Types of Rocks Essay

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Rocks will start within one form and change through the course of time into another rock type. There are three different types of rocks in the world: igneous, sedimentary, and metamorphic. Each type of rock is formed in its own unique way. First, igneous rocks can be intrusive or extrusive. Intrusive igneous rocks, also known as plutonic rocks, are formed inside earth's crust. Molten rock rises and fills openings or melts overlaying rocks. This eventually hardens creating an intrusive igneous rock. An extrusive igneous rock, or volcanic rock, is created by magma rising the earth's surface. Magma is known as lava when it reaches the surface. After the magma reaches the surface, it runs down and cools. As the lava cools it hardens and becomes an extrusive igneous rock. Igneous rocks are classified depending on the texture and mineral or chemical content. Intrusive rocks cool very slowly due to the high insulation they have. When rocks cool slowly, it forms rather large and well developed crystals. Some extrusive rocks cool and solidify in the air after an explosive volcanic eruption. When lava cools quickly, the crystal in the rock are not well developed and usually unseen. Examples of igneous rocks are: obsidian, pumice, basalt, and granite (Igneous Rocks). Next, sedimentary rocks are formed at or near the earth's surface. Weathered pieces of pre-existing rocks and organic debris make up a sedimentary rock. This is the type of rock that fossils are found in. Sedimentary rocks are classified into three separate groups: clastic, chemical, or organic. Clasitic rocks are classified by their grain size. Shale, siltstone, sandstone, and conglomerate. Mineral solutions form chemical sedimentary rocks. They are classified by thier mineral composition. Rock salt, rock gypsum, and crystalline limestone are common chemical sedimentary rocks. Organic sedimentary rocks are

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