Weathering, Soil, and Sedimentary Rocks - The Changing Earth: Exploring Geology and Evolution - page 133

Most evidence of prehistoric life in the form of fossils
is found in sedimentary rocks.
Weathering is important in the origin and concen-
tration of some resources, and sediments and sedi-
mentary rocks are resources themselves or contain
resources such as petroleum and natural gas.

or dissolved. Some of this weathered material may accumulate
and be further modifi ed to form soil. Much of it, however, is
removed by erosion , which is the wearing away of soil and
rock by geologic agents such as running water. This eroded
material is transported elsewhere by running water, wind,
glaciers, and marine currents and is eventually deposited as
sediment , the raw material for sedimentary rocks.
Earth's crust is composed mostly of crystalline rock , a
term that refers loosely to metamorphic and igneous rocks,
except those made up of pyroclastic materials. Nevertheless,
sediment and sedimentary rocks, making up perhaps only
5% of the crust, are by far the most common materials in
surface exposures and in the shallow subsurface. They cover
approximately two-thirds of the continents and most of the
seafl oor, except spreading ridges. All rocks are important in
deciphering Earth history, but sedimentary rocks have a spe-
cial place in this endeavor because they preserve evidence of
surface processes responsible for them, as well as most fossils,
which are evidence of prehistoric life.
Weathering and erosion have yielded many areas of
exceptional scenery, including Bryce Canyon and Arches
National Parks, both in Utah (

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INTRODUCTION
All rocks at or near Earth's surface, as well as rocklike substances
such as pavement and concrete in sidewalks, bridges, and foun-
dations, decay and crumble with age. In short, they experience
weathering , defi ned as the physical breakdown and chemical
alteration of Earth materials as they are exposed to the atmo-
sphere, hydrosphere, and biosphere. Actually, weathering is a
group of physical and chemical processes that alter Earth mate-
rials so that they are more nearly in equilibrium with a new set
of environmental conditions. Many rocks form within the crust
where little or no oxygen or water is present, but at or near the
surface they are exposed to both, as well as to lower tempera-
ture and pressure and the activities of organisms.
During weathering, the rock acted on by weathering,
or parent material , is disaggregated to form smaller pieces
(

Figure 6.2 and the chap-
ter opening photograph). In addition to interesting land-
scapes, weathering is also responsible for the origin of
some natural resources, aluminum ore, for example, and it

◗

◗ Figure 6.1), and some of its constituent minerals are altered

◗ Figure 6.1 Weathering of Granite

a This exposure of granite has been so
thoroughly weathered that only a few spherical
masses of the original rock are visible.

b Closeup view of the weathered material. Mechanical weathering
has predominated, so the particles are mostly small pieces of granite
and materials such as quartz and feldspars.