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THE EARTHS INTERIOR

Seismic Waves: ReflectionRefraction

Divisions of the EarthIsostasyGravity

Magnetic FieldHeat Flow Origin of HeatPlanetary Comparisons

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Study of the Earths Interior = GEOPHYSICS (usesseismic waves, gravity, magnetism, heat flow).

THE EARTHS INTERIOR

SEISMIC WAVES :Provide information ondivisions of the Earth.

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Seismic Reflection : defines boundaries betweenlayers of different densities.

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Seismic Refraction : bending of seismic waves due tochanges in density.

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Note : increasing densitywith depth causesrefraction even withouta boundary.

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Seismic waves define 3basic divisions of the

Earth

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Crust : outer layer or scum that floats.

Lithosphere : Crust and upper mantle.

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Box 17.1.1

Continental Drilling

Southern Germany: 10 kmKola Peninsula, Russia: 12 km

Both = lots of circulating fluids.

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MANTLE : ~2,900 km thick.

Low Velocity Zone : (~100 km)separates lithosphere from

Asthenosphere (100-250 km thick).

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Mesosphere : solid, below asthenosphere . Higher velocities due tohigher pressure, which produces higher pressure (more closely

packed) mineral phases.

Base of outer core ~5,150 km.

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Mantle Minerals :Mg,Fe silicates (olivine,

pyroxenes, garnet,spinel).

2 mm

Garnet Peridotite

Higher densitythan crust seismic waves movefaster.

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P-wave shadow due to refraction between liquid outercore and the mantle above and solid inner core below.

Crust ~2.7-3.0 g cm3

Mantle ~3.3-3.5 g/cm 3.

Outer core ~10 g/cm 3;

Inner Core ~12-13 g/cm3

.

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CORE : Liquid outer core S-waves do not propagatethrough.

Composition : Fe + minor Ni .

Evidence :

Earths density~5.5 g/cm 3 (speedof rotation).

Magnetic fieldrequires Fe.

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Further Evidence of Core Composition:

Meteorites iron meteorites

(Fe,Ni metal) = fragmentsof destroyed planets?

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Isostasy

Concept lighter, less dense continental crust floatshigher on the mantle than the denser oceanic crust(equilibrium).

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Isostatic readjustment inmountain belts.

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Isostatic readjustment also due to de-glaciation .

Crustal Rebound when ice is removed and the crust

continues to rise.

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Crustal Rebound

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Isostatic readjustment also due to magmatic underplating

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GRAVITY Force of gravity increases between 2 objects with an increase inmass of either one.

Use a gravity meter to explore local variations in rock density:(mass = density x volume).

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Dense rock =metal ores used in

exploration.

Less dense = cavitiesor sediment (i.e., saltdomes) used in oil

exploration.

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Also use gravity to explore isostatic equilibrium.

If gravity meter is flown across amountain range with no change, it is inisostatic equilibrium, because the

mountain range has a deep root.

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Uplift may be due to mantle(dense rock) upwelling gives abroad regional positive anomaly.

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The largest negative gravity anomaliesare found over ocean trenches out ofisostatic equilibrium.

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Magnetic Field Near rotational poles, 11.5

inclination.Polar wander through geologictime or do they?

Intensity varies with time.

Igneous rocks record direction +intensity at time of formation. Look atsequence of lava flows find N-S

reversals.

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Magnetic FieldIgneous rocks record direction + intensity at time of formation.Look at sequence of lava flows find N-S reversals.

Magnetism frozen in when lava drops below Curie Point this is 580C for magnetite.

Science of Paleomagnetism .

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Magnetic reversals recordedin ocean flow it is like a tape

recorder. Record back to lateJurassic.

Use magnetometer tomeasure strength of theEarths magnetic field.

Rocks differ in magnetismdependent upon their contentof Fe-bearing minerals,particularly magnetite .

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Positive Magnetic Anomaly : magnetic field strength above theavera e vice versa for a ne ative anomal .

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Dependent upon rocks & structure.

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DynamoTheory : rotationof solid mantle

around liquidouter corecauses slippage& sets upcurrent.

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HEAT FLOWGeothermal gradient: ~25C/km at surface and changes

with depth

Base of crust: ~800- 1,200C

Core-mantle boundary: ~3,500- 5,000C

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HEAT FLOWHeat escaping (volcanoes,

hot springs, geysers etc.).

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Thinner insulating crust over oceans is offset by moreradioactive elements in continental crust.

f

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Origin of Heat Initial Cooling of Earth dissipation of accretionaryheat (kinetic energy).

Conversion of gravitational energy to heat(convective overturn, differentiation = frictionalheating).

Deceleration of the Earth (tidal friction, internalfriction or inertia).

Radioactive Decay ( 238 U, 235 U, 232 Th, 87Rb, 40K).Effect varies with time as the amount of radioactiveelements decreases. Important for age-dating of rocks.

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HEAT FLOW

Volcanic EruptionPrediction

Exploration

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HEAT FLOW

Conduction : thermal vibration of constituentatoms.For most rocks this is low (i.e., 400 km in 4.5

billion years).

Convection : flow of heat due to movement ofmaterial.Requires different densities (high temperature

gradient and high coefficient of thermalexpansion).Implications: hotspots moving upward.

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Blue = cold (fast seismic velocities).

Red = hot (slow seismicvelocities).

Recycling

Seismic Tomography : earthquake waves + computers to study planar

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Seismic Tomography : earthquake waves + computers to study planarcross sections of the mantle after large EQs.

PLAN

CROSS SECTION

l

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Hotspots or PlumesTransfer of heat across a boundary.

Seismic data suggest plumes may be linked to returnof crust via subduction.

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Diameter Density Crust Mantle Core Geology(km) (g cm -3) (km) (km) (km)

Earth 12,756 5.52 7-60 2,900 3,470 Still Alive

Mercury 4,880 5.44 50-100 500-600 1750 Dead ~3Ga

Venus 12,102 5.2 ? ? ? Still Alive

Mars 6,786 3.93 25-70 2,700 2,500 Dead?-4,150 -4,000

Moon 3,476 3.36 50-100 ~1,000 ? Dead

Planetary Comparisons

0158

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