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How do rocks form?

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Quick Answer

Rocks are formed in three primary ways, and each method produces rocks with different characteristics. Rocks are either classified as sedimentary, igneous or metamorphic, depending on the manner in which they were formed. Most rocks encountered in everyday life are sedimentary, although some igneous rocks, such as granite, are also common.

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Sedimentary rocks are formed over thousands or millions of years as fine particles are compressed to form stones. As the layers accumulate, they impart incredible pressure on