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Sedimentary Rocks Chapter 4 Section 4
Sedimentary Rocks � Sediments are loose materials like rock fragments, mineral grains, and bits of shell. � Sediments come from already-existing rocks that are weathered and eroded. � Sedimentary rocks form when sediments are pressed and cemented together, or when minerals form from solutions. � Sedimentary rocks form as layers. The oldest layers are at the bottom because they were deposited first. -Sometimes the forces within the Earth overturn layers of rock, and the oldest are no longer on the bottom.
Sedimentary Rocks �over 75% of the surface are sedimentary rocks!
Classifying Sedimentary Rocks. �Sedimentary rocks are classified by their composition and the manner in which they are formed. �Sedimentary rocks are classified as detrital (clastic), chemical, or organic (nonclastic).
Detrital Sedimentary Rocks �Detrital, or clastic sedimentary rocks are made from the broken fragments of other rocks. These loose sediments are compacted and cemented together to form solid rock. �The process that breaks rocks into smaller pieces is called weathering, . -Rocks are weathered by things such as air water and ice. -Sediments are classified by size. Clay is the smallest sediment, followed by silt, then sand, and then gravel, the largest sediment.
Detrital Sedimentary Rocks � Erosion moves sediments to a new location where they are deposited. Pressure from the upper layers pushes down on the lower layers of sediment. � If the sediments are small, they can stick together to form a solid rock. The process is called compaction. � If the sediments are large, pressure can’t make them stick together. Cementation occurs when minerals like quartz and calcite, are deposited by water between pieces of sediment. The minerals act like glue and form a detrital sedimentary rock. � Conglomerate, sandstone, siltstone, and shale are all detrital sedimentary rocks named for the shapes and sizes of sediments that formed them.
Examples of Detrital Sedimentary Rocks
Compaction and Cementation
Chemical Sedimentary Rocks �Chemical sedimentary rocks form when dissolved minerals come out of solution. �Minerals collect when seas or lakes evaporate. The deposits of minerals that come out of solution form sediments and rock. �Gypsum and rock salt form when water that is rich in dissolved salts evaporates. Halite forms rock salt.
Chemical Sedimentary Rocks �Calcium carbonate is carried in solution in ocean water. When calcium carbonate comes out of solution it forms calcite. Limestone is a chemical sedimentary rock that is formed as crystals of calcite grow together. �Large areas of the United States have limestone bedrock because seas covered much of the country throughout geologic history.
Chemical Sedimentary Rock Example
Organic Sedimentary Rocks �Organic sedimentary rocks are made of the remains of once living things. �Fossil-rich limestone is made from the remains of once living organisms. -If a rock is made completely out of shell fragments that you can see it is called coquina. -Chalk is another organic sedimentary rock made of microscope shells.
Organic Sedimentary Rocks �Coal is an organic sedimentary rock that is formed when pieces of dead plants are buried under other sediments in swamps. The sediments are compacted over millions of years to form coal. �Chemical and organic sedimentary rocks are also called nonclastic sedimentary rocks, because they are not made up of broken fragments of other rocks.
Organic Sedimentary Rock Example