Sedimentary Rocks
The most common rocks on Earth’s surface are sedimentary rocks. These rocks are formed from particles that have been deposited by water, wind, glaciers, or landslides. These particles, or sediments, become compacted or cemented together over time. Sedimentary rocks form at or near the surface of Earth. They usually are easy to recognize because the sediments are deposited in layers. The arrangement of layers is known as stratification. Any rock that contains fossils is almost certainly a sedimentary rock. Fossils are the remains or any other evidence of living organisms, such as plants and animals, preserved in rock. Fossils are found in sedimentary rocks because when an organism dies, it can become buried in loose sediments. Although the soft parts of the organisms decay, the hard parts may become preserved as fossils. For example, the shells of clams and mussels are often found in sandstone, limestone, and shale. Even if the entire organism decays, an impression might be left in the rock. Plants and animal impressions can be seen when the rock is split apart. Sedimentary rocks can be classified as fragmental, organic, or chemical. The most common sedimentary rocks are fragmental, or clastic, rocks, which are formed from the weathered fragments of other rocks. The fragments are then carried by natural processes, such as running water. What makes the particles stick together? Water in oceans, lakes, and the ground contains substances that act as cement to hold the sediment together. These natural cements, including silica, calcite, and iron oxide, bind the fragments together into rock. The color of the rock is related to the cement. Silica or calcite causes the rocks to be gray or white. Iron oxide causes red or brown rocks. The properties of fragmental rocks depend on the size of sediment from which they are formed. Sediment ranges in size from large pebbles to microscopic flakes. Clay, for examples, is very fine sediment that is abundant in the most common...
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