Explain the importance of sedimentary rocks and summarize the part of the rock cycle that pertains to sediments and sedimentary rocks. List the three categories of sedimentary rocks.
Most of the solid Earth consists of igneous and metamorphic rocks. Geologists estimate that these two categories represent 90 to 95 percent of the outer 16 kilometers (10 miles) of the crust. Nevertheless, most of Earth’s solid surface consists of either sediment or sedimentary rock.
About 75 percent of land areas are covered by sediments and sedimentary rocks. Across the ocean floor, which represents about 70 percent of Earth’s solid surface, virtually everything is covered by sediment. Igneous rocks are
exposed only at the crests of mid-ocean ridges and in some volcanic areas. Thus, while sediment and sedimentary rocks make up only a small percentage of Earth’s crust, they are concentrated at or near the surface—the interface among the geosphere, hydrosphere, atmosphere, and biosphere.
Because of this unique position, sediments and the rock layers that they eventually form contain evidence of past conditions and events at the surface.Based on the compositions, textures, structures,and fossils in sedimentary rocks,experienced geologists can decipher clues that provide insights into past climates, ecosystems, and ocean environments. Furthermore, by studying sedimentary rocks, geologists can reconstruct the configuration of ancient landmasses and the locations and compositions of long-vanished mountain systems. In short, this group of rocks provides geologists with much of the basic information needed to reconstruct the details of Earth history (Figure 1).
The study of sedimentary rocks has economic significance as well. Coal,
which provides a significant portion of our electrical energy, is classified as a sedimentary rock. Moreover, other major energy sources—including oil, natural gas, and uranium—are derived from sedimentary rocks. So are major sources of iron, aluminum, manganese, and phosphate fertilizer, plus numerous materials that are essential to the construction industry, such as cement and aggregate.
Sediments and sedimentary rocks are also the primary reservoir of groundwater. Thus, having an understanding of this group of rocks and the processes that form and modify them is basic to locating and maintaining supplies of many important resources.
Origins of sedimentary rocks
Like other rocks, the sedimentary rocks that we see around us and use in so many different ways have their origin in the rock cycle. Figure 2 illustrates the portion of the rock cycle that occurs near Earth’s surface—the part that pertains to sediments and sedimentary rocks. A brief overview of these processes provides a useful perspective:
- Weathering begins the process. It involves the physical disintegration and chemical decomposition of preexisting igneous, metamorphic, and sedimentary rocks. Weathering generates a variety of products subject to erosion, including various solid particles and ions in solution. These are the raw materials for sedimentary rocks.
- Soluble constituents are dissolved and carried away by runoff and groundwater. Solid particles are frequently moved downslope by gravity, a process termed mass wasting, before running water, groundwater, wave activity, wind, and glacial ice remove them. These agents of transport, covered in detail in later chapters, move these materials from the sites where they originated to locations where they accumulate. The transport of sediment is usuallyintermittent. For example, during a flood, a rapidly moving river moves large quantities of sand and gravel. As the floodwaters recede, particles are temporarily deposited, only to be moved again by a subsequent flood.
- Deposition of solid particles occurs when wind and water currents slow down and as glacial ice melts. The word sedimentary actually refers to this process. It is derived from the Latin sedimentum, which means “to settle,” a reference to solid material settling out of a fluid (water or air). The mud on the floor of a lake, a delta at the mouth of a river, a gravel bar in a stream bed, the particles in a desert sand dune, and even household dust are examples.
- The deposition of material dissolved in water is not related to the strength of water currents. Rather, ions in solution are removed when chemical or temperature changes cause material to crystallize and precipitate (solidify out of a liquid solution) or when organisms remove dissolved material to build hard parts such as shells.
- As deposition continues, older sediments are buried beneath younger layers and gradually converted to sedimentary rock (lithified) by compaction and
cementation. This and other changes are referred to as diagenesis (dia = change; genesis = origin), a collective term for all the changes (short of metamorphism) that take place in texture, composition, and other physical properties after sediments are deposited.
Because there are a variety of ways that the products of weathering are transported, deposited, and transformed into solid rock, geologists recognize three categories of sedimentary rocks. As this overview reminds
us, sediment has two principal sources. First, it may be an accumulation of material that originates and is transported as solid particles derived from both mechanical and chemical weathering. Deposits of this type are termed detrital, and the sedimentary rocks they form are called detrital sedimentary rocks.
The second major source of sediment is soluble material produced largely by chemical weathering. When these ions in solution are precipitated by either inorganic or biological processes, the material is known as chemical
sediment, and the rocks formed from it are called chemical sedimentary rocks.
The third category is organic sedimentary rocks, which form from the carbon-rich remains of organisms.
The primary example is coal. This black combustible rock consists of organic carbon from the remains of plants that died and accumulated on the floor of a swamp. The bits and pieces of undecayed plant material that constitute the “sediments” in coal are quite unlike the weathering products that make up detrital and chemical sedimentary rocks.