Explain the three types of sedimentary rocks, where they form, and how they provide information about Earth’s history.

The desert in today’s southwestern United States was once a large, shallow sea and series of large freshwater lakes that dried up and re-formed many times. Rivers carried dissolved and fragmented rock material into these water bodies and deposited it as sediments. Many of these sediments were rich in hematite (or other iron oxides). Later, the sediments were lithified and then exposed through erosion, in many places forming dramatic and brightly colored scenery (Figure 13.11).

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There are three types of sedimentary rock: clastic sedimentary rock, organic sedimentary rock, and chemical sedimentary rock. These three types are distinguished by the kinds of sediments of which they are composed, the environment in which they form, and the processes that form them (Table 13.1). Clastic sedimentary rock is composed of broken pieces of other rocks; the sizes of those pieces determine the kind of clastic sedimentary rock that forms (see Figure 9.4). Organic sedimentary rock is composed mostly of organic material derived from ancient organisms or their shells. Chemical sedimentary rock is formed as dissolved minerals are precipitated out and as water evaporates.

Sedimentary rocks form only in depositional environments, which are places where sediments accumulate. The sedimentary rock formations we see in today’s landscapes reflect the environments in which their sediments accumulated and lithified long ago. There are many depositional environments on Earth’s surface. Figure 13.12 shows some of the more geographically widespread environments in which sediments accumulate.

The processes that form sedimentary rocks operate under the principle of uniformitarianism (see Section 11.2). These slow and gradual processes are difficult to observe on human time scales, but nonetheless operate in the world around us today. Crunch the Numbers asks you to calculate how long it takes for sediments on the seafloor to develop into sedimentary rock.

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Although 96% of Earth’s crust is composed of igneous and metamorphic rocks, most of the rocks exposed on the crust’s surface are sedimentary rocks (Table 13.2). About 99% of those sedimentary rocks are shale, sandstone, or limestone. Shale is a clastic sedimentary rock formed from clay-sized particles. Sandstone is a clastic sedimentary rock composed chiefly of quartz sand grains. Limestone is a chemical sedimentary rock composed mostly of calcite. Figure 13.13 shows the modern environments associated with these three types of sedimentary rock.

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Human society is built on natural resources. Most natural resources must either be mined or grown. In the case of minerals and rocks, they must be mined. Among the sedimentary rocks, coal, shale, and some chemical sedimentary rocks are particularly important in this regard.

Buried Sunshine: Coal

The most economically valuable sedimentary rock is coal. Coal is an organic rock that is formed from the remains of terrestrial wetland forests. The two main types of coal are bituminous coal and anthracite coal. Bituminous coal is an organic sedimentary rock formed from peat, and anthracite coal is a metamorphic rock that forms as bituminous coal is subjected to great heat and pressure.

Coal forms from ancient deposits of peat, a brownish-black, heavy soil that forms in wetlands from the partially decomposed remains of plants. When peat is compacted and heated by being buried deeply beneath other sediments, it can lithify into coal (Figure 13.14).

Coal plays a central role in today’s world economies, and about 36% of the energy consumed in the United States comes from coal. Unfortunately, many types of coal mining are environmentally destructive, particularly the widespread practice of coal mountaintop removal in West Virginia, described in Picture This.

Petroleum

Petroleum (or oil) is not a mineral or rock because it is a liquid. It deserves special mention here, however, because it is found in association with sedimentary rocks. In addition, it is an economically vital material resource for humanity today.

Petroleum is a hydrocarbon, meaning that it is composed of chains or rings of molecules made of hydrogen and carbon atoms. Like coal, it is derived from the organic remains of ancient organisms. Whereas coal is formed from ancient terrestrial wetland forests, petroleum is formed from the remains of ancient marine zooplankton and phytoplankton.

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As much as modern society depends on coal, petroleum is equally or even more important to our everyday lives, accounting for about 25% of the U.S. energy supply. On average, each person in the United States uses about 70 L (20 gal) of petroleum each day.

Sedimentary rocks are often associated with deposits of petroleum and natural methane gas. Shale can form a porous reservoir, where natural gas and petroleum accumulate. On the other hand, if shale is nonporous, it can form a cap rock, trapping petroleum and natural gas beneath the ground.

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Some porous shale can hold natural gas within its pore spaces. Such “shale gas” is playing an increasingly prominent role in the North American energy supply. Shale gas currently provides about 35% of the natural gas supplies in the United States, and this figure is growing rapidly due to the current boom in shale gas extraction. This controversial topic is explored in the Geographic Perspectives at the end of this chapter.

Evaporites

Some chemical sedimentary rocks have significant economic value. One such group of chemical sedimentary rocks is the evaporites, which are used in many applications, ranging from lithium ion batteries (lithium) to construction materials (gypsum) to seasonings (table salt). An evaporite is a deposit of one or more minerals resulting from the repeated evaporation of water from a basin. Figure 13.15 highlights one particularly important evaporite deposit, Salar de Uyuni in Bolivia.

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Sedimentary rocks reveal much about Earth’s ancient life and environments because it is in these rocks that fossils are preserved. Fossils are the remains or the impressions of organisms preserved in sedimentary rock. Figure 13.16 explores the use of fossils to study Earth’s ancient history.

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