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Plate Tectonics

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New Evidence For Wegener’s Theoryof Continental Drift

Wegener’s theory of continental drift was out of favor with the scientificcommunity until new technology provided evidence to support hisideas.

Seismographs revealed a pattern of volcanoes andearthquakes

Radiometric dating of rocks revealed a surprisinglyyoung oceanic crust.

Echo sounders revealed the shape of the Mid-AtlanticRidge and other features on the sea floor.

Locations ofshallow anddeep focusearthquakes

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Benioff Zone is an area of increasingly deeperseismic activity, inclined from the trenchdownward in the direction of the island arc.

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Rock typesand glacialfeaturesmatch upacross thecontinents.

Seafloor spreading was anidea proposed in 1960 toexplain the features of theocean floor. It explained thedevelopment of the seafloorat the Mid-Atlantic Ridge.Convection currentsin the mantle wereproposed as the force thatcaused the ocean to growand the continents to move.

The Mid-Atlantic Ridgeconforms to the shape ofthe adjacent continents.The inset shows the centralrift.

Seafloor Spreading - A Key Idea

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Sea-Floor Spreading

Provides a mechanism for continentaldrift

Sea floor moves away from oceanic ridgeat rate of 1 - 6 cm/yr

Plunges beneath continent or island arc -subduction

Driving force may be mantle convection

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Paleomagnetism

Magnetic minerals align to magnetic field whenrocks solidify

Can determine orientation of magnetic field inpast by measuring magnetism “locked” in rocks

Orientation and dip of past magnetic fields giveevidence of (apparent) polar wander

Different continents give different polarwander paths

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Paleomagnetism: strips of alternating magnetic polarity at spreading regions.

Confirmation of the Theory of PlateTectonics

The patterns of paleomagnetism support plate tectonic theory. The moltenrocks at the spreading center take on the polarity of the planet while they arecooling. When Earth’s polarity reverses, the polarity of newly formed rockchanges.

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The ideas of continental drift and seafloor spreading were tied together inthe theory of plate tectonics. Main points of the theory include:

• Earth’s outer layer is divided into lithosphericplates

• Earth’s plates “float” on the asthenosphere

• Plate movement is powered by convectioncurrents in the asthenosphere, and thedownward pull of a descending plate’s leadingedge.

Tying It All Together - The TheoryOf Plate Tectonics

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Evidence for Plate Motion

Magnetic anomalies - predict rate ofmotion and sea floor ages

Seismicity along transform faults Direct measurement

The Major Lithospheric Plates

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Types of Plate Boundaries Divergent Transform Convergent

– Ocean-ocean– Ocean-continent– Continent-continent

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The lithospheric plates interact with the neighboring plates in severalways.

Divergent plate boundaries – Boundaries between plates movingapart, further classified as:

Divergent oceanic crust – for example, the Mid-Atlantic Ridge

Divergent continental crust - for example, the Rift Valley of EastAfrica.

Plate Boundaries

Extension of divergentboundaries causes splittingand rifting.

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Divergent Boundary

Mid-oceanic ridge develops Oceanic ridge - site of formation of new

oceanic crust Initial rifting, extension, faulting Creates new ocean basin Passive continental margins face ridge;

marine sediment covers rift zone edges

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Transform Boundaries

Plates slide past each otherUsually occur at oceanic ridgeCauses offset of ridgesMay intersect a continent

San Andreas Fault

Transform plate boundaries - locations where crustal plates movepast one another, for example, the San Andreas fault.

Plate Boundaries

Translation at transform boundaries causes shear.

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Convergent Plate Boundaries - Regions where plates are pushingtogether can be further classified as:

Oceanic crust toward continental crust - for example, the west coast ofSouth America

Oceanic crust toward oceanic crust - occurring in the northern Pacific

Continental crust toward continental crust – one example is theHimalayas

Plate Boundaries

Compressionat convergentboundariesproducesbuckling andshortening.

Convergent: Ocean-Ocean

Trench and volcanic arc form - convex toward subducting plate

Benioff Zone of earthquakesAndesitic volcanism in volcanic arcAccretionary wedge of ocean sediment

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Ocean-Continent Convergence

Active continental marginSubduction of oceanic lithosphere beneath

continental lithosphere - trenchMagmatic arc and intrusions - andesitic

volcanoes on the continentBenioff Zone

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Convergent: Continent-Continent

Two continents approach each otherOcean disappearsContinents collide, neither are subductedCrust is thickenedForms mountain belt in continental interior

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Motion of Plate Boundaries

Boundaries move as well as platesRidge crests may jump to a new positionConvergent boundaries can migrate or

jumpTransform boundaries can shift

What Causes Plate Motions

Convection in mantleDeep mantle vs. two layer convection

Convection due to plate motionRidge push; slab pull; trench suction

Mantle plumes and Hot SpotsHawaii and Yellowstone volcanism

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Hot Spots: Surface expression of plumes of magma.

Hot Spots

(A volcanic island chain can form when a plate passes over a hot spotand a stationary mantle plume.

Atolls and Guyots: Coral formations and submerged volcanic mountains.

Guyots were once volcanic peaks above sea level. They were eroded bywave action as they sank beneath the surface of the water.

Atolls and Guyots

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History of Continental Positions

Pangaea split up about 200 m.y. agoNew ocean formed - Atlantic OceanPacific Ocean is becoming smaller as

Atlantic Ocean becomes largerContinents have been in motion for at least

2 b.y.

Plate Tectonics Explains:

Ocean Ridge System - high heat flow, basalt eruptions, rift valley, shallow focus earthquakes

Oceanic Trenches - low heat flow, deepfocus earthquakes, andesitic volcanism

Age of the Sea Floor - youngest at ridge,oldest at trench; no sediments at ridgecrest

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Wilson Cycle refers to the sequence of events leadingto the formation, expansion, contracting and eventualelimination of ocean basins.

Stages in basin history are:Embryonic - rift valley forms as continent begins to split.Juvenile - sea floor basalts begin forming as continental

fragments diverge.Mature - broad ocean basin widens, trenches eventually

develop and subduction begins.Declining - subduction eliminates much of sea floor and

oceanic ridge.Terminal - last of the sea floor is eliminated and

continents collide forming a continental mountainchain.

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