Continental Drift & Plate Continental Drift & Plate TectonicsTectonics

Chapter 3

Continental drift - observationsContinental drift - observations 1620, Francis Bacon noticed that S America 1620, Francis Bacon noticed that S America

and Africa appear to fit together and and Africa appear to fit together and proposed that they had once been joined proposed that they had once been joined (as were Europe and Africa)(as were Europe and Africa)

1885, Edward Suess, S American and Africa 1885, Edward Suess, S American and Africa fit and similarities in fossils found on these fit and similarities in fossils found on these continentscontinents

Fit got even better if continental shelves Fit got even better if continental shelves were included.were included.

No mechanismsNo mechanisms

Evidence that continents were Evidence that continents were togethertogether

Fit of continentsFit of continents EcosystemsEcosystems

– Rock weathering (e.g., glacial weathering in Rock weathering (e.g., glacial weathering in current tropics)current tropics)

– Fossils (similar across continents, coal in Fossils (similar across continents, coal in Antarctica)Antarctica)

Continental drift proposedContinental drift proposed In 1910, Alfred Wegner proposed the idea of continental drift. In 1910, Alfred Wegner proposed the idea of continental drift.

Proposed a super-continent, Pangaea, existed 225 MY ago Proposed a super-continent, Pangaea, existed 225 MY ago (about the time of dinosaurs in the Cretaceous period), and (about the time of dinosaurs in the Cretaceous period), and that continental movements caused friction resulting in that continental movements caused friction resulting in volcanic activity.volcanic activity.– N part was Laurasia, S part was Gondwanaland, partially separated N part was Laurasia, S part was Gondwanaland, partially separated

by the Tethys Seaby the Tethys Sea Proposed mechanism – centrifugal effect from earth’s Proposed mechanism – centrifugal effect from earth’s

spinning and tidal drag from combined effects of sun and spinning and tidal drag from combined effects of sun and moon.moon.

Hang-up was the mechanism.Hang-up was the mechanism. Idea largely discounted.Idea largely discounted. Lack of understanding of the mantle and isostatic support of Lack of understanding of the mantle and isostatic support of

crustal material.crustal material.

Post-WWIIPost-WWII

New instruments make new observations New instruments make new observations possible. Lines of evidencepossible. Lines of evidence– SeismologySeismology– Volcanoes, hot spotsVolcanoes, hot spots– Sediment distributionSediment distribution– Heat flow patterns from earthHeat flow patterns from earth– MagnetismMagnetism

Additional observationsAdditional observations

Better understanding of earth’s structure – Better understanding of earth’s structure – seismologyseismology

Charts of earthquake and volcanic activity (Pacific Charts of earthquake and volcanic activity (Pacific Ring of Fire)Ring of Fire)

1925, mid-Atlantic ridge was mapped1925, mid-Atlantic ridge was mapped Seismological evidence of deformable, non-rigid Seismological evidence of deformable, non-rigid

upper mantle (asthenosphere)upper mantle (asthenosphere) Post WWII, Radiometric dating of ocean crust Post WWII, Radiometric dating of ocean crust

(max. age of 200 MY old! Young relative to (max. age of 200 MY old! Young relative to continental crust more than 4 BY old)continental crust more than 4 BY old)

Seismic events worldwide between 1977-1986.

Seismic activitySeismic activity

Earthquakes and volcanoes near trenchesEarthquakes and volcanoes near trenches Earthquakes at mid-ocean ridgesEarthquakes at mid-ocean ridges

– Plus transform faulting because of rotational stressPlus transform faulting because of rotational stress Focus of earthquakes is below the surface in the Focus of earthquakes is below the surface in the

crust or mantle; epicenter is the site on the earth crust or mantle; epicenter is the site on the earth surface above the focussurface above the focus– Shallow focus - < 75 km (e.g., transform quakes)Shallow focus - < 75 km (e.g., transform quakes)– Deep focus - > 300 km (e.g., trenches)Deep focus - > 300 km (e.g., trenches)

Plate formation and destruction explains Plate formation and destruction explains worldwide seismic distributionworldwide seismic distribution

More observationsMore observations

Detailed mapping of ocean floor crust and Detailed mapping of ocean floor crust and sediments – post WWII, echo soundingsediments – post WWII, echo sounding– Ridge conformed to coastsRidge conformed to coasts– Sediments thicker near continents and thicker Sediments thicker near continents and thicker

near mid-ocean ridges.near mid-ocean ridges. Mantle studies – International Geophysical Mantle studies – International Geophysical

Year in 1957.Year in 1957. Lithosphere isostatically balanced on Lithosphere isostatically balanced on

partially melted upper layerpartially melted upper layer

Age of rocks and sedimentsAge of rocks and sediments

Rocks increase in age with increasing Rocks increase in age with increasing distance from the mid-ocean ridgedistance from the mid-ocean ridge

Sediment cover increases with distance Sediment cover increases with distance from the ridge (older rocks exposed to from the ridge (older rocks exposed to sediment rain for longer periods) – this isn’t sediment rain for longer periods) – this isn’t foolproof because of sedimentation from foolproof because of sedimentation from land and movement of sediments.land and movement of sediments.

