Continental Drift. Basic Premise ► At one point in history all continents were combined in one big supercontinent ► For some reason the continent split.

Continental DriftContinental Drift

Basic PremiseBasic Premise

►At one point in history all continents At one point in history all continents were combined in one big were combined in one big supercontinentsupercontinent

►For some reason the continent split For some reason the continent split apart and the smaller land masses apart and the smaller land masses slowly drifted to there current slowly drifted to there current positionspositions

Early IdeaEarly Idea

►Continental Drift had been suggested Continental Drift had been suggested by numerous scientistsby numerous scientists

Edward Seuss (1800)Edward Seuss (1800) Frank Taylor (1910)Frank Taylor (1910) Alfred Wegner (1912)Alfred Wegner (1912) Alexander du Toit (1937)Alexander du Toit (1937)

What would make people think What would make people think this?this?

EvidenceEvidence

► Edward SeussEdward Seuss noted similarities between the Late Paleozoic noted similarities between the Late Paleozoic

plant fossils plant fossils GlossopterisGlossopteris flora flora and evidence and evidence for glaciation in the rock sequences of for glaciation in the rock sequences of ► IndiaIndia►AustraliaAustralia►South Africa South Africa ►South AmericaSouth America

He proposed the name He proposed the name GondwanalandGondwanaland Still couldn’t provide processStill couldn’t provide process

EvidenceEvidence

►Frank TaylorFrank Taylor►lateral movement of continents formed lateral movement of continents formed

mountain ranges a continent broke apart at mountain ranges a continent broke apart at the Mid-Atlantic Ridge to form the Atlantic the Mid-Atlantic Ridge to form the Atlantic Ocean.Ocean.

►Supposedly, tidal forces pulled formerly polar Supposedly, tidal forces pulled formerly polar continents toward the equator, when Earth continents toward the equator, when Earth captured the Moon about 100 million years captured the Moon about 100 million years agoago

Alfred WegenerAlfred Wegener

► Proposed that all landmasses were originally Proposed that all landmasses were originally united into a supercontinent united into a supercontinent He named the continent He named the continent PangaeaPangaea from the from the

Greek meaning “all land”Greek meaning “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

►Shorelines of continents fit togetherShorelines of continents fit together matching marine, nonmarine and glacial matching marine, nonmarine and glacial

rock sequences of Pennsylvanian to rock sequences of Pennsylvanian to Jurassic age for all five Gondwana Jurassic age for all five Gondwana continents including Antarcticacontinents including Antarctica

►Mountain ranges and glacial deposits Mountain ranges and glacial deposits match up when continents are united into match up when continents are united into

a single landmassa single landmass

The EvidenceThe Evidence

► Fossil EvidenceFossil Evidence

The EvidenceThe Evidence

► Fossil EvidenceFossil Evidence

The EvidenceThe Evidence

►Geologic EvidenceGeologic Evidence Mountain RangesMountain Ranges

The EvidenceThe Evidence

► Climatic EvidenceClimatic Evidence Glacial evidenceGlacial evidence

Additional Support for Additional Support for Continental DriftContinental Drift

►Alexander du ToitAlexander du Toit (South African (South African geologist, 1937) geologist, 1937) Proposed that a northern landmass he Proposed that a northern landmass he

called called LaurasiaLaurasia consisted of present-day consisted of present-day ►North AmericaNorth America►GreenlandGreenland►Europe Europe ►and Asia (except India).and Asia (except India).

Provided additional fossil evidence for Provided additional fossil evidence for Continental driftContinental drift

Still Problems?Still Problems?

