Plate Tectonics

Plate TectonicsPlate Tectonics The theory --the surface of the Earth is The theory --the surface of the Earth is

broken into large plates. The size and broken into large plates. The size and position of these plates change over position of these plates change over time. The edges of these plates, where time. The edges of these plates, where they move against each other, are they move against each other, are sites of intense geologic activity, such sites of intense geologic activity, such as earthquakes, volcanoes, and as earthquakes, volcanoes, and mountain building. mountain building.

Plate tectonics is a combination of two Plate tectonics is a combination of two earlier ideas, continental drift and sea-earlier ideas, continental drift and sea-floor spreading. Continental drift is the floor spreading. Continental drift is the movement of continents over the movement of continents over the Earth's surface and in their change in Earth's surface and in their change in position relative to each other. Sea-position relative to each other. Sea-floor spreading is the creation of new floor spreading is the creation of new oceanic crust at mid-ocean ridges and oceanic crust at mid-ocean ridges and movement of the crust away from the movement of the crust away from the

mid-ocean ridges.mid-ocean ridges. http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.htmlhttp://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.html

Earth StructureEarth Structure

Mexico quake-- magnitude at 7.0, and epicenter was in Mexico quake-- magnitude at 7.0, and epicenter was in the western Pacific state of Michoacan. Its depth was about the western Pacific state of Michoacan. Its depth was about 40 miles40 miles

Indonesia-- earthquake off coast of Indonesia Wednesday, Indonesia-- earthquake off coast of Indonesia Wednesday, April 11 with magnitude of 8.6, sparking tsunami warningsApril 11 with magnitude of 8.6, sparking tsunami warnings

LithosphereLithosphere

Consists of continental, Consists of continental, oceanic and upper part of oceanic and upper part of mantlemantle

Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3

Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3 Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere

Continental Drift EvidenceContinental Drift Evidence

– Geographic fit of continentsGeographic fit of continents– FossilsFossils– MountainsMountains– GlaciationGlaciation

Evidence supporting the idea that the continents had drifted.

Researchers noted geographic fit of continents– e.g. Africa and S. America– Atlantic formed by separation of Africa from S.

America Seuss, 1885, proposed super continent by studying fossils, rocks, mountains

Wegener and Taylor, early 1900’s, proposed continental drift and Pangaea

Continental Drift Continental Drift Geographic FitGeographic Fit

Continents seem to fit together like pieces of a puzzleContinents seem to fit together like pieces of a puzzle

Continental DriftContinental DriftFossilsFossils

Similar distribution of fossils such as the Similar distribution of fossils such as the MesosaurusMesosaurus

Continental DriftContinental DriftMountainsMountains

Mountain ranges match across oceansMountain ranges match across oceans

Continental DriftContinental DriftGlaciationGlaciation

Glacial ages and climate evidenceGlacial ages and climate evidence

ObjectionsObjectionsto theto the

Continental Drift ModelContinental Drift Model

Wegener did not Wegener did not provide a plausible provide a plausible mechanism to explain mechanism to explain how the continents how the continents could have drifted.could have drifted.

Seafloor SpreadingSeafloor Spreading

Continental drift reexamined in 1960’s with new Continental drift reexamined in 1960’s with new informationinformation

– World seismicity

New theory developed – Seafloor spreading Supporting evidence for seafloor spreading

Theory combining continental drift and seafloor spreading termed “Plate Tectonics”

– Volcanism– Age of seafloor

– Heat flow

– Paleomagnetism

Seafloor SpreadingSeafloor Spreading

New sea floor created at the mid-ocean ridge and New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenchesdestroyed in deep ocean trenches

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingWorld SeismicityWorld Seismicity

Earthquake distribution matches plate boundariesEarthquake distribution matches plate boundaries

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingVolcanismVolcanism

Volcanoes match some plate boundaries; some are hot spotsVolcanoes match some plate boundaries; some are hot spots

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingAge of SeafloorAge of Seafloor

Youngest sea floor is at mid-ocean ridgeYoungest sea floor is at mid-ocean ridge Oldest sea floor away from mid-ocean ridgeOldest sea floor away from mid-ocean ridge

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingPaleomagnetismPaleomagnetism

When rocks cool at the Earth’s surface, they record Earth’s When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity)magnetic field (normal or reverse polarity)

Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingPaleomagnetismPaleomagnetism

Paleomagnetic studies indicate alternating Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the stripes of normal and reverse polarity at the mid-ocean ridge.mid-ocean ridge.

