Ch. 8: Exploring Earth’s Interior You are here
Ch. 8: Exploring Earth’s Interior
You are here
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