Earth Science 3.3 Sedimentary Rocks Sedimentary Rocks Sedimentary Rocks begin to form when existing rocks are broken down into sediments. These sediments, which consists of mainly of weathered rock debris, are often transported to other places by wind or water, ice or gravity. When these sediments are dropped, they eventually become compacted and harden to form sedimentary rock. Formation of Sedimentary Rocks Sedimentary rocks form when solids settle out of a fluid water or air. Some sediments are dropped by moving water, some when winds blow them from one location to another. The sandstone arch at right has been eroded by wind over centuries. Weathering Recall that weathering is any process that breaks rocks into sediments. Weathering is often the first step in the formation of sedimentary rocks. Chemical weathering takes place when minerals in rocks change into new substances. Weathering Weathering also takes place when physical forces; wind and water, breaks rocks into smaller pieces. Weathered sediments don’t usually stay in one place. Instead wind, water, ice or gravity carries away the fragments to new locations. Erosion Erosion involves weathering and the removal of rock. When an agent of weathering; water wind, ice or gravity, looses energy and slows down it’s movement, it drops the sediments. This process, the dropping of the sediments, is called deposition. The Mississippi Delta was formed by deposition as the Mississippi River slows down as it arrives at the ocean. Deposition Deposition is the process by which an agent of erosion loses energy and drops the sediment it is carrying. When this happens, sediments are deposited according to size and weight. The larger sediments, such as rounded pebbles, are deposited first. Deposition Smaller sediments, like grains of sand, are carried farther and deposited last. Some sediments are so small they are carried great distances before being deposited. The Mississippi Delta was formed from all the materials transported by the waters of the river carrying sediments downstream. Compaction and Cementation After sediments are deposited, they often become lithified, or turned to rock. Two processes both contribute to this process of layers of sediment hardening (lithifying) into rock: ○ compaction ○ cementation Compaction Compaction, is a process that Compaction is caused by the squeezes or compacts sediment. weight of sediments. During compaction , much of the water in the sediment is driven out. Cementation Cementation takes place when dissolved minerals are deposited in the tiny spaces among the sediments. Rocks formed by cementation can have mixed appearances. Cementation The conglomerate at right is formed when larger rocks are cemented together by finer material. The sandstone at right has a smooth fine grain as it is the result of layers of fine sanding compacting over centuries. Classification of Sedimentary Rock Just like igneous rocks, sedimentary rocks can be classified into two main groups depending on their formation Clastic Sedimentary Chemical Sedimentary Clastic Sedimentary Rocks The most common materials found in clastic Clastic sedimentary rocks can be grouped sedimentary rocks are clay and quartz. “according to the size of the sediments in the rocks”. When rounded, gravel sized or larger rocks make up the rock, the rock is said to be a conglomerate. When particles are angular the rock is called breccia, and if the rock is fine sand-sized grains it is called sandstone. Chemical and Biochemical Sedimentary Rock Chemical sedimentary rocks form when substances dissolved in water precipitate, or separate, from water solutions. This precipitation usually happens when water evaporates or boils away, leaving a solid product. Examples of this type of rock includes some limestones, rock salt, chert, flint, and rock gypsum Biochemical Sediments About 90 % of limestones are formed from biochemical sediments Such sediments are the shells and skeletal remains of organisms that settle to the ocean floor. In coquina, you can see the shells cemented together. Another biochemical rock is chalk, the material we use to write on blackboards. To the right you see the famous White Cliffs of Dover, comprised of mainly white chalk. Features of Sedimentary Rocks Many unique features of Each layer records a period of sedimentary deposit. sedimentary rocks are clues to how, when, and where the rocks formed. The newest layers are on the bottom and the oldest are near the top. Features of Sedimentary Rocks Ripple marks or mud cracks found Fossils are sometimes found in in a layer indicate that water existed in this area at a certain time period; that it may have been an ocean or lake at one time past. these layers and the age of the fossil can be determined in part by the period of formation of the rock layer the fossil is found within. Summery In summary, our Key Points are: ○ Sedimentary rocks are rocks that form because of 4 major processes. Weathering produces particles called sediments. Wind, water, ice and gravity erode and deposit these sediments. Over time, the sediments are compacted and cemented to form rocks. Sedimentary Rocks can be classified according to how they form.