel and discovered to be parallel and symmetrical with the oceanic ridgessymmetrical with the oceanic ridges

Confirmation of Hess’s Confirmation of Hess’s HypothesisHypothesis

Seafloor spreading theory Seafloor spreading theory indicates that indicates that oceanic crust is geologically young oceanic crust is geologically young

because because it forms during spreading it forms during spreading and is destroyed during subductionand is destroyed during subduction

Radiometric dating confirms Radiometric dating confirms the oldest oceanic crust the oldest oceanic crust is less than 180 million years oldis less than 180 million years old

whereas oldest continental crust whereas oldest continental crust is 3.96 billion yeas oldis 3.96 billion yeas old

Oceanic Crust Is YoungOceanic Crust Is Young

Age of Ocean BasinsAge of Ocean Basins

Plate tectonic theoryPlate tectonic theory is based on is based on the simple model thatthe simple model that the lithosphere is rigidthe lithosphere is rigid it consists of oceanic and continental it consists of oceanic and continental

crust with upper mantlecrust with upper mantle it consists of variable-sized pieces it consists of variable-sized pieces

called called platesplates with plate regions containing with plate regions containing

continental crust continental crust up to 250 km thickup to 250 km thick

and plate regions containing oceanic and plate regions containing oceanic crust crust up to 100 km thickup to 100 km thick

Plate TectonicsPlate Tectonics

Plate MapPlate Map

Numbers represent average rates of relative Numbers represent average rates of relative movement, cm/yrmovement, cm/yr

The lithospheric plates overlie The lithospheric plates overlie hotter and weaker semiplastic hotter and weaker semiplastic asthenosphereasthenosphere

Movement of the platesMovement of the plates results from some type of heat-transfer results from some type of heat-transfer

system within the asthenosphere system within the asthenosphere As plates move over the As plates move over the

asthenosphereasthenosphere they separate, mostly at oceanic they separate, mostly at oceanic

ridgesridges they collide, in areas such as oceanic they collide, in areas such as oceanic

trenches trenches where they may be subducted back where they may be subducted back

into the mantleinto the mantle

Plate Tectonics and Plate Tectonics and BoundariesBoundaries

Divergent plate boundariesDivergent plate boundaries or or spreading ridges,spreading ridges, occur occur where plates are separating where plates are separating and new oceanic lithosphere is forming.and new oceanic lithosphere is forming.

Crust is extendedCrust is extended thinned and fracturedthinned and fractured

The magmaThe magma originates from partial melting of the originates from partial melting of the

mantlemantle is basalticis basaltic intrudes into vertical fractures to form intrudes into vertical fractures to form

dikesdikes or is extruded as lava flowsor is extruded as lava flows

Divergent BoundariesDivergent Boundaries

Successive injections of magma Successive injections of magma cool and solidifycool and solidify form new oceanic crustform new oceanic crust record the intensity and orientation record the intensity and orientation of Earth’s magnetic fieldof Earth’s magnetic field

Divergent boundaries most commonly Divergent boundaries most commonly occur along the crests of oceanic ridges occur along the crests of oceanic ridges such as the Mid-Atlantic Ridgesuch as the Mid-Atlantic Ridge

Ridges haveRidges have rugged topography resulting from displacement rugged topography resulting from displacement of rocks along large fracturesof rocks along large fractures shallow earthquakesshallow earthquakes


Ridges also have Ridges also have high heat flowhigh heat flow and basaltic flows or pillow lavasand basaltic flows or pillow lavas


Pillow lavas Pillow lavas havehave a distinctive a distinctive

bulbous bulbous shape shape resulting resulting from from underwater underwater eruptionseruptions

Divergent boundaries are also present Divergent boundaries are also present under continents during the early stages under continents during the early stages of continental breakupof continental breakup


Beneath a Beneath a continent, continent, magma magma

wells up, wells up, and and

the crust is the crust is initially initially elevated, elevated, stretched stretched and and

thinnedthinned

The stretching produces fractures and The stretching produces fractures and rift valleys.rift valleys.

