The Rock on the Bottom By Trista L. Pollard 1 As I laid on a rock, I received such a shock, Another rock upon me lay, Dried and hardened throughout the day, Before I knew it, many more had lain upon me, Many strata as far as the eye can see... If sedimentary rocks could talk, would they complain about carrying the weight of other rocks on their backs? Or would they speak about the many years of history their layers have witnessed? Since geologists do not have the privilege of studying "talking" rocks, they must rely on their observations of the layers and their formation to tell the true story. 2 A sedimentary rock's life begins with the sediment. Our planet's surface is constantly worn down by physical weathering and erosion. The wind and rain chip away at the rocks on the surface, breaking them into smaller fragments. These smaller fragments of rocks, minerals, and organic material are carried by running water, waves, and glacial ice to other areas. As the sediment is moved, it continues to be broken down physically or chemically altered. The composition of sediment is based on the source of its materials. Over time, the sediment is deposited in different areas. It's the build-up of this sediment that creates the sedimentary rock. The solid materials settle out of the fluid or water. In fact, the word "sedimentary" comes from the Latin word sedimentum, which means "settling." Debris from weathered bedrock, particles of sand from dunes, mud from swamp floors, and surprisingly, household dust all become part of the sediment train. Now that the sediment has been deposited, it goes through the processes of compaction and cementation. 3 As new sediment piles on top of old sediment, the layers on the bottom are compacted. The weight of the overlying layers causes the lower layers to be squeezed and their volume to decrease. As more layers and time passes, the layers become cemented together by mineral deposits. These mineral deposits precipitate from the water and settle into the pore spaces between the sediment grains. These minerals provide the glue that holds the solid sedimentary rock together. This process takes place over many years. So it's 4 no wonder that geologists study sedimentary rocks for clues about our planet's past. Sedimentary rocks are classified into three main groups based on how they form and their composition. These groups are chemical sedimentary rocks, organic sedimentary rocks, and clastic sedimentary rocks. When scientists study sedimentary rocks, they find that accumulating sediment has two main sources: solid particles from weathered rocks and solid material produced from chemical weathering. Chemical sedimentary rocks form when dissolved minerals precipitate out of water. This occurs because the chemical concentration of the water and minerals changes as the water flows. Minerals also precipitate when evaporation occurs. Once the water evaporates, dissolved materials remain. These remaining minerals form rocks or evaporites. Common evaporates are gypsum and halite. 5 Organic sedimentary rocks form when the remains of plants and animals are buried before they decay. When plant remains are buried in places where there is very little oxygen in the water, such as swamps, the plants will not decay as quickly. Over many years the accumulating plant remains are compacted and cemented into carbonated matter. This is the process that forms coal. Rocks made from the remains of animals fall into the category of limestone. Chemical limestones form when chemicals precipitate out of the water. Organic limestones are made from marine animals like coral, clams, oysters, and plankton. These shelled organisms use minerals, calcite and aragonite from the sea water to build their shells. After the organisms die, their shells settle to the ocean floor. Over time, these shells accumulate and their layers become compacted and cemented. Chalk, another type of limestone, is also formed from the shells of marine animals. 6 Clastic sedimentary rocks are formed from the fragments of other rocks. These fragments are transported from their source by wind, water, and ice. Eventually, these fragments are deposited, compacted, and cemented into new sedimentary rocks. As with other types of sedimentary rocks, this process takes a long time. Geologists classify clastic sedimentary rocks according to their sediment or particle sizes. Conglomerate rocks have fragments that can be big as boulders or as tiny as mud particles. In fact, what gives conglomerate rocks their distinctive appearance is the combination of large round pebbles that are cemented together with finer sediments or minerals. Breccia rocks differ from conglomerate rocks because they have angular fragments and sharp corners. Like conglomerates, breccias have larger fragments that are cemented together by finer 7 fragments. In both types of rocks you can clearly see the pieces of sediment. On the opposite end of the spectrum, there are clastic rocks that are made of sand-sized grains. Sandstone, which also contains the mineral quartz, is formed when sand-sized grains are cemented together. Within the sandstones there are pores between the sand grains. These pores allow groundwater, natural gas, and other fluids to move through the rock. Shale has flaky clay-sized particles that have been compacted and cemented. The flat layers found in shale can be easily broken apart. Sedimentary rocks like sandstone, limestone, shale, and breccias have provided a lot of information about the Earth's surface. As geologists have learned, it's the rock on the bottom that holds the clues to our planet's history. 8 Copyright © 2013 edHelper Name _____________________________ Date ___________________ The Rock on the Bottom 1. Breccia is a form of ______. Clastic sedimentary rock Chemical sedimentary rock None of the below Organic sedimentary rock 2. How do chemical limestone sedimentary rocks differ from organic limestone sedimentary rocks? 3. Based on context clues, what does 4. What are evaporites and how are the word debris mean? 5. Compare compaction and cementation. they formed? 6. How do geologists classify sedimentary rocks? Name _____________________________ Date ___________________ The Rock on the Bottom 7. True/False: Sandstone rarely contains the mineral quartz as part of its composition. 8. Conglomerate rocks have a combination of ______. Angular fragments and finer grains of sediment Large round pebbles cemented together with finer grains of sediment Flaky clay-sized particles and finer sediment Smaller round pebbles cemented together with finer grains of sediment