Earth’s StructureEarth’s Structure

Origin of the EarthOrigin of the Earth

• Meteors and Asteroids Meteors and Asteroids bombarded the Earthbombarded the Earth

• Gravitational compressionGravitational compression

• Density Stratified planetDensity Stratified planet

Earth’s InteriorEarth’s Interior• CoreCore

– densedense– Iron and NickelIron and Nickel– Inner CoreInner Core - solid - solid– Outer CoreOuter Core - liquid - liquid

– Less dense than coreLess dense than core– Iron and Magnesium silicatesIron and Magnesium silicates– Mostly solidMostly solid– Upper mantle is partially moltenUpper mantle is partially molten

• MantleMantle

– Outermost layerOutermost layer– Very thin and rigidVery thin and rigid– ContinentalContinental – granite – granite

– Density = 2.8 g/cmDensity = 2.8 g/cm33

– OceanicOceanic – basalt – basalt– Density = 3.0 g/cmDensity = 3.0 g/cm33

• CrustCrust

Evidence of Internal StructureEvidence of Internal Structure

Granite• DensityDensity

– calculate density of Earthcalculate density of Earth– Speculate on probable compositionsSpeculate on probable compositions

Basalt

• MeteoritesMeteorites– Use composition and age to Use composition and age to

determine composition and determine composition and age of Earthage of Earth

• Seismic wavesSeismic waves– Travel times and direction Travel times and direction

give indication of internal give indication of internal structure of Earth structure of Earth

Types of Seismic WavesTypes of Seismic Waves

• S wavesS waves

• P wavesP waves– Primary wavesPrimary waves

– Push and pull movementPush and pull movement

– Travel fastest (Travel fastest (~ 6 km/sec)~ 6 km/sec)

– Travel thru solids and liquidsTravel thru solids and liquids

– Secondary wavesSecondary waves

– Move side-to-sideMove side-to-side

– Slower (~ 4 km/sec)Slower (~ 4 km/sec)

– Travel thru solids onlyTravel thru solids only

Seismic Waves Through EarthSeismic Waves Through Earth

LithosphereLithosphere

• Consists of continental, Consists of continental, oceanic and upper part oceanic and upper part of mantleof mantle

• Continents composed of Continents composed of granite-type rock, quartz and granite-type rock, quartz and feldspar minerals, feldspar minerals, density+2.8g/cmdensity+2.8g/cm33

• Oceanic crust formed of Oceanic crust formed of basalt; basalt rich in basalt; basalt rich in iron/magnesium minerals, iron/magnesium minerals, density+3.0 g/cmdensity+3.0 g/cm33

• Lithosphere is rigid layer of Lithosphere is rigid layer of crust and mantle overlying crust and mantle overlying partially-molten partially-molten asthenosphereasthenosphere

Continental Drift EvidenceContinental Drift Evidence

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

• Evidence supporting the idea that the continents had Evidence supporting the idea that the continents had drifteddrifted.

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

AmericaAmerica• Seuss, 1885, proposed super continent by studying Seuss, 1885, proposed super continent by studying

fossils, rocks, mountainsfossils, rocks, mountains

• Wegener and Taylor, early 1900’s, proposed Wegener and Taylor, early 1900’s, proposed continental drift and Pangaeacontinental 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

Continental Drift ModelContinental Drift ModelProblemsProblems

• Alfred WegenerAlfred Wegener

– Did not provide a Did not provide a plausible mechanism plausible mechanism to explain how to explain how continents driftedcontinents drifted

– Presented research Presented research to professionalsto professionals

Seafloor SpreadingSeafloor Spreading

•Continental drift reexamined in 1960’s with Continental drift reexamined in 1960’s with new informationnew information

– World World seismicityseismicity

• New theory developed – Seafloor spreadingNew theory developed – Seafloor spreading

• Supporting evidence for seafloor spreadingSupporting evidence for seafloor spreading

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

– VolcanismVolcanism– Age of seafloorAge of seafloor

– Heat flowHeat flow

– PaleomagnetismPaleomagnetism

Seafloor SpreadingSeafloor Spreading

• New sea floor created at the mid-ocean New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenchesridge and destroyed 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 Volcanoes match some plate boundaries; some are hot spotsare 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 When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity)Earth’s magnetic field (normal or reverse polarity)

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

Evidence for Seafloor SpreadingEvidence for Seafloor SpreadingPaleomagnetismPaleomagnetism

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

Seafloor SpreadingSeafloor SpreadingHeat FlowHeat Flow

Seafloor SpreadingSeafloor SpreadingConvection CurrentsConvection Currents

• In 1960, proposed as driving force to move In 1960, proposed as driving force to move continentscontinents

Theory of Plate TectonicsTheory of Plate Tectonics

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

Principles of Plate TectonicsPrinciples of Plate Tectonics

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

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

•Plate boundaries have high degree of Plate boundaries have high degree of tectonic activitytectonic activity– mountain buildingmountain building– earthquakesearthquakes– volcanoesvolcanoes

Plate BoundariesPlate BoundariesThree typesThree types

• DivergentDivergent• ConvergentConvergent• TransformTransform

Plate BoundariesPlate BoundariesDivergentDivergent

• Plates move away from each otherPlates move away from each other

• New crust is being formedNew crust is being formed

Divergent Plate BoundariesDivergent Plate BoundariesExamplesExamples

East African RiftEast African Rift Mid-Atlantic Ocean RidgeMid-Atlantic Ocean Ridge

Plate BoundariesPlate BoundariesConvergentConvergent

• Plates are moving toward each Plates are moving toward each otherother

Three Types:Three Types:

• Ocean-continentOcean-continent

• Ocean-oceanOcean-ocean

• Continent-continentContinent-continent

• Crust is being destroyedCrust is being destroyed

Convergent Plate Convergent Plate BoundariesBoundaries

ExamplesExamples 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 FaultSan Andreas Fault

Calexico, California

Carrizo Plains, Central California