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What is inside the Earth?
It was the study of seismic refraction and seismic reflection that enabled scientists to plot the
three main zones of the Earth’s interior.
-The crust is the outer layer of rock, which forms a thin skin on Earth’s surface.
o Below the crust lies the mantle, a thick shell of rock that separates the crust
above from the core below. The core is the central zone of earth. It is probably
metallic and the source of earth’s magnetic field.
The Crust:
Studies of seismic waves have shown:
1. That the crust is thinner beneath the oceans than beneath the continents.
2. That seismic waves travel faster in oceanic crust than in continental crust.
Because of this velocity differences, it is assumed that the two types of crust are made up of
different kinds of rock.
Seismic P waves travel through oceanic crust at about 7km per second, which is also the speed
at which they travel through basalt and gabbro (the coarse-grained equivalent of basalt) .
Samples of rocks taken from the sea floor by oceanographic ships verify that the upper part of the
oceanic is basalt and suggest that the lower part is gabbro. The oceanic crust averages 7 km in
thickness, varying from 5 to 8 km.
Seismic P waves travel more slowly through continental crustabout 6 km per second, the same
speed at which they travel through granite and gneiss. Continental crust is often calledgranitic”,
but the term should be put in quotation marks because most of the rocks exposed on land are not
The continental crust is highly variable and complex, consisting of a crystalline basement
composed of granite, other plutonic rocks, gneisses, and schists all capped by a layer of
sedimentary rocks, like icing on a cake. Since a single rock term cannot accurately describe crust
that varies so greatly in composition, some geoscientists use the term felsic—rocks high in
feldspar and silicon for continental crust and mafic—rock high in magnesium and iron for
oceanic crust.
Continental crust is much thicker than oceanic crust, averaging 30 to 50 km in thickness, though it
varies from 10 to 70 km. seismic waves show that the crust is thickest under geologically young
mountain ranges, such as the Andes and the Himalaya bulging downward as a mountain root into
the mantle. The continental crust is also less dense than oceanic crust, a fast that is important in
plate tectonics.
The boundary that separates the crust from the mantle beneath it is called the Mohorovicic
discontinuity ( Moho for short).
The Mantle: Because of the way seismic waves pass through the mantle, geoscientists interpret it
to be made of solid rock. Localized magma chambers of melted rock may occur as isolated
pockets of liquid in both the crust and the upper mantle, but most of the mantle seems to be solid.
Because P waves travel at about 8 km per second in the upper mantle, it appears that the mantle
is a different type of rock from either oceanic crust of continental crust. The best hypothesis that
geoscientists can make about the composition of the upper mantle is that it consists of ultramafic
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