Rift ValleyRift Valley

During this During this stage, stage, magma magma

typicallytypically intrudes into intrudes into

the fracturesthe fractures and flows and flows

onto the onto the valley floorvalley floor

Example: East Example: East African Rift African Rift ValleyValley

Narrow SeaNarrow Sea As spreading proceeds, some rift valleys As spreading proceeds, some rift valleys

will continue to lengthen and deepen untilwill continue to lengthen and deepen until

the the continental continental crust crust eventually eventually breaksbreaks

a narrow a narrow linear sea is linear sea is formed,formed,

separating separating two two continental continental blocksblocks

Examples: Examples: Red Sea Red Sea Gulf of Gulf of

CaliforniaCalifornia

Modern DivergenceModern Divergence View looking down the View looking down the

Great Rift Valley of Great Rift Valley of Africa.Africa.

Little Magadi Little Magadi soda lakesoda lake

OceanOcean As a newly created narrow sea As a newly created narrow sea

continues to spread, continues to spread, it may eventually become it may eventually become an expansive ocean basinan expansive ocean basin such as the such as the

Atlantic Ocean Atlantic Ocean basin is today,basin is today, separating separating

North and South North and South America America

from Europe and from Europe and Africa Africa

by thousands of by thousands of kilometerskilometers

Atlantic Ocean BasinAtlantic Ocean Basin

Europe Europe

AfricaAfrica

North North AmericaAmerica

South South AmericaAmerica

Atlantic Atlantic

Ocean Ocean basinbasin

Thousands

Thousands of of

kilometers

kilometers

An Example of Ancient An Example of Ancient RiftingRifting

What features in the rock record can What features in the rock record can geologists use to recognize ancient geologists use to recognize ancient rifting?rifting? faultsfaults dikesdikes sillssills lava flowslava flows thick sedimentary thick sedimentary

sequences within sequences within rift valleysrift valleys

Example:Example: Triassic fault-block Triassic fault-block

basins in eastern basins in eastern USUS

Ancient Ancient RiftingRifting

Palisades of Palisades of Hudson RiverHudson River

sillsill

These Triassic fault These Triassic fault basins basins mark the zone of rifting mark the zone of rifting between North America between North America

and Africaand Africa

They contain They contain thousands of thousands of meters of meters of continental continental sediment sediment

and are riddled and are riddled with dikes and with dikes and sillssills

Convergent BoundariesConvergent Boundaries

Older crust must be destroyed Older crust must be destroyed at convergent boundaries at convergent boundaries so that Earth’s surface area remains the so that Earth’s surface area remains the

samesame Where two plates collide, Where two plates collide,

subductionsubduction occurs occurs when an oceanic plate when an oceanic plate descends beneath the margin of another descends beneath the margin of another

plateplate The subducting plate The subducting plate

moves into the asthenospheremoves into the asthenosphere is heatedis heated and eventually incorporated into the mantleand eventually incorporated into the mantle

Convergent BoundariesConvergent Boundaries Convergent boundaries are Convergent boundaries are

characterized bycharacterized by deformation deformation volcanism volcanism mountain buildingmountain building metamorphismmetamorphism earthquake activityearthquake activity valuable mineral depositsvaluable mineral deposits

Convergent boundaries are of three Convergent boundaries are of three types:types: oceanic-oceanicoceanic-oceanic oceanic-continentaloceanic-continental continental-continentalcontinental-continental

Oceanic-Oceanic Oceanic-Oceanic BoundaryBoundary

When two oceanic plates converge, When two oceanic plates converge, one is subducted beneath the other one is subducted beneath the other along an along an oceanic-oceanic plate boundaryoceanic-oceanic plate boundary forming an forming an oceanic trenchoceanic trench and a and a subduction complexsubduction complex

composed of composed of slices of slices of folded and folded and faulted faulted sedimentssediments

and oceanic and oceanic lithospherelithosphere

scraped off scraped off the the descending descending plateplate

Volcanic Island ArcVolcanic Island Arc As the plate subducts into the mantle, As the plate subducts into the mantle,

