CHAPTER 5 Weathering and Soils
124
Earth’ s External
Processes
Weathering and Soil
External versus Internal Processes
Weathering, mass wasting, and erosion are
called external processes because they
occur at or near Earth’ s surface and are
powered by energy from the Sun. External
processes are a basic part of the rock cycle
because they are responsible for transforming solid rock into sediment.
To the casual observer, the face of
Earth may appear to be without change,
unaffected by time. In fact, 200 years ago,
most people believed that mountains, lakes,
and deserts were permanent features of an
GEODe
ESSENTIALS
OF GEOLOGY
Earth that was thought to be no more than
a few thousand years old. Today we know
that Earth is 4.6 billion years old and that
mountains eventually succumb to weathering and erosion, lakes fill with sediment or
are drained by streams, and deserts come
and go with changes in climate.
Earth is a dynamic body. Some parts
of Earth’ s surface are gradually elevated by
mountain building and volcanic activity.
These internal processes derive their
energy from Earth’ s interior. Meanwhile,
opposing external processes are continually
breaking rock apart and moving the debris
to lower elevations. The latter processes
include:
1. Weathering—the physical breakdown
(disintegration) and chemical alteration
(decomposition) of rocks at or near
Earth’ s surface.
2. Mass wasting—the transfer of rock
and soil downslope under the
influence of gravity.
3. Erosion—the physical removal of
material by mobile agents such as
water, wind, or ice.
In this chapter we will focus on rock
weathering and the products generated by
this activity. However, weathering cannot
be easily separated from mass wasting and
erosion, because as weathering breaks rocks
apart, mass wasting and erosion remove the
rock debris. This transport of material by
mass wasting and erosion further disintegrates and decomposes the rock.
FIGURE 5.1 Arizona’s Monument Valley. When weathering accentuates differences in rocks, spectacular
landforms are sometimes created. As the rock gradually disintegrates and decomposes, mass wasting
and erosion remove the products of weathering. (Photo by Michael Collier)
124
Earth’ s External
Processes
Weathering and Soil
External versus Internal Processes
Weathering, mass wasting, and erosion are
called external processes because they
occur at or near Earth’ s surface and are
powered by energy from the Sun. External
processes are a basic part of the rock cycle
because they are responsible for transforming solid rock into sediment.
To the casual observer, the face of
Earth may appear to be without change,
unaffected by time. In fact, 200 years ago,
most people believed that mountains, lakes,
and deserts were permanent features of an
GEODe
ESSENTIALS
OF GEOLOGY
Earth that was thought to be no more than
a few thousand years old. Today we know
that Earth is 4.6 billion years old and that
mountains eventually succumb to weathering and erosion, lakes fill with sediment or
are drained by streams, and deserts come
and go with changes in climate.
Earth is a dynamic body. Some parts
of Earth’ s surface are gradually elevated by
mountain building and volcanic activity.
These internal processes derive their
energy from Earth’ s interior. Meanwhile,
opposing external processes are continually
breaking rock apart and moving the debris
to lower elevations. The latter processes
include:
1. Weathering—the physical breakdown
(disintegration) and chemical alteration
(decomposition) of rocks at or near
Earth’ s surface.
2. Mass wasting—the transfer of rock
and soil downslope under the
influence of gravity.
3. Erosion—the physical removal of
material by mobile agents such as
water, wind, or ice.
In this chapter we will focus on rock
weathering and the products generated by
this activity. However, weathering cannot
be easily separated from mass wasting and
erosion, because as weathering breaks rocks
apart, mass wasting and erosion remove the
rock debris. This transport of material by
mass wasting and erosion further disintegrates and decomposes the rock.
FIGURE 5.1 Arizona’s Monument Valley. When weathering accentuates differences in rocks, spectacular
landforms are sometimes created. As the rock gradually disintegrates and decomposes, mass wasting
and erosion remove the products of weathering. (Photo by Michael Collier)