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A Guide to Shale, Limestone and Other Sedimentary Rocks

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Sedimentary rocks are one of three types of rocks found on the Earth’s surface. Sedimentary rocks are formed from sediments or little pieces of rock, such as sand, gravel, clay, and silt. These fragments are called claysts. Other types of sedimentary rocks may contain minerals. In addition, sedimentary rocks may also consist of decomposed plant or animal matter that has accumulated over time. A large percentage of these sedimentary rocks contain fossilized remains. This excites geologists because it gives clues about Earth’s history around the time they were formed. It can take millions of years to form sedimentary rock.

Classifications

Sedimentary rock falls into one of three classifications: clastic, biochemical, and chemical. Clastic sedimentary rocks are formed by weathering and erosion, which produce broken fragments or sediments from other rocks. These sediments are transported by water, air, or glaciers and then dropped. Biochemical sedimentary rocks are formed when biological elements take part in the change of rocks. For instance, limestone is created from calcium-rich skeletons of coral and mollusks. Biochemical sedimentary rocks can be found at the bottom of a lake or ocean, where marine life dies and decomposes into existing sedimentary rocks. Chemical sedimentary rocks are formed from changes in chemical processes. These types of sedimentary rocks are usually formed in bodies of water with high concentrations of minerals. Some examples of chemical sedimentary rocks include halite and dolomite. Dolomite is formed as a result of the crystallization of limestone under extreme temperatures.

Formation

Sedimentary rocks are formed when a river, ocean, or other body of water carries fragments of rock to an area where they settle. In geology, the settling of sediments is called deposition. The deposited fragments of rock build up in layers, a process known as sedimentation. Over time, the top layer of sediment squashes the sediments at the bottom. Geologists refer to this process as compaction. This process leaves holes that allow water to pass through the sedimentary rock. Next, the water squeezes between the fragments of rocks and crystals to form different salts or crystals. These crystals cement the fragments of rock together. This process is called cementation. All of these processes contribute toward the formation of sedimentary rock. It may take millions of years to complete the formation of sedimentary rock.

Composition

Sedimentary rocks are composed of sediment. The composition of the sediment depends on the environment or area of deposition. Clastic rocks contain clasts or fragments of rocks or minerals, such as clay, silt, sand, and gravel. Biochemical sedimentary rocks are formed from the accumulation of organic matter, such as decomposed animal and plant material. For instance, coal is created by the accumulation of decomposed plant matter. They may also contain fossil fragments. Chemical sedimentary rocks are composed of different minerals, such as halite, quartz, gypsum, and calcite.

Properties

Sedimentary rocks have different characteristics or properties that make them different from other rocks. The color of sedimentary rock depends largely on the element iron and its two major oxides. Depending on environmental conditions and composition, iron oxide can produce a gray, greenish, reddish, or brownish color. The presence of organic matter can color a sedimentary rock black or gray. Dark rocks rich in organic matter are often shales. The size, form, and orientation of sedimentary rock define its texture. The texture of sedimentary rock can be observed by the smoothness and shape of the grain. Sedimentary rocks may have mineralogical properties that make them unique. The structures of sedimentary rocks also vary. These are usually determined by the agent of transportation, such as air, water, or glacier. The structures of sedimentary rocks may appear as horizontal layers or stratification, symmetrical and asymmetrical ripples, crossbeds, and mudcracks.

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