►Most geologists did not accept the idea Most geologists did not accept the idea of moving continentsof moving continents No one could provide a suitable No one could provide a suitable

mechanism to explain how continents mechanism to explain how continents could move over Earth’s surfacecould move over Earth’s surface

Then WWIIThen WWII

► Interest in continental drift only Interest in continental drift only revived whenrevived when 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 younggeologically young

Earth’s Magnetic FieldEarth’s Magnetic Field

► Similar to a giant dipole Similar to a giant dipole magnetmagnet magnetic poles essentially magnetic poles essentially

coincide with the coincide with the geographic polesgeographic poles

Result from different Result from different rotation of outer core and rotation of outer core and mantlemantle

Strength and orientation of the Strength and orientation of the magnetic field variesmagnetic field varies

► inclination and strength increase inclination and strength increase from the equator to the polesfrom the equator to the poles weak and horizontal at the equatorweak and horizontal at the equator strong and vertical at the polesstrong and vertical at the poles

PaleomagnetismPaleomagnetism► PaleomagnetismPaleomagnetism is a remnant magnetism is a remnant magnetism

in ancient rocks in ancient rocks

►When magma cools below the When magma cools below the Curie Point, Curie Point, magnetic, iron-bearing minerals align with magnetic, iron-bearing minerals align with Earth’s magnetic field.Earth’s magnetic field.

Records the direction and strength of Earth’s Records the direction and strength of Earth’s magnetic field magnetic field

Records the direction of Earth’s magnetic poles Records the direction of Earth’s magnetic poles at the time of the rock’s formationat the time of the rock’s formation

Polar WanderingPolar Wandering

► In 1950s, research revealed In 1950s, research revealed that paleomagnetism of ancient rocks showed that paleomagnetism of ancient rocks showed

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

► Magnetic poles apparently moved.Magnetic poles apparently moved. Their trails were called Their trails were called polar wandering paths.polar wandering paths. Different continents had different paths.Different continents had different paths.

Polar Wandering PathsPolar Wandering Paths

The best explanation is stationary poles and moving continents

Magnetic ReversalsMagnetic Reversals

► Earth’s present magnetic field is called Earth’s present magnetic field is called normalnormal, , with magnetic north near the north geographic pole with magnetic north near the north geographic pole and magnetic south near the south geographic poleand magnetic south near the south geographic pole

► At various times in the past, Earth’s magnetic field At various times in the past, Earth’s magnetic field has completely has completely reversedreversed, , magnetic south near the north geographic pole magnetic south near the north geographic pole magnetic north near the south geographic polemagnetic north near the south geographic pole

► The condition for which Earth’s magnetic field is in this orientation The condition for which Earth’s magnetic field is in this orientation is called a is called a magnetic reversalmagnetic reversal

Magnetic ReversalsMagnetic Reversals

►Measuring paleomagnetism and Measuring paleomagnetism and dating continental lava flows dating continental lava flows lead to: lead to: the realization that magnetic the realization that magnetic

reversals existedreversals existed the establishment of a magnetic the establishment of a magnetic

reversal time scalereversal time scale

Mapping the OceansMapping the Oceans

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

worldworld

►The Mid-Atlantic RidgeThe Mid-Atlantic Ridge is the best knownis the best known and divides Atlantic Ocean basin and divides Atlantic Ocean basin in two nearly equal partsin two nearly equal parts

The Mid Atlantic RidgeThe Mid Atlantic Ridge

Sea Floor SpreadingSea Floor Spreading

► 1962, Harry Hess proposed the hypothesis 1962, Harry Hess proposed the hypothesis of of seafloor spreadingseafloor 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 cooling where new crust forms from upwelling and cooling magmamagma

►the new crust moves laterally away from the ridgethe new crust moves laterally away from the ridge the mechanism to drive seafloor spreading was the mechanism to drive seafloor spreading was

thermal convectionthermal convection cells in the mantle cells in the 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

Conformation for Hess Conformation for Hess (Finally…)(Finally…)

► In addition to mapping mid-ocean ridges, In addition to mapping mid-ocean 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 from is a deviation from the average strength of Earth’s Magnetic the average strength of Earth’s Magnetic fieldfield

Conformation for HessConformation for Hess

► The magnetic anomalies were discovered The magnetic anomalies were discovered to be striped ridges that are parallel and to be striped ridges that are parallel and symmetrical to the Oceanic Ridgesymmetrical to the Oceanic Ridge