it is heated and partially meltedit is heated and partially melted generating magma of ~ andesitic composition generating magma of ~ andesitic composition that rises to the surface that rises to the surface because it is less dense than the surrounding because it is less dense than the surrounding

mantle rocksmantle rocks

At the At the surface of surface of the non-the non-subducting subducting plate, plate, the magma the magma

forms a forms a volcanic volcanic island arcisland arc

Oceanic-Oceanic Plate Oceanic-Oceanic Plate BoundaryBoundary

A A back-arc basinback-arc basin forms in some cases forms in some cases of fast subduction. of fast subduction. The lithosphere on the landward side of the The lithosphere on the landward side of the

island arc island arc is stretched and thinned is stretched and thinned

Example: Japan Example: Japan SeaSea

Oceanic-Continental Oceanic-Continental BoundaryBoundary

An An oceanic-continental plate boundaryoceanic-continental plate boundary occurs when a denser oceanic plate occurs when a denser oceanic plate subducts under less dense continental subducts under less dense continental

lithospherelithosphere Magma generated by subduction Magma generated by subduction

rises into the continental crust to form large rises into the continental crust to form large igneous bodiesigneous bodies

or erupts or erupts to form a to form a volcanic volcanic arcarc of of andesitic andesitic volcanoesvolcanoes

Example: Example: Pacific Pacific coast of coast of South South AmericaAmerica

Where the Nazca plate in the Pacific Ocean Where the Nazca plate in the Pacific Ocean is subducting under South Americais subducting under South America the Peru-Chile Trench marks subduction sitethe Peru-Chile Trench marks subduction site and the Andes Mountains are the volcanic arcand the Andes Mountains are the volcanic arc

Oceanic-Continental Oceanic-Continental BoundaryBoundary

Andes Andes MountainMountainss

Continent-Continent Continent-Continent BoundaryBoundary

Two approaching continents are initially Two approaching continents are initially separated by ocean floor that is being subducted separated by ocean floor that is being subducted under one of them, which, thus, has a volcanic arcunder one of them, which, thus, has a volcanic arc

When the 2 continents collide When the 2 continents collide the continental lithosphere cannot subductthe continental lithosphere cannot subduct

Its density Its density is too low,is too low, although although

one one continent continent may may partly partly slide slide under the under the otherother

Continent-Continent Continent-Continent BoundaryBoundary

When the 2 continents collide When the 2 continents collide they weld together at a they weld together at a continent-continent-

continent plate boundarycontinent plate boundary, , where an interior mountain belt forms where an interior mountain belt forms

consisting ofconsisting of deformed deformed sedimentarsedimentary rocksy rocks

igneous igneous intrusionsintrusions

metamorphimetamorphic rocksc rocks

fragments fragments of oceanic of oceanic crust crust

EarthquakEarthquakes occur es occur herehere

Continental-Continental Continental-Continental BoundaryBoundary

Example: Himalayas in central AsiaExample: Himalayas in central Asia Earth’s youngest and highest mountain Earth’s youngest and highest mountain

systemsystem resulted from collision between India and resulted from collision between India and

AsiaAsia began 40 to 50 million years agobegan 40 to 50 million years ago and is still continuingand is still continuing

HimalayaHimalayass

Recognizing Ancient Recognizing Ancient Convergent BoundariesConvergent Boundaries

How can former subduction zones How can former subduction zones be recognized in the rock record?be recognized in the rock record? Andesitic magma erupted, Andesitic magma erupted,

forming island arc volcanoes and continental forming island arc volcanoes and continental volcanoesvolcanoes

The subduction complex results in The subduction complex results in a zone of intensely deformed rocksa zone of intensely deformed rocks between the trench and the area of igneous between the trench and the area of igneous

activityactivity Sediments and submarine rocksSediments and submarine rocks

are folded, faulted and metamorphosedare folded, faulted and metamorphosed making a chaotic mixture of rocks termed a making a chaotic mixture of rocks termed a

mélangemélange Slices of oceanic lithosphere may be accreted Slices of oceanic lithosphere may be accreted

to the continent edge and are called to the continent edge and are called ophiolitesophiolites

OphioliteOphiolite Ophiolites Ophiolites

consist of consist of layers layers representing representing

parts of the parts of the oceanic crust oceanic crust and upper and upper mantle.mantle.

