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Lecture 25-26 Earth's Structure(1)

36 l there are porhons of the earths crust that shck

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Unformatted text preview: and therefore densiHes 25 l༆  So the S- wave shadow suggested that the centre of the Earth was liquid, since shear waves can’t travel through a liquid. l༆  P- waves by contrast can travel directly through the centre of the Earth l༆  But there is also a region of the Earth, where P- waves are not seen, something called a P- wave shadow l༆  This one is not as big, between about 105º and 142º l༆  Why is this? l༆  Well it turns out that it is completely consistent with the S- wave observaHon. 26 27 •  Remembering Snell’s Law makes it easier to understand why there is a P- shadow •  Seismic waves bend inward towards the core, at the interface between the mantle, because the liquid is more dense than the mantle 28 l༆  The Earth’s crust is much 29 thinner than shown here, in the range of 20 to 80 km thick l༆  More like the width of the shell of an egg compared to the diameter of the egg. l༆  About 1 % of the Earth’s volume is the crust = about 0.4% of the Earth’s weight. l༆  Mantle makes up about 85% of volume and core about 15% 3...
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