Ch. 8: Exploring Earth’s Interior

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Today’s Lecture (Chapter 8):Deep Interior of the EarthEarth LayersSeismic Tomography

A look beneath Yellowstone…

Journey to the center of the Earth?

• Earth’s radius = 6,370 km (about 4,000 miles)

• Continental crust about 40 km on average• Deep drill holes less than 7 km• Deepest drill hole = 12.3 km (Kola Well,

northern Russia)

Reflection

Refraction

Seismic waves “bounce” (reflect) and “bend” (refract) at sharp boundaries

Example: inner-outer core boundary

Waves also change speed when material changes (density or composition)

Inge Lehmann (1888-1993) discovered the inner/outer core boundary

(“Lehmann discontinuity”)

Constant Velocity Earth

Velocity Increasing with Depth

Layers with Different Velocity

Are there both P- and S-wave Shadow Zones?

P and S wave shadow zones

Why are thereBoth P and S waveShadow zones?

Other ways we know what’s down there

• Lab experiments on rocks at high pressure & temperature

• Deep rocks brought up by volcanoes• Computer models

Diamond-anvil Cell

Computer model of mantle convection

So, what IS down there?

“Ogres are like onions …”

Earth’slayered structure

Crust

• Continents• 30-40 km thick• Up to 70 km thick under big mountain belts• Rich in felsic rocks

• Oceans • Only about 6 km thick• Rich in mafic rocks

Western United States Crustal Thickness

Gilbert and Sheehan, 2004

B. Mantle : 30 – 3000 km• Mantle: ultra-mafic rocks

• Lithosphere• Uppermost mantle AND crust together• Averages about 100 km thick• Strong and rigid (bends but doesn’t flow)

• Asthenosphere (“sphere of weakness”)• Middle part of the mantle• Hot enough to flow and convect

•Lower Mantle•Fastest seismic wave speeds in the Earth

Grand, 1989

Red = slow(hot or wet)Blue = fast(cold)

What regionsare hot?

c. Core: 3000 km to center• Iron rich

– very dense; 3-4 x surface rocks

• Outer core : liquid

• Inner core : solid

Changes in P-and S- wave Velocity Reveal Earth’s Internal Layers

Which Earth layer is the thinnest?A. CrustB. MantleC. Outer coreD. Inner core

Filtered Backprojection

Filtered Backprojection

y

z

x

x-position

Count rate

z

y

Reconstruction of a slice from projectionsexample = myocardial perfusion, left ventricle, long axis

Reconstruction of a slice from projectionsexample = myocardial perfusion, left ventricle, long axis

Where are the fastest P wave velocities in the Earth?A. CrustB. Upper MantleC. Lower MantleD. Outer CoreE. Inner Core

Where do we see slow seismic wave speeds (velocities)?

HotLow densityMeltWet (at shallow depth saturated is faster)

Where do we see fast seismic wave speeds (velocities)?

ColdRigidHigh DensityDry(depends on depth)

What holds up the Rocky Mountains?A. Thick crustB. Low densities in the crustC. Low densities in the mantleD. All of the above