Seafloor SpreadingSeafloor SpreadingHeat FlowHeat Flow

In 1960, convection currents were proposed as driving In 1960, convection currents were proposed as driving force to move continentsforce to move continents

Theory of Plate TectonicsTheory of Plate Tectonics

John Tuzo Wilson combined ideas of continental drift and John Tuzo Wilson combined ideas of continental drift and seafloor spreading into “Plate Tectonics”seafloor spreading into “Plate Tectonics”

Principles of Plate TectonicsPrinciples of Plate Tectonics

Earth’s outermost layer composed of thin rigid Earth’s outermost layer composed of thin rigid plates moving horizontallyplates moving horizontally

Plates interact with each other along their edges (plate boundaries)

Plate boundaries have a high degree of tectonic activity– mountain building– earthquakes– volcanoes

Plate BoundariesPlate BoundariesThree typesThree types

Plate BoundariesPlate BoundariesDivergentDivergent

Plates move away from each otherPlates move away from each otherNew crust is being formedNew crust is being formed

Divergent Plate BoundariesDivergent Plate BoundariesExamplesExamples

East African Rift Mid-Atlantic Ocean Ridge

Plate BoundariesPlate BoundariesConvergentConvergent

Plates are moving toward each otherPlates are moving toward each otherCrust is being destroyedCrust is being destroyed

Three Types:Three Types:

Ocean-continentOcean-continent

Ocean-oceanOcean-ocean

Continent-continentContinent-continent

Convergent Plate BoundariesConvergent Plate BoundariesExamplesExamples

Mount Lassen, California

Andes, South America

Mount Fuji, Japan

Plate BoundariesPlate BoundariesTransformTransform

Plates slide past one anotherPlates slide past one another Crust is neither created nor destroyedCrust is neither created nor destroyed

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Transform Plate BoundariesTransform Plate BoundariesExamplesExamples

San Andreas Fault

Calexico, California

Carrizo Plains, Central California

Stop HereStop Here

Origin of the EarthOrigin of the Earth

Meteors and Meteors and Asteroids bombarded Asteroids bombarded the Earththe Earth

Gravitational compressionGravitational compression

Density Stratified planetDensity Stratified planet

Earth’s InteriorEarth’s Interior

Crust Crust Outermost layerOutermost layer ContinentalContinental – granite-type, – granite-type,

density=2.8 g/cm density=2.8 g/cm OceanicOceanic – basalt, density=3.0 g/cm – basalt, density=3.0 g/cm Rigid Rigid

CoreCore– densedense– Nickel and IronNickel and Iron– Inner coreInner core – solid – solid– Outer coreOuter core – liquid – liquid

MantleMantle– Less dense than coreLess dense than core– Iron and Magnesium silicatesIron and Magnesium silicates– Partially moltenPartially molten

Evidence of Internal StructureEvidence of Internal Structure

Granite Density

– calculate density of Earth– Speculate on probable compositions

Basalt

Meteorites– Use composition and age to

determine composition and age of Earth

Seismic waves– Travel times and direction

give indication of internal structure of Earth

Types of Seismic WavesTypes of Seismic Waves

S wavesS waves– Secondary, shaking, side-to-sideSecondary, shaking, side-to-side– Slower (average 4 km/sec)Slower (average 4 km/sec)– Travel through solids onlyTravel through solids only

P waves– Primary, pressure, push and pull– Travel fastest of the seismic waves (average 6 km/sec)– Travels through solids and liquids

Seismic Waves Through EarthSeismic Waves Through Earth