the earth s interior structure - lecture 1 · earth’s interior: samples plummer 1st cdn edition,...
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The Earth’sinterior structure - Lecture 1
How do we know what’s inside the Earth?• samples from inside the Earth (not many)• indirect observations of the Earth’s interior seismic, gravity, magnetic, and heat flow measurements• today (mostly): seismic studies of Earth’s interior
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EARTH’S INTERIOR: SAMPLES
Plummer 1st Cdn edition, Ch. 4 pp 107-109, plus Box 4.4
(page 117)
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KTB borehole -- drilled to a depth of 10 km in GermanyRussians have drilled a 12-km-deep borehole
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Convecting Mantle
Kimberlites: samples of the upper mantle
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Kimberlites: contain samples of the upper mantle (from >150 km deep)
Box 4.4, page 118 (Plummer 1st Canadian edition)
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Mantle xenoliths: samples of the upper mantle from volcanoes
http://www.ees.nmt.edu/condie/MantleXenolith_NM.jpg http://www.calstatela.edu/faculty/acolvil/interior.html
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EVIDENCE FROM SEISMIC WAVES
Plummer 1st Cdn. edition, Ch. 4 pp. 109-112
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Refracted and Reflected Waves
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Fig. 4.01
Seismic Reflection
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Seismic Reflection
T53
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Crustal Seismology: Seismic Reflection
Lithoprobe Seismic ReflectionNovember 2000
Vibroseis trucksStewart-Cassiar HwyNorthwest BC
pp 110-111, Canadian edition of text
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Active sourcereflection seismology:
Petroleum industry’sprimary explorationtechnique
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• energy reflects off sharp boundaries between rocks with strong contrasts in density and seismic velocity
• reflection is widely used by oil industry to find hydrocarbon traps in sedimentary basins
Seismic Reflection
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Fig. 4.2
Seismic Refraction
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Fig. 4.3
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Effect of material change on paths of Refracted and reflected waves
layer with slow v
layer with faster v
layer with even faster v
With no change in properties, no refractions, no reflections
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Slower
Faster
Curvature(refraction)
of the energy
Velocitygraduallyincreasingwith depth
seismic raypath bends
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• energy is transmitted through layers and takes a curved path back to the surface without a reflection
• this happens because seismic velocity increases gradually with depth
• seismic velocities in the Earth generally increase with depth due to effects of pressure
Seismic Refraction
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EARTH’S INTERNAL STRUCTURE
Plummer et al. Canadian Edition Ch. 4 pp. 112-119
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Fig. 4.5
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• Earth structure dominantly radial due to – pressure: rearrange atoms to form denser
minerals, for example, in the mantle
– compositional differentiation (denser elements sink to the center)
• main compositional layers are:– crust (0 to 30 km, on average)
– mantle (30 to 2900 km)
– core (2900 to 6370 km)
Earth’s Radial Structure
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• some important rheologic layers are:– solid lithosphere (0 to 150 km)
– gooey asthenosphere (150 to 300 km)
– liquid outer core (2900 to 5150 km)
– solid inner core (5150 to 6370 km)
Earth’s Radial Structure
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Figure 4.6
P-wave velocities in crust and upper mantleMUCH lowervelocities nearEarth’s surface
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Table 4.1 in Canadian edition of text
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Fig. 4.7
P- and S- wave velocities inside the Earth
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• upper mantle (30-660 km)– 8 km/s : peridotite
– seismic velocity variations due to lithosphere, asthenosphere (gooey), olivine phase changes
• lower mantle (660-2900 km)– steady increase in velocity
– probably same composition as upper mantle
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Wait! How do we know this??
We measure the arrival times of different seismic waves, and compare them with what we would expect from calculations for a model Earth.