why do sedimentary rocks form in layers

There are three main types of rock: sedimentary,
igneous and metamorphic. A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation. The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation. These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.

What are they like? Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are: The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed. The forces of weather break rocks into small pieces that are carried away and deposited elsewhere. These small pieces are often deposited in shallow seas or lakes as sediments. As the layers of deposits pile up, perhaps over millions of years, pressure from the weight of the sediments above turns the lower layers into solid rock. Sand may turn into sandstone; silt and clay become shale. Such rock, made of sediment, is called sedimentary rock. Geologists believe that sedimentary rock was forming three and a half billion years ago.

Sedimentary rock is formed when mineral matter of plants and animals settle out of water and, less commonly, air or ice. The most common materials for sedimentary rocks are fossils, formed when sediment covered dead plants and animals as the sediment changed into rock. The remains are outlines of the dead plants and animals. Some limestone is made entirely of fossils, microscopic sea life, and is deposited in oceans. Sedimentary rock covers about three fourths of the land area, and most of the ocean floor. Where the earth's crust is deformed or eroded, large areas of buried sedimentary rock may be exposed. In some places, such as the mouths of rivers, the sedimentary rock is 12,000 meters thick. After thousands of years, sedimentary rock is formed by many compact layers of rock building up pressure from water and the weight of other layers of overlying rock squeezing the rock until it molds together.

Water that trickles slowly through layers of coarse sand and gravel, deposits mineral cement around these particles, cementing the layers together to form rock. The layers, which vary from one another in composition or texture, distinguish sedimentary rock from igneous and metamorphic rock. For sedimentary rock to bed or to form broad, flat layers from the collection of grains of clay, silt, or sand settling in river valleys, or on the bottoms of lakes and oceans, there has to be a parent rock for the sediment to form around. This parent rock can be any type of rock, igneous, metamorphic, or sedimentary.

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