The sediments The sediments includeinclude graywackesgraywackes black shalesblack shales chertscherts

Ophiolites are Ophiolites are key to key to detecting old detecting old subduction subduction zoneszones

Transform BoundariesTransform Boundaries The third type of plate boundary is a The third type of plate boundary is a

transform plate boundarytransform plate boundary where plates slide laterally past each where plates slide laterally past each

otherother roughly parallel to the direction of plate roughly parallel to the direction of plate

movementmovement Movement results inMovement results in

zone of intensely shattered rockzone of intensely shattered rock numerous shallow numerous shallow

earthquakesearthquakes The majority of The majority of

transform faults transform faults connect two oceanic connect two oceanic

ridge segmentsridge segments and are marked by and are marked by

fracture zonesfracture zones

fracture fracture zonezone

Transform BoundariesTransform Boundaries

Other kinds of Other kinds of transform plate transform plate boundariesboundaries connect two connect two

trenchestrenches or connect a or connect a ridge to a trenchridge to a trench

or even a ridge or or even a ridge or trench to another trench to another transform faulttransform fault

Transforms can Transforms can also extend into also extend into continentscontinents

Transform BoundariesTransform Boundaries

separates the Pacific plate separates the Pacific plate from the North American from the North American plateplate

connects ridges inconnects ridges in Gulf of California Gulf of California

Example: San Andreas Example: San Andreas Fault, CaliforniaFault, California

Many of the earthquakes Many of the earthquakes in California result from in California result from movement along this movement along this faultfault

with the Juan de Fuca and with the Juan de Fuca and Pacific platesPacific plates

Hot Spots and Mantle Hot Spots and Mantle PlumesPlumes

Hot spotsHot spots are locations where are locations where stationary columns of magmastationary columns of magma originating deep within the mantle, originating deep within the mantle,

called called mantle plumesmantle plumes slowly rise to the surfaceslowly rise to the surface

Mantle plumes remain stationaryMantle plumes remain stationary although some evidence suggests they although some evidence suggests they

may move may move When plates move over themWhen plates move over them

hot spots leave trailshot spots leave trails of extinct, progressively older volcanoesof extinct, progressively older volcanoes called called aseismic ridgesaseismic ridges which record the movement of the plateswhich record the movement of the plates

Hot Spots and Mantle Hot Spots and Mantle PlumesPlumes

Example: Emperor Seamount-Example: Emperor Seamount-Hawaiian Island chainHawaiian Island chain

plate plate movememovementnt

Age Age increaseincreasess

How Is Plate Motion How Is Plate Motion Determined?Determined?

Rates of plate movement can be Rates of plate movement can be calculated in several wayscalculated in several ways SedimentSediment

determine the age of sediment that isdetermine the age of sediment that is immediately above any portion of oceanic crust immediately above any portion of oceanic crust divide the distance from the spreading ridge by divide the distance from the spreading ridge by

the agethe age gives average rate of movement relative to the gives average rate of movement relative to the

ridgeridge LEAST ACCURATE METHODLEAST ACCURATE METHOD

Plate Movement Plate Movement MeasurementsMeasurements

Seafloor magnetic anomaliesSeafloor magnetic anomalies measure the distance of the magnetic anomaly measure the distance of the magnetic anomaly

in seafloor crust from the spreading ridge in seafloor crust from the spreading ridge divide by the age of the anomalydivide by the age of the anomaly

The present The present average average rate of rate of movement, movement, relative relative motion, and motion, and the average the average rate of rate of motion in motion in the past the past can be can be determined.determined.

