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Inner core Mohorovi cic discontinuity, or the “Moho” for short. The bound-
ary is a zone where seismic P-waves increase in velocity because
Outer core of changes in the composition of the materials. The increase
occurs because the composition on both sides of the boundary
is different. The mantle is richer in ferromagnesian minerals
and poorer in silicon than the crust.
Studies of the Moho show that the crust varies in thickness
around Earth’s surface. It is thicker under the continents and
thinner under the oceans.
The age of rock samples from Earth’s continents has been
compared with the age samples of rocks taken from the seafloor
by oceanographic ships. This sampling has found the continen-
tal crust to be much older, with parts up to 3.8 billion years old.
By comparison, the oldest oceanic crust is less than 200 million
years old.
Comparative sampling also found that continental crust is
3
a less dense, granite-type rock with a density of about 2.7 g/cm .
Oceanic crust, on the other hand, is made up of basaltic rock
3
with a density of about 3.0 g/cm . The less dense crust behaves
as if it were floating on the mantle, much as less dense ice floats
Mantle
on water. There are exceptions, but in general, the thicker, less
Crust dense continental crust “floats” in the mantle above sea level,
and the thin, dense oceanic crust “floats” in the mantle far below
FIGURE 18.3 The structure of Earth’s interior.
sea level (Figure 18.4).

crust from the center part, which is called the core. The follow- EXAMPLE 18.1
ing discussion starts on Earth’s surface, at the crust, and then
What is the height at which the oceanic crust floats in the mantle?
digs deeper and deeper into Earth’s interior. (Assume average oceanic crust thickness of 7.0 km.)

THE CRUST SOLUTION
Seismic studies have found that Earth’s crust is a thin skin that The position at which crust floats in the mantle can be estimated from
covers the entire Earth, existing below the oceans as well as the thickness of the crust and the relative densities of the crust and
making up the continents. According to seismic waves, there are mantle. The height of the crust floating above the mantle is equal to
differences between the crust making up the continents and the the thickness of the crust minus the thickness times the density ratio
crust beneath the oceans (Table 18.1). These differences are that (h = z crust − z crust ( ρ crust )/ρ mantle ).
(1) the oceanic crust is much thinner than the continental crust z crust = 7.0 km _
ρ
crust

( ρ
and (2) seismic waves move through the oceanic crust faster than _ h = z crust − z crust mantle )
g

they do through continental crust. The two types of crust vary ρ mantle = 3.3 g
_
3
c m 3.0
because they are made up of different kinds of rock. _ ( 3.3 )
3
_
cm
g
g
The boundary between the crust and the mantle is marked ρ crust = 3.0 = 7.0 km − 7.0 km _
3
cm
3
by a sharp increase in the velocity of seismic waves as they pass c m
h = ?
from the crust to the mantle. Today, this boundary is called the _
g

c m
3.0 _ _ 3
= 7.0 − 7.0 ( ) cm

g
3.3 _

c m 3
TABLE 18.1
= 7.0 − 7.0(0.91) km
Comparison of oceanic crust and continental crust
= 7.0 − 6.4 km
Oceanic Crust Continental Crust = 0.6 km
Age Less than 200 Up to 3.8 billion years
million years old EXAMPLE 18.2
Thickness 5–8 km 10–75 km (6–47 mi) What is the height at which the continental crust floats in the mantle?
(3–5 mi) What is the difference between the height of the continental crust and
Density 3.0 g/cm 3 2.7 g/cm 3 the height of the oceanic crust? (Assume z crust = 35 km.)
Composition Basalt Granite, schist, gneiss (Answer: 6.3 km; 5.7 km.)
458 CHAPTER 18 Plate Tectonics 18-4