HotspotsHotspots

Sources of magma other than those that drive the Sources of magma other than those that drive the main convection cellsmain convection cells

Sources static while lithosphere moves over themSources static while lithosphere moves over them Erupt as volcanoes for awhile and form chains Erupt as volcanoes for awhile and form chains

(e.g., Hawaiian Islands)(e.g., Hawaiian Islands) Youngest mountain in the chain is closest to the Youngest mountain in the chain is closest to the

hot spot (big island, Hawaii, most active)hot spot (big island, Hawaii, most active) Emperor Seamount chain formed from same hot Emperor Seamount chain formed from same hot

spot long agospot long ago

Seafloor spreadingSeafloor spreading 1960 Harry Hess and Robert Dietz proposed seafloor spreading1960 Harry Hess and Robert Dietz proposed seafloor spreading Mechanism – conveyor belts of oceanic crust moving up at center and Mechanism – conveyor belts of oceanic crust moving up at center and

down at edges; analogous to a convection cell in waterdown at edges; analogous to a convection cell in water New crust formed at Mid-Atlantic Ridge (hot and less dense) and New crust formed at Mid-Atlantic Ridge (hot and less dense) and

spreads outward (cools, shrinks and collects sediments)spreads outward (cools, shrinks and collects sediments) Continents carried alongContinents carried along

Mid-Atlantic Ridge

Mass balanceMass balance

Earth either expanding or there isEarth either expanding or there is Consumption of crustConsumption of crust Wadati-Benioff zones – subduction zones Wadati-Benioff zones – subduction zones

where crust is being consumedwhere crust is being consumed– Oceanic trenches – continental crust less dense Oceanic trenches – continental crust less dense

and floats on oceanic crust which dives downand floats on oceanic crust which dives down– Violent so, very seismically activeViolent so, very seismically active– No direct exchange of mantle material but a No direct exchange of mantle material but a

window into mantle compositionwindow into mantle composition

MagnetismMagnetism Earth behaves like there’s a giant magnet inside.Earth behaves like there’s a giant magnet inside.

– Magnetic N pole in Hudson BayMagnetic N pole in Hudson Bay– Magnetic S pole opposite in the PacificMagnetic S pole opposite in the Pacific

Outer liquid core (Ni, Fe) rotating around inner Outer liquid core (Ni, Fe) rotating around inner solid core. Rotation calculated at 1 mm/sec (90 solid core. Rotation calculated at 1 mm/sec (90 m/d)m/d)

When magnetic materials melt and then solidify in When magnetic materials melt and then solidify in the presence of a magnetic field, solid material the presence of a magnetic field, solid material lines up with prevailing field.lines up with prevailing field.

Dating of magnetized rocks (radiometric dating) Dating of magnetized rocks (radiometric dating) reveals that rocks of different ages sometimes reveals that rocks of different ages sometimes have opposite magnetic orientationhave opposite magnetic orientation

Magnetism (continued)Magnetism (continued)

Earth’s magnetic field must have reversed in Earth’s magnetic field must have reversed in the pastthe past

Causes are unknownCauses are unknown History of magnetic polarity and reversals History of magnetic polarity and reversals

recorded in volcanic (basaltic) rocksrecorded in volcanic (basaltic) rocks Ocean crust is high in basaltOcean crust is high in basalt Magnetometers towed across ocean floor Magnetometers towed across ocean floor

revealed large scale pattern of alternating revealed large scale pattern of alternating polarity parallel to mid-ocean ridge (stripes!)polarity parallel to mid-ocean ridge (stripes!)

If pole moved and not the continents(would have to have been in different places at the same time)

If continents moved

Magnetic stripesMagnetic stripes

Not evenly spacedNot evenly spaced Stripes used to calculate rates of seafloor Stripes used to calculate rates of seafloor

spreadingspreading– E.g., rocks 1000 km from ridge are 50 MY oldE.g., rocks 1000 km from ridge are 50 MY old

1000km/50,000,000 = 2 cm/year1000km/50,000,000 = 2 cm/year

Magnetism asideMagnetism aside Reversals take many years as field collapse and Reversals take many years as field collapse and

reorientreorient May allow cosmic radiation to penetrate normal May allow cosmic radiation to penetrate normal

magnetic field and cause changes in surface living magnetic field and cause changes in surface living organisms during these periods (fossil records?)organisms during these periods (fossil records?)