Plate Position Plate Position ReconstructionReconstruction

Reconstructing plate positionsReconstructing plate positions to determine the plate and continent to determine the plate and continent

positions at the time of an anomalypositions at the time of an anomaly move the anomaly back to the spreading move the anomaly back to the spreading

ridgeridge Since Since

subduction subduction destroys destroys oceanic oceanic crustcrust

this kind of this kind of reconstructreconstruction cannot ion cannot be done be done earlier than earlier than the oldest the oldest oceanic oceanic crustcrust

Plate Movement Plate Movement MeasurementsMeasurements

Satellite-laser rangingSatellite-laser ranging bounce laser beams from a station on one plate bounce laser beams from a station on one plate off a satellite, to a station on another plateoff a satellite, to a station on another plate measure the elapsed timemeasure the elapsed time after sufficient time has passed to detect after sufficient time has passed to detect

motionmotion measure the elapsed time againmeasure the elapsed time again use the difference in elapsed times to calculate use the difference in elapsed times to calculate the rate of movement between the two platesthe rate of movement between the two plates

Hot spotsHot spots determine the age of rocks and their distance determine the age of rocks and their distance

from a hot spotfrom a hot spot divide the distance by the agedivide the distance by the age this gives the motion relative to the hot spot so this gives the motion relative to the hot spot so (possibly) the absolute motion of the plate(possibly) the absolute motion of the plate

Plate Movement at Hot Plate Movement at Hot SpotSpot

What Is the Driving What Is the Driving Mechanism of Plate Mechanism of Plate

Tectonics?Tectonics? Most geologists accept some type of Most geologists accept some type of

convective heat system convective heat system as the basic cause as the basic cause of plate motionof plate motion

In one possible In one possible model, model, thermal thermal

convection cells convection cells are restricted to are restricted to

the the asthenosphereasthenosphere


Tectonics?Tectonics? In a second model, the entire mantle In a second model, the entire mantle

is involved in thermal convection.is involved in thermal convection. In both models, In both models,

spreading ridges spreading ridges mark the rising mark the rising limbs of limbs of neighboring neighboring convection cells convection cells

trenches occur trenches occur where the where the convection cells convection cells descend back descend back into Earth’s into Earth’s interiorinterior


Tectonics?Tectonics? In addition to a thermal convection system, In addition to a thermal convection system, some geologists think that movement may be some geologists think that movement may be

aided byaided by ““slab-pull” slab-pull”

the slab is cold the slab is cold and dense and and dense and pulls the platepulls the plate

““ridge-push”ridge-push” rising magma rising magma

pushes the pushes the ridges up ridges up

and gravity and gravity pushes the pushes the oceanic oceanic lithosphere lithosphere away from the away from the ridge and ridge and toward the toward the trenchtrench

How Are Plate How Are Plate Tectonics and Tectonics and

Mountain Building Mountain Building Related?Related? An An orogenyorogeny is an episode is an episode

of intense rock deformation or mountain of intense rock deformation or mountain buildingbuilding

It results from compressive forces It results from compressive forces related to plate movementrelated to plate movement

During subduction, During subduction, sedimentary and volcanic rocks sedimentary and volcanic rocks are folded and faulted along the plate marginare folded and faulted along the plate margin

Most orogenies occur along oceanic-Most orogenies occur along oceanic-continental continental or continental-continental plate boundariesor continental-continental plate boundaries

How Are Plate How Are Plate Tectonics and Tectonics and

Mountain Building Mountain Building Related?Related?

Ophiolites are evidence of ancient Ophiolites are evidence of ancient convergent plate boundariesconvergent plate boundaries

The The Wilson CycleWilson Cycle describes the describes the relationship between mountain relationship between mountain building and the opening and closing building and the opening and closing of ocean basins.of ocean basins.

Terrane TectonicsTerrane Tectonics

TerranesTerranes differ from neighboring regions differ from neighboring regions in their fossil content, in their fossil content, stratigraphy, structural trends, stratigraphy, structural trends, and paleomagnetismand paleomagnetism

They probably formed elsewhere They probably formed elsewhere were carried great distances as parts of were carried great distances as parts of

other plates other plates until they collided with other terranes or until they collided with other terranes or

continentscontinents Numerous terranes have been identified Numerous terranes have been identified

in mountains of the North American in mountains of the North American Pacific coast region Pacific coast region

How Does Plate How Does Plate Tectonics Affect the Tectonics Affect the Distribution of Life?Distribution of Life?