Magnetism and Sea Floor Magnetism and Sea Floor SpreadingSpreading

Age of Oceanic CrustAge of Oceanic Crust

► Seafloor spreading theory indicates that Seafloor spreading theory indicates that oceanic crust is geologically young because oceanic crust is geologically young because it forms during spreading it forms during spreading and is destroyed during subductionand is destroyed during subduction

► Radiometric dating confirms the youth Radiometric dating confirms the youth of the oceanic crustof the oceanic crust and reveals that the youngest oceanic crust and reveals that the youngest oceanic crust occurs at mid-ocean ridges occurs at mid-ocean ridges and the oldest oceanic crust and 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

Plate Tectonics (the Unifying Plate Tectonics (the Unifying Theory)Theory)

►A A unifying theoryunifying theory is one that helps is one that helps explain a broad range of diverse explain a broad range of diverse

observations observations interpret many aspects of a science on a interpret many aspects of a science on a

grand scalegrand scale Relates many seemingly unrelated Relates many seemingly unrelated

phenomenaphenomena

►Plate tectonics is a unifying theory for Plate tectonics is a unifying theory for geology.geology.

Plate TectonicsPlate Tectonics

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

mountainsmountains location of location of

continents continents location of ocean location of ocean

basinsbasins► It influencesIt influences

atmospheric and oceanic circulation, and climateatmospheric and oceanic circulation, and climate geographic distribution, evolution and extinction of geographic distribution, evolution and extinction of

organismsorganisms distribution and formation of resourcesdistribution and formation of resources

The Theory of Plate TectonicsThe Theory of Plate Tectonics

► Plate tectonic theoryPlate tectonic theory is based on a simple model is based on a simple model the lithosphere is rigid a structurethe lithosphere is rigid a structure it consists of variable-sized pieces called it consists of variable-sized pieces called plates plates that that

move as a unitmove as a unit► Plates can be either Continental or OceanicPlates can be either Continental or Oceanic

Oceanic Plates consist of oceanic crust and upper mantleOceanic Plates consist of oceanic crust and upper mantle Continental Plates consist of continental crust and upper mantleContinental Plates consist of continental crust and upper mantle

Regions containing continental crust are up to 250 km Regions containing continental crust are up to 250 km thickthick

Regions containing oceanic crust are up to 100 km Regions containing oceanic crust are up to 100 km thickthick

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

How it all worksHow it all works

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

Movement of the asthenosphere results from some Movement of the asthenosphere results from some type of heat-transfer system within the type of heat-transfer system within the asthenosphere and causes the plates above to moveasthenosphere and causes the plates above to move

► As plates move over the asthenosphere they: As plates move over the asthenosphere they: Separate, mostly at oceanic ridgesSeparate, mostly at oceanic ridges Collide, in areas such as oceanic trenches where they Collide, in areas such as oceanic trenches where they

may be subducted back into the mantlemay be subducted back into the mantle Slide past each other along transform faultsSlide past each other along transform faults

Divergent Plate BoundariesDivergent Plate Boundaries► Divergent plate boundariesDivergent plate boundaries

occur where plates are separating and new oceanic occur where plates are separating and new oceanic lithosphere is forming.lithosphere is forming.

► Crust bulges due to magma, is extended thinned and Crust bulges due to magma, is extended thinned and fracturedfractured The magmaThe magma

► originates from partial melting of the mantleoriginates from partial melting of the mantle► is basaltic in compositionis basaltic in composition► intrudes into vertical fractures to form intrudes into vertical fractures to form dikesdikes► some rises to the surface and is extruded as lava flowssome rises to the surface and is extruded as lava flows

Divergent BoundariesDivergent Boundaries

► Successive injections of magma Successive injections of magma cool and solidify to form new oceanic crustcool and solidify to form new oceanic crust As magma cools it records the intensity and As magma cools it records the intensity and

orientation of Earth’s magnetic fieldorientation of Earth’s magnetic field

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

Ridges haveRidges have► rugged topography resulting from displacement of rocks along rugged topography resulting from displacement of rocks along

large fractureslarge fractures► shallow earthquakesshallow earthquakes

Features of Ridges (divergent Features of Ridges (divergent boundaries)boundaries)

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

Divergent BoundariesDivergent Boundaries

►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

when magma wells up the crust is initially elevated, stretched and thinned

RiftingRifting

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

► ExamplesExamples AfricaAfrica

EvidenceEvidence

► What features in the rock What features in the rock record can geologists use record can geologists use to recognize ancient to recognize ancient rifting?rifting?