Magnetosphere protects us from solar wind (radiation)Magnetosphere protects us from solar wind (radiation) Cause of reversals is unknownCause of reversals is unknown Current polarity has lasted 730,000 yrCurrent polarity has lasted 730,000 yr 170 reversals in last 76 MY170 reversals in last 76 MY Decrease in field of 7% in the last 160 yr which would Decrease in field of 7% in the last 160 yr which would

predict a reversal in 1500-2000 years!predict a reversal in 1500-2000 years!

Seafloor spreadingSeafloor spreading

1963 – Vine and Matthews proposed that magnetic 1963 – Vine and Matthews proposed that magnetic patterns were created by seafloor spreadingpatterns were created by seafloor spreading– New ocean floor formed by vulcanism at mid-ocean ridgeNew ocean floor formed by vulcanism at mid-ocean ridge– Rock solidifies and takes on polarity of prevailing fieldRock solidifies and takes on polarity of prevailing field– Matching patterns on each side of spreading centerMatching patterns on each side of spreading center– Stripes of alternating polarity with increasing age and Stripes of alternating polarity with increasing age and

distance from the ridge axisdistance from the ridge axis– Reversal stripe in sea floor correlated with age-dated Reversal stripe in sea floor correlated with age-dated

reversals on landreversals on land– Youngest rocks at mid-ocean ridges and oldest at Youngest rocks at mid-ocean ridges and oldest at

margins and on continentsmargins and on continents

Plate tectonicsPlate tectonics

1965 – John Wilson. Lithospheric plates floating 1965 – John Wilson. Lithospheric plates floating on asthenosphereon asthenosphere– Plates diverge when heated mantle (asthenosphere) Plates diverge when heated mantle (asthenosphere)

becomes less dense & rises upbecomes less dense & rises up– Plates converge where cool, dense crustal rocks collide Plates converge where cool, dense crustal rocks collide

and are pulled downand are pulled down 1966-1967 Debate1966-1967 Debate 1968 1968 Glomar ChallengerGlomar Challenger drilled deep-crustal cores drilled deep-crustal cores Reexamination of scientific disciplines – e.g., Reexamination of scientific disciplines – e.g.,

similar fossils on different continents, coal in similar fossils on different continents, coal in AntarcticaAntarctica

Plate movementPlate movement Plate movement is slow about 5 cm/yr (2 Plate movement is slow about 5 cm/yr (2

inches) & powered by mantle’s heatinches) & powered by mantle’s heat Crust is formed and consumedCrust is formed and consumed Plate boundariesPlate boundaries

– ConvergenceConvergence– DivergenceDivergence– TransformTransform

Seven major lithospheric platesEach have continental or continental plus ocean crust

Ridges, faults and trenches form boundaries

Some highlightsSome highlights

Lithospheric plates float on asthenosphereLithospheric plates float on asthenosphere Asthenosphere is where convection cells Asthenosphere is where convection cells

occuroccur Lithospheric plates move past each other Lithospheric plates move past each other

but are not created and destroyed as is but are not created and destroyed as is oceanic crustoceanic crust

Wegner’s date for Pangaea not so bad!

The Earth: # of Millions of Years Ago

0

550

220

190

136

65

Grand Canyon

Plate boundaries

Major features of platesMajor features of plates

Why no trench on the west coast of N Why no trench on the west coast of N America? Probably used to be and then America? Probably used to be and then Pacific plate changed directionPacific plate changed direction

Current spreading has lasted about 200 MY Current spreading has lasted about 200 MY but, earth is older so previous history but, earth is older so previous history depends on fossils & rocks on landdepends on fossils & rocks on land

Take home pointsTake home points Lithosphere versus asthenosphere also the MohoLithosphere versus asthenosphere also the Moho Continental drift, seafloor spreading and plate tectonicsContinental drift, seafloor spreading and plate tectonics Continental drift – fit of continents & fossil evidenceContinental drift – fit of continents & fossil evidence Seafloor spreading – theory supported by several lines Seafloor spreading – theory supported by several lines

of evidence made possible by post WWII technology of evidence made possible by post WWII technology (magnetism, seismology, sedimentology, heat flow, (magnetism, seismology, sedimentology, heat flow, distribution of volcanoes and hot spots)distribution of volcanoes and hot spots)

Plate tectonics – provided a mechanismPlate tectonics – provided a mechanism Types and features of plate boundariesTypes and features of plate boundaries