Plate Tectonics—part 2

Alfred Wegener did Alfred Wegener did not know the earth’s not know the earth’s composition, but by composition, but by the 1960’s scientists the 1960’s scientists were able to revise were able to revise Wegener’s hypothesis Wegener’s hypothesis into a into a theorytheory now now known as Plate known as Plate Tectonics.Tectonics.

Post 1950’s evidence

Convection currentsConvection currents Mid-ocean ridge & trenchesMid-ocean ridge & trenches Age of continent rocks vs. seafloor rocksAge of continent rocks vs. seafloor rocks Earthquake/volcano and satellite dataEarthquake/volcano and satellite data

Directions and rates of plate motions

Plate Tectonics

The lithosphere is The lithosphere is composed of 7 major composed of 7 major and 13 minor plates.and 13 minor plates.

A plate can be A plate can be composed of composed of continental and/or continental and/or oceanic crust.oceanic crust.

Continental crustContinental crust--felsicfelsic

Oceanic crustOceanic crust-mafic-mafic

14.3 Differences Between Continental & Oceanic Lithosphere

OceanicOceanic – – younger (180 million years or less)younger (180 million years or less) more densemore dense ~7 km thick~7 km thick

ContinentalContinental – – older (4 billion years older (4 billion years

old)old) less denseless dense ~35-40 km thick~35-40 km thick

Plate Tectonics

The areas where plates The areas where plates are in contact with are in contact with each other are called each other are called plate boundariesplate boundaries..

There are There are threethree types types of plate boundaries.of plate boundaries.

Plate Boundaries (type 1)

DivergentDivergent-two plates are -two plates are moving away from each moving away from each other. New crust is formed. other. New crust is formed. MostMost divergent boundaries divergent boundaries are on the ocean floor. are on the ocean floor. These are marked by mid-These are marked by mid-ocean ridges.ocean ridges.

Tensional force is applied Tensional force is applied to rocks at a divergent to rocks at a divergent boundary. (tension is what boundary. (tension is what pulls them apart)pulls them apart)

Geographic Examples

The East African rift – a divergent boundary on land

Plate Boundaries (type 2) ConvergentConvergent- two plates are - two plates are

moving towards each other. moving towards each other. (which applies a (which applies a “compressional force”)“compressional force”) Oceanic/continentalOceanic/continental

oceanic crust has a higher oceanic crust has a higher density than Continental density than Continental crust, therefore it sinks crust, therefore it sinks below the continental below the continental crust. This is called a crust. This is called a subuction zonesubuction zone..

Volcanoes, mountains Volcanoes, mountains and trenches form.and trenches form.

Plate Boundaries (type 2)

Continental/continentalContinental/continental neither plate is neither plate is subducted. In this subducted. In this case, both edges are case, both edges are crumpled and uplifted crumpled and uplifted producing large producing large mountain ranges.mountain ranges.

Himalayan Himalayan MountainsMountains (India is (India is colliding with Asia)colliding with Asia)

Plate Boundaries (type 2)

Oceanic/oceanicOceanic/oceanic These are rare and These are rare and create deep create deep trenches.trenches.

Marianas TrenchMarianas Trench deepest trench in deepest trench in the world.the world.

Plate Boundaries (type 3)

Transform Fault Transform Fault BoundariesBoundaries- two plates - two plates grind past each other. grind past each other. (which applies a “shearing” (which applies a “shearing” force)force) They do not slide easily-They do not slide easily-

they stick and slip.they stick and slip. When these plates slip When these plates slip

they create earthquakes.they create earthquakes. San Andreas Fault in San Andreas Fault in

CaliforniaCalifornia

MOUNTAIN BUILDING

DeformationDeformation The bending, tilting The bending, tilting

and breaking of and breaking of Earth’s Crust. Earth’s Crust.

MOUNTAIN BUILDING

StressStress Force applied to rock material from Force applied to rock material from

Isostasy.Isostasy. StrainStrain

Change in shape or volume of rocks from Change in shape or volume of rocks from rocks being squeezed, twisted or pulled rocks being squeezed, twisted or pulled apart.apart.

MOUNTAIN BUILDING

FoldingFolding Permanent deformation or bending of a Permanent deformation or bending of a

rock under stress.rock under stress. FaultingFaulting

Break in rock along which rocks on Break in rock along which rocks on either side of the break have moved.either side of the break have moved.

MOUNTAIN BUILDING

MOUNTAIN BUILDING

FaultedFaulted Faulting breaks Earth’s Faulting breaks Earth’s

crust into large blocks crust into large blocks and those blocks and those blocks become tilted.become tilted.

Ex. Sierra Nevada Ex. Sierra Nevada MountainsMountains

MOUNTAIN BUILDING

FoldedFolded Plates collide Plates collide

(Convergent (Convergent boundary)boundary)

Ex. Appalachian Ex. Appalachian Mtns, HimalayasMtns, Himalayas

MOUNTAIN BUILDING DomeDome

Molten rock pushes up rock Molten rock pushes up rock layers on Earth’s surface creating layers on Earth’s surface creating a dome. Over time this top layer a dome. Over time this top layer erodes away leaving the igneous erodes away leaving the igneous rock exposed.rock exposed.

Ex. Black Hills, South DakotaEx. Black Hills, South Dakota

MOUNTAIN BUILDING

VolcanicVolcanic Molten rock that Molten rock that

has erupted onto has erupted onto Earth’s Surface.Earth’s Surface.

Cascade Mtns., Cascade Mtns., Mid-Atlantic Mid-Atlantic RidgeRidge