► faultsfaults► dikesdikes► sillssills► lava flowslava flows► thick sedimentary thick sedimentary

sequences within rift sequences within rift valleysvalleys

Example:Example:► Triassic age fault basins in Triassic age fault basins in

eastern USeastern US

Convergent Plate BoundariesConvergent Plate Boundaries

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

► Where two plates collide, if at least one is Where two plates collide, if at least one is oceanic, oceanic, subductionsubduction occurs occurs During subduction, oceanic plate descends beneath During subduction, oceanic plate descends beneath

the margin of another platethe margin of another plate► the subducting plate moves into the the subducting plate moves into the asthenosphereasthenosphere is is

heated and is incorporated into the mantleheated and is incorporated into the mantle

Convergent BoundariesConvergent Boundaries

► Convergent boundaries are characterized by:Convergent boundaries are characterized by: deformation - folding and faultingdeformation - folding and faulting andesitic volcanism andesitic volcanism (except at continental collisions)(except at continental collisions) mountain buildingmountain building metamorphismmetamorphism earthquake activityearthquake activity important mineral depositsimportant mineral deposits

► Three types of Convergent boundaries Three types of Convergent boundaries oceanic-oceanicoceanic-oceanic oceanic-continentaloceanic-continental continental-continental continental-continental (continental collisions)(continental collisions)

Oceanic-OceanicOceanic-Oceanic►When two oceanic plates converge, one is When two oceanic plates converge, one is

subducted beneath the other along an oceanic-subducted beneath the other along an oceanic-oceanic plate boundaryoceanic plate boundary

an an oceanic trenchoceanic trench forms forms a a subduction complexsubduction complex forms forms

composed of slices of folded and faulted sediments and oceanic lithosphere scraped off the subducting plate

Volcanic ArcsVolcanic Arcs

► As the plate subducts into the mantle, it is heated As the plate subducts into the mantle, it is heated and partially melted generating magma of an and partially melted generating magma of an andesiticandesitic composition composition the magma rises to the surface because it is less the magma rises to the surface because it is less

dense than the surrounding mantle rocksdense than the surrounding mantle rocks At the surface of the non-subducting plate, the At the surface of the non-subducting plate, the

magma forms a magma forms a volcanic island arcvolcanic island arc

Back-arc basinBack-arc basin

► A A back-arc basinback-arc basin forms in some cases of forms in some cases of fast subduction when the lithosphere on fast subduction when the lithosphere on the landward side of the island arc is the landward side of the island arc is stretched and thinned stretched and thinned

Oceanic-ContinentalOceanic-Continental► An An oceanic-continental plate boundary oceanic-continental plate boundary occurs when occurs when

a denser oceanic plate subducts under less dense a denser oceanic plate subducts under less dense continental lithospherecontinental lithosphere

► Magma generated by subduction Magma generated by subduction rises into the continental crust to form large igneous bodiesrises into the continental crust to form large igneous bodies or erupts to form a or erupts to form a volcanic arcvolcanic arc of andesitic volcanoes of andesitic volcanoes

► Example: Pacific coast of South America (Andes Mountains, Peru)Example: Pacific coast of South America (Andes Mountains, Peru)

Continental-ContinentalContinental-Continental► Two approaching continents are initially separated by ocean floor Two approaching continents are initially separated by ocean floor

that is being subducted under one of them, which, thus, has a that is being subducted under one of them, which, thus, has a volcanic arcvolcanic arc

