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OVERVIEW

As discussed in chapter 19, the movement of Earth’s plates produces mountains, folded structures, and other surface
features. These are produced through volcanic activity and as a result of stress, which results in folded and faulted
structures. Vulcanism and diastrophism are constructive forces of Earth that result in a building up of the surface,
an increase in the elevation of the land. There is also a destructive side, however, a side that goes mostly unseen. The
elevated land is now subjected to a sculpturing and tearing down of its surface.
Sculpturing of Earth’s surface takes place through agents and processes acting so gradually that humans are
usually not aware that it is happening. Sure, some events such as a landslide or the movement of a big part of a
beach by a storm are noticed. But the continual, slow, downhill drift of all the soil on a slope or the constant shift of
grains of sand along a beach are outside the awareness of most people. People do notice the muddy water moving
rapidly downstream in the swollen river after a storm, but few are conscious of the slow, steady dissolution of
limestone by acid rain percolating through it. Yet it is the processes of slow moving, shifting grains and bits of rocks,
and slow dissolving that will wear down the mountains, removing all the features of the landscape that you can see.
Each of the agents that wear down Earth’s surface—gravity, moving water, glaciers, and wind—has its own
way of removing and redepositing the fragments of the land. Each produces a set of characteristic sculpturing
and depositional features. Thus, it is possible to recognize how a particular landscape formed in the past, even
though different agents may be working today. This chapter is about the decomposition and sculpturing changes
that occur in the landscape. Knowledge of these changes can be used to account for some of the varied and
interesting scenery that you can observe across the countryside, telling you something about the history of the
region (Figure 20.1).

20.1 WEATHERING, EROSION, and necessary in (1) the rock cycle, (2) the formation of soils,
and (3) the movement of rock materials over Earth’s surface.
AND TRANSPORTATION
Weathering is important in the rock cycle because it produces
A mountain of solid granite on the surface of Earth might ap- sediment, the raw materials for new rocks. It is important in
pear to be a very solid, substantial structure, but it is always the formation of soils because soil is an accumulation of rock
undergoing slow and gradual changes. Granite on the surface is fragments and organic matter. Weathering is also important
exposed to and constantly altered by air, water, and other agents because it reduces the size of rock particles, preparing the rock
of change. It is altered both in appearance and in composition, materials for transport by wind or moving water. Before the
slowly crumbling and then dissolving in water. Smaller rocks process of weathering, the rock is mostly confined to one loca-
and rock fragments are moved downhill by gravity or streams, tion as a solid mass.
exposing more granite that was previously deeply buried. The Weathering breaks down rocks physically and chemically,
process continues, and ultimately—over much time—a moun- and this breaking down can occur while the rocks are station-
tain of solid granite is reduced to a mass of loose rock frag- ary or while they are moving. The process of physically remov-
ments and dissolved materials. The photograph in Fig ure 20.2 ing weathered materials is called erosion. Weathering prepares
is a snapshot of a mountain-sized rock mass in a stage some- the way for erosion by breaking solid rock into fragments. The
where between its formation and its eventual destruction to fragments are then eroded, physically picked up by an agent
rock fragments. Can you imagine the length of time that such such as a stream or a glacier. After they are eroded, the materi-
a process requires? als are then removed by transportation. Transportation is the
movement of eroded materials by agents such as rivers, gla ciers,
wind, or waves. The weathering process continues during trans-
portation. A rock being tumbled downstream, for example, is
physically worn down as it bounces from rock to rock. It may be
20.2 WEATHERING
chemically altered as well as it is bounced along by the moving
The slow changes that result in the breaking up, the crum- water. Overall, the combined action of weathering and erosion
bling, and the destruction of any kind of solid rock are called wears away and lowers the elevated parts of Earth and sculpts
weathering. The term implies changes in rocks from the action their surfaces.
of the weather, but it actually includes chemical, physical, and There are two basic kinds of weathering that act to break
biological processes. These weathering processes are important down rocks: mechanical weathering and chemical weathering.

502 CHAPTER 20 Shaping Earth’s Surface 20-2