Present distribution of plants and animals Present distribution of plants and animals is largely controlled by climate is largely controlled by climate and geographic barriersand geographic barriers

Barriers create Barriers create biotic provinces biotic provinces each province is a region characterized each province is a region characterized by a distinctive assemblage of plants and by a distinctive assemblage of plants and

animalsanimals Plate movements largely control barriersPlate movements largely control barriers

When continents break up, new provinces When continents break up, new provinces formform

When continents come together, fewer When continents come together, fewer provinces resultprovinces result

As continents move north or south they move As continents move north or south they move across temperature barriersacross temperature barriers

How Does Plate How Does Plate Tectonics Affect the Tectonics Affect the Distribution of Life?Distribution of Life?

Physical barriers Physical barriers caused by plate caused by plate movements includemovements include intraplate volcanoesintraplate volcanoes island arcsisland arcs mid-ocean ridgesmid-ocean ridges mountain rangesmountain ranges subduction zonessubduction zones

Example: Example: Isthmus of Isthmus of Panama Panama creates a creates a barrier to barrier to marine marine organismsorganisms

CaribbCaribbeanean

PacificPacific

Plate Tectonics and the Plate Tectonics and the Distribution of Natural Distribution of Natural

ResourcesResources Plate movements influence the formation Plate movements influence the formation

and distribution of some natural resources and distribution of some natural resources such assuch as petroleumpetroleum natural gasnatural gas some mineral depositssome mineral deposits

Metal resources related to igneous and Metal resources related to igneous and associated hydrothermal activity includeassociated hydrothermal activity include coppercopper goldgold leadlead

silversilver tin tin zinczinc


ResourcesResources Magma generated by subduction can Magma generated by subduction can

precipitate and concentrate metallic oresprecipitate and concentrate metallic ores Example: copper Example: copper

deposits in westerndeposits in westernAmericasAmericas

Bingham Mine in Utah Bingham Mine in Utah is a huge open-pit is a huge open-pit copper minecopper mine


ResourcesResources Another place where hydrothermal activity Another place where hydrothermal activity

can generate rich metal deposits can generate rich metal deposits is divergent boundariesis divergent boundaries

Example: island of Cyprus in the Example: island of Cyprus in the MediterraneanMediterranean Copper concentrations there formed as a result Copper concentrations there formed as a result of precipitation adjacent to hydrothermal vents of precipitation adjacent to hydrothermal vents along a divergent plate boundaryalong a divergent plate boundary

Example: Red SeaExample: Red Sea copper, gold, iron, lead, silver ,and zinc copper, gold, iron, lead, silver ,and zinc

deposits deposits are currently forming as sulfides in the Red are currently forming as sulfides in the Red

Sea, Sea, a divergent boundarya divergent boundary

http://www.google.com/url?http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&sourcsa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=10&ved=0CGkQFjAJ&url=he=web&cd=10&ved=0CGkQFjAJ&url=http%3A%2F%2Fwww.wvup.eduttp%3A%2F%2Fwww.wvup.edu%2Fecrisp%2FGeology%2Fecrisp%2FGeology%2520101%2520chapter2%2520Plate%2520101%2520chapter2%2520Plate%2520tectonics.ppt&ei=NTXfT73mEMT%2520tectonics.ppt&ei=NTXfT73mEMT66QGJlqC9Cw&usg=AFQjCNFt6R76qn_66QGJlqC9Cw&usg=AFQjCNFt6R76qn_UTLBgEZHtW6wzLw5BHQ&sig2=LbikSUTLBgEZHtW6wzLw5BHQ&sig2=LbikSmCWlm7usxoO1Kz37wmCWlm7usxoO1Kz37w

Plate Tectonics: A Unifying Theory Geology 101 Chapter 2.

Documents

Transcript of Plate Tectonics: A Unifying Theory Geology 101 Chapter 2.