► When the 2 continents collide When the 2 continents collide Density of the plates are equal so no subduction occurs, though Density of the plates are equal so no subduction occurs, though

one may wedge beneath the otherone may wedge beneath the other

► The plates are The plates are weldedwelded together at a continent-continent plate together at a continent-continent plate boundary, boundary, along the site of former subduction an interior mountain belt along the site of former subduction an interior mountain belt

forms consisting offorms consisting of► deformed sedimentary rocksdeformed sedimentary rocks► igneous intrusionsigneous intrusions► metamorphic rocksmetamorphic rocks► fragments of oceanic crustfragments of oceanic crust

Continental-ContinentalContinental-Continental

Identifying Convergent Identifying Convergent BoundariesBoundaries

Andesitic magma erupted, Andesitic magma erupted, ► forming island arc volcanoes and continental volcanoesforming island arc volcanoes and continental volcanoes

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

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

OhiolitesOhiolites

►Ophiolites consist of Ophiolites consist of layers layers representing parts of representing parts of

the oceanic crust the oceanic crust and upper mantle.and upper mantle.

► The sediments includeThe sediments include graywackegraywacke black shaleblack shale chertchert

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

Transform BoundariesTransform Boundaries► Occur where plates slide Occur where plates slide

laterally past each otherlaterally past each other roughly parallel to the roughly parallel to the

direction of plate movementdirection of plate movement

► Movement results inMovement results in zone of intensely shattered zone of intensely shattered

rockrock numerous shallow numerous shallow

earthquakesearthquakes

► The majority of transform faults The majority of transform faults connect two oceanic ridge connect two oceanic ridge

segmentssegments and are at fracture zonesand are at fracture zones

Hot SpotsHot Spots

► Hot spotsHot spots are locations where stationary columns of magma, are locations where stationary columns of magma, originating deep within the mantle, called originating deep within the mantle, called mantle plumes, mantle plumes, slowly rise to the surface.slowly rise to the surface.

► Mantle plumes remain stationaryMantle plumes remain stationary► although some evidence suggests they may move somewhatalthough some evidence suggests they may move somewhat

► When plates move over them, hot spots leave trails of extinct When plates move over them, hot spots leave trails of extinct progressively older volcanoes called progressively older volcanoes called aseismic ridges aseismic ridges which which record the movement of the platesrecord the movement of the plates

The Mechanism…The Mechanism…

► Most geologists accept Most geologists accept some type of convective some type of convective heat system as the basic heat system as the basic cause of plate motioncause of plate motion

► In one possible model, In one possible model, thermal convection cells thermal convection cells are restricted to the are restricted to the asthenosphereasthenosphere

The MechanismThe Mechanism

► In a second model, the In a second model, the entire mantle is involved entire mantle is involved in thermal convection.in thermal convection.

► In both models, In both models, spreading ridges mark the spreading ridges mark the

rising limbs of neighboring rising limbs of neighboring convection cells convection cells

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

The MechanismThe Mechanism

► In addition to thermal In addition to thermal convection cells, some convection cells, some geologists think that geologists think that movement may be aided movement may be aided byby ““slab-pull” slab-pull”

► the slab is cold and dense the slab is cold and dense and pulls the plateand pulls the plate

““ridge-push”ridge-push”► rising magma pushes the rising magma pushes the

ridges up ridges up ► and gravity pushes the and gravity pushes the

ocean floor toward the ocean floor toward the trenchtrench

Plate Tectonics and LifePlate Tectonics and Life

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

► Barriers create Barriers create biotic provinces biotic provinces each province is a region characterized by a distinctive each province is a region characterized by a distinctive

assemblage of plants and animalsassemblage of plants and animals

► Plate movements largely control barriersPlate movements largely control barriers When continents break up, new provinces formWhen continents break up, new provinces form When continents come together, fewer provinces resultWhen continents come together, fewer provinces result As continents move north or south they move across As continents move north or south they move across

temperature barrierstemperature barriers