Chapter 4 Notes

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4 Apr 2011
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Chapter 4
THE EARTHS INTERIOR
-earth has a radius of 6,370 km
-we can only look at the crust; cannot go any further
-deep parts can be studied indirectly through geophysics
-geophysics includes the study of seismic waves and earths magnetic field, gravity, and heat
What Can We Learn From The Study of Seismic Waves?
-nuclear bomb explosions also cause seismic waves
-seismic reflection: important way of learning about the Earths interior; the return of some
of the energy of seismic waves to the Earths surface after the waves bounce off a rock
boundary
-seismic refraction: the bending of seismic waves as they pass from one material to another
-boundaries between rock layers are usually distinct enough to be located by seismic
refraction techniques
-seismic station 2 receives waves from two pathways: a direct path and a refracted path
-more info on this page 107
What is Inside the Earth?
-crust, mantle, core
The Crust
-the crust is thinner under the oceans than the continents
-seismic waves travel faster in oceanic crust than in continental crust
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-oceanic crust is basalt (mafic)
-continental is granite (felsic)
-Mohorovicic Discontinuity: the boundary that separates the crust from the mantle
The Mantle
-possibly made of ultramafic rock: like peridotite
-the crust and uppermost mantle together form the lithosphere: the outer shell of earth
that is relatively strong and brittle; makes up the plates of plate tectonics
-it is about 70km thick under oceans and possibly 125- 250 km under continents
-asthenosphere: 200km down; rocks probably closer to melting points because of pressure,
not heat
-important that some rocks may be melted here because:
1) it may represent a zone where magma is likely to be generated
2) the rocks here may have relatively little strength and therefore are likely to flow
-which can help the plates move around
The Core
-we got our evidence of the core from seismic wave data
-P-wave shadow; not all p-waves reach the other side of the earth, they are stopped by
something
-s-wave shadow is larger than p-wave shadow
-so this implies that the earths core is liquid, or at least acts like a liquid
-core has two parts: a liquid outer core and a solid inner core
Composition of the Core
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