CHAPTER 1 An Introduction to Geology
24
D I D Y O U K N O W ?
We have never sampled the mantle or
core directly. The structure of Earth’s
interior is determined by analyzing
seismic waves from earthquakes. As
these waves of energy penetrate
Earth’s interior, they change speed and
are bent and reflected as they move
through zones having different
properties. Monitoring stations around
the world detect and record this
energy.
zone the lithosphere is mechanically
detached from the layer below. The result
is that the lithosphere is able to move
independently of the asthenosphere, a fact
we will consider later in the chapter and in
Chapter 15.
It is important to emphasize that the
strength of various Earth materials is a
function of both their composition and the
temperature and pressure of their environment. You should not get the idea that the
entire lithosphere behaves like a brittle
solid similar to rocks found on the surface.
Rather, the rocks of the lithosphere get progressively hotter and weaker (more easily
deformed) with increasing depth. At the
depth of the uppermost asthenosphere, the
rocks are close enough to their melting
temperature (some melting may actually
occur) that they are very easily deformed.
Thus, the uppermost asthenosphere is
weak because it is near its melting point,
just as hot wax is weaker than cold wax.
THE LOWER MANTLE. From a depth of
660 kilometers (nearly 410 miles) to the
top of the core, at a depth of 2900
kilometers (1800 miles), is the lower
mantle. Because of an increase in pressure
(caused by the weight of the rock above)
the mantle gradually strengthens with
depth. Despite their strength however, the
rocks within the lower mantle are very hot
and capable of very gradual flow.
Earth’ s Core
The composition of the core is thought to
be an iron-nickel alloy with minor amounts
of oxygen, silicon, and sulfur—elements
that readily form compounds with iron. At
the extreme pressure found in the core, this
iron-rich material has an average density of
nearly 11 g/cm
3 and approaches 14 times
the density of water at Earth’ s center.
The core is divided into two regions
that exhibit very different mechanical
strengths. The outer core is a liquid layer
2270 kilometers (1410 miles) thick. It is
the movement of metallic iron within this
zone that generates Earth’ s magnetic field.
The inner core is a sphere having a radius
of 1216 kilometers (754 miles). Despite its
higher temperature, the iron in the inner
core is solid due to the immense pressures
that exist in the center of the planet.
C O N C E P T C H E C K 1 . 1 0
List and briefly describe Earth’s compositional layers.
Contrast the lithosphere and the
asthenosphere.
The Face of Earth
The two principal divisions of Earth’ s surface are the continents and the ocean basins
(FIGURE 1.27). A significant difference
between these two areas is their relative
levels. The continents are remarkably flat
features that have the appearance of
plateaus protruding above sea level. With
an average elevation of about 0.8 kilometer
(0.5 mile), continents lie close to sea level,
except for limited areas of mountainous
terrain. By contrast, the average depth
of the ocean floor is about 3.8 kilometers
(2.4 miles) below sea level.
The elevation difference between the
continents and ocean basins is primarily
the result of differences in their respective
densities and thicknesses. Recall that the
continents average 35 to 40 kilometers in
thickness and are composed of granitic
rocks having a density of about 2.7 g/cm
3
.
The basaltic rocks that comprise the
oceanic crust average only 7 kilometers
thick and have an average density of about
3.0 g/cm
3
. Thus, the thicker and less dense
continental crust is more buoyant than the
oceanic crust. As a result, continental crust
2
1
but may exceed 70 kilometers (40 miles) in
some mountainous regions such as the
Rockies and Himalayas. Unlike the oceanic
crust, which has a relatively homogeneous
chemical composition, the continental crust
consists of many rock types. Although the
upper crust has an average composition of
a granitic rock called granodiorite, it varies
considerably from place to place.
Continental rocks have an average
density of about 2.7 g/cm
3
, and some have
been discovered that are 4 billion years old.
The rocks of the oceanic crust are younger
(180 million years or less) and denser
(about 3.0 g/cm
3
) than continental rocks.*
Earth’ s Mantle
More than 82 percent of Earth’ s volume is
contained in the mantle, a solid, rocky shell
that extends to a depth of nearly 2900
kilometers (1800 miles). The boundary
between the crust and mantle represents a
significant change in chemical composition.
The dominant rock type in the uppermost
mantle is peridotite, which is richer in the
metals magnesium and iron than the
minerals found in either the continental
or oceanic crust.
THE UPPER MANTLE. The upper mantle
extends from the crust-mantle boundary
down to a depth of about 660 kilometers
(410 miles). The upper mantle can be
divided into two different parts. The top
portion of the upper mantle is part of the
stiff lithosphere, and beneath that is the
weaker asthenosphere.
The lithosphere (sphere of rock)
consists of the entire crust and uppermost
mantle and forms Earth’ s relatively cool,
rigid outer shell. Averaging about 100 kilometers in thickness, the lithosphere is more
than 250 kilometers thick below the oldest
portions of the continents. Beneath this stiff
layer to a depth of about 350 kilometers
lies a soft, comparatively weak layer known
as the asthenosphere (“weak sphere”). The
top portion of the asthenosphere has a temperature/pressure regime that results in a
small amount of melting. Within this weak
*Liquid water has a density of 1 g/cm
3
; therefore,
the density of basalt is three times that of water.
24
D I D Y O U K N O W ?
We have never sampled the mantle or
core directly. The structure of Earth’s
interior is determined by analyzing
seismic waves from earthquakes. As
these waves of energy penetrate
Earth’s interior, they change speed and
are bent and reflected as they move
through zones having different
properties. Monitoring stations around
the world detect and record this
energy.
zone the lithosphere is mechanically
detached from the layer below. The result
is that the lithosphere is able to move
independently of the asthenosphere, a fact
we will consider later in the chapter and in
Chapter 15.
It is important to emphasize that the
strength of various Earth materials is a
function of both their composition and the
temperature and pressure of their environment. You should not get the idea that the
entire lithosphere behaves like a brittle
solid similar to rocks found on the surface.
Rather, the rocks of the lithosphere get progressively hotter and weaker (more easily
deformed) with increasing depth. At the
depth of the uppermost asthenosphere, the
rocks are close enough to their melting
temperature (some melting may actually
occur) that they are very easily deformed.
Thus, the uppermost asthenosphere is
weak because it is near its melting point,
just as hot wax is weaker than cold wax.
THE LOWER MANTLE. From a depth of
660 kilometers (nearly 410 miles) to the
top of the core, at a depth of 2900
kilometers (1800 miles), is the lower
mantle. Because of an increase in pressure
(caused by the weight of the rock above)
the mantle gradually strengthens with
depth. Despite their strength however, the
rocks within the lower mantle are very hot
and capable of very gradual flow.
Earth’ s Core
The composition of the core is thought to
be an iron-nickel alloy with minor amounts
of oxygen, silicon, and sulfur—elements
that readily form compounds with iron. At
the extreme pressure found in the core, this
iron-rich material has an average density of
nearly 11 g/cm
3 and approaches 14 times
the density of water at Earth’ s center.
The core is divided into two regions
that exhibit very different mechanical
strengths. The outer core is a liquid layer
2270 kilometers (1410 miles) thick. It is
the movement of metallic iron within this
zone that generates Earth’ s magnetic field.
The inner core is a sphere having a radius
of 1216 kilometers (754 miles). Despite its
higher temperature, the iron in the inner
core is solid due to the immense pressures
that exist in the center of the planet.
C O N C E P T C H E C K 1 . 1 0
List and briefly describe Earth’s compositional layers.
Contrast the lithosphere and the
asthenosphere.
The Face of Earth
The two principal divisions of Earth’ s surface are the continents and the ocean basins
(FIGURE 1.27). A significant difference
between these two areas is their relative
levels. The continents are remarkably flat
features that have the appearance of
plateaus protruding above sea level. With
an average elevation of about 0.8 kilometer
(0.5 mile), continents lie close to sea level,
except for limited areas of mountainous
terrain. By contrast, the average depth
of the ocean floor is about 3.8 kilometers
(2.4 miles) below sea level.
The elevation difference between the
continents and ocean basins is primarily
the result of differences in their respective
densities and thicknesses. Recall that the
continents average 35 to 40 kilometers in
thickness and are composed of granitic
rocks having a density of about 2.7 g/cm
3
.
The basaltic rocks that comprise the
oceanic crust average only 7 kilometers
thick and have an average density of about
3.0 g/cm
3
. Thus, the thicker and less dense
continental crust is more buoyant than the
oceanic crust. As a result, continental crust
2
1
but may exceed 70 kilometers (40 miles) in
some mountainous regions such as the
Rockies and Himalayas. Unlike the oceanic
crust, which has a relatively homogeneous
chemical composition, the continental crust
consists of many rock types. Although the
upper crust has an average composition of
a granitic rock called granodiorite, it varies
considerably from place to place.
Continental rocks have an average
density of about 2.7 g/cm
3
, and some have
been discovered that are 4 billion years old.
The rocks of the oceanic crust are younger
(180 million years or less) and denser
(about 3.0 g/cm
3
) than continental rocks.*
Earth’ s Mantle
More than 82 percent of Earth’ s volume is
contained in the mantle, a solid, rocky shell
that extends to a depth of nearly 2900
kilometers (1800 miles). The boundary
between the crust and mantle represents a
significant change in chemical composition.
The dominant rock type in the uppermost
mantle is peridotite, which is richer in the
metals magnesium and iron than the
minerals found in either the continental
or oceanic crust.
THE UPPER MANTLE. The upper mantle
extends from the crust-mantle boundary
down to a depth of about 660 kilometers
(410 miles). The upper mantle can be
divided into two different parts. The top
portion of the upper mantle is part of the
stiff lithosphere, and beneath that is the
weaker asthenosphere.
The lithosphere (sphere of rock)
consists of the entire crust and uppermost
mantle and forms Earth’ s relatively cool,
rigid outer shell. Averaging about 100 kilometers in thickness, the lithosphere is more
than 250 kilometers thick below the oldest
portions of the continents. Beneath this stiff
layer to a depth of about 350 kilometers
lies a soft, comparatively weak layer known
as the asthenosphere (“weak sphere”). The
top portion of the asthenosphere has a temperature/pressure regime that results in a
small amount of melting. Within this weak
*Liquid water has a density of 1 g/cm
3
; therefore,
the density of basalt is three times that of water.