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.
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 crust—about 6 km per second, the same
speed at which they travel through granite and gneiss. Continental crust is often called “granitic”,
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
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
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
rock such as peridotite. Ultra mafic rock is dense igneous made up chiefly of ferromagnesian
minerals such as olivine and pyroxene. Some ultramafic rocks contain garnet, and all of them lack
The crust and uppermost mantle together form the lithosphere, the outer shell of earth that is
relatively strong and brittle. The lithosphere makes up the plates of plate tectonics theory.
The lithosphere averages about 70 km thick beneath oceans and maybe 125 to 250 km thick
beneath continents. Its lower boundary is marked by curious mantle layer in which seismic waves
Generally , seismic waves increase in velocity with depth as increasing pressure alters the
properties of the rock. Beginning at a depth of 70-125km however, seismic waves travel more
slowly than they do in shallower layers, and so this zone has been called the low-velocity zone.
This zone extending to a depth of perhaps 200 km, is called the asthenophsere. The rocks in this
zone may be closerto their melting point than the rocks above or below the zone. (the rocks are
probably not hotter than the rocks below—melting points are controlled by pressure as well as
temperature.) Some geoscientists thinkthat these rocks may actually be partially melted, forming
a crystal- and-liquid slush; a very small percentage of liquid in this asthenosphere could help
explain some of its physical properties.
If the rocks of the asthenosphere are close to their melting point, this zone may be important for
1. It may represent a zone where magma is likely to be generated and
2. The rocks here may have relatively little strength and therefore are likely to flow. If mantle
rocks in the asthenosphere are weaker than they are in the overlying lithosphere, then the
asthenosphere can deform easily by plastic flow. Plates of brittle lithosphere probably
move easily over the asthenosphere, which may act as a lubricating layer below.
There is widespread agreement on the existence and depth of the asthenosphere under oceanic
crust, but considerable disagreement about asthenosphere continental crust.
Data from seismic flection and refraction indicate several concentric layers in the mantle, with
prominent boundaries at 400 and 670 km it is doubtful that the layering is due to the presence of
several different kinds of rock . Most geoscientists think that the chemical composition of the
mantle rock is about the same throughout the mantle . because pressure increases with depth
into the earth, the boundaries between mantle layers possibly represent depths at which pressure
collapses the internal structure of certain minerals into denser minerals. For example at a
pressure equivalent to a depth of about 670 km , the mineral olivine should collapse into the
denser structure of the mineral perovskite. If the boundaries between mantle layers represent
pressure- caused transformations of minerals, the entire mantle may have the same chemical
composition throughout, although not the same mineral composition. However, some
geoscientists think that the 670km boundary represents a chemical change as well as a physical
change and separates the upper mantle from the chemically different lower mantle below.
Seismic-wave data provide the primary evidence for the existence of the core of Earth. (See
chapter 3 for a discussion of seismic P and S waves.) Seismic waves do not reach certain areas
on the opposite side of the Earth from a large earthquake. Figure 4.8 shows how seismic P waves
spread out from a quake until, at 103 degrees of arc (11,500 km) from the epicentre, they
suddenly disappear from seismograms. At more than 142 degrees (15,500 km) from the
epicenter, P waves reappear on seismograms. The region between 103 degrees and 142
degrees, which lacks P waves, is called the P-wave shadow zone.
The P-wave shadow zone can be explained by the refraction of P waves when they
encounter the core boundary deep within Earth’s interior. Because the paths of P waves can be
accurately calculated the size and shape of the core can be determined also. In figure 4.8, notice
that Earth’s core deflects the P waves and, in effect, “casts a shadow” where their energy does
not reach the surface. In other words, P waves are missing within the shadow zone because they
have been bent (refracted) by the core.
The chapter on earthquakes explains the while P waves can travel through solids and fluids,
S waves can travel only through solids. As figure 4.9 shows, an s-wave shadow zone also exists
and is larger than the P-wave zone. Direct S waves are not recorded in the entire region more
than 103 degrees away from the epicentre. The S-wave shadow zone seems to indicate that S
waves do not travel through the core at all. If this is true, it implies that the core of Earth is a liquid,
or at least acts like a liquid.
The way in which P waves are refracted with Earth’s core (as shown by careful analysis of
seismograms) suggests that the core has two parts, a liquid outer core and a solid inner core
Composition of the Core
When evidence from astronomy and seismic-wave studies is combined with what we know about
the properties of materials, it appears that the Earth’s core is made of metal—not silicate rock—
and that this metal is probably iron (along with a minor amount of oxygen, silicon, sulphur, or
nickel). How did geoscientists arrive at this conclusion?
The overall density of the Earth is 5.5 gm/cm3, based on calculations from Newton’s law of
gravitational attraction. The crustal rocks are relatively low density, from 2.7 gm/cm3 for granite to
3.0 gm/cm3 for basalt. The ultramafic rock thought to make up the mantle probably has a density
of 3.3 gm/cm3 in the upper mantle, although rock pressure should raise this value to about 5.5
gm/cm3 at the base of the mantle (figure 4.7).
If the crust and the mantle, which have approximately 85 percent of the Earth’s volume, are at
or below the average density of the Earth, then the core must be very heavy to bring the average
up to 5.5 gm/cm3.
Calculations show that the core has to have a density of about 10 gm/cm3 at the core-mantle
boundary, increasing to 12 or 13 gm/cm3 at the centre of Earth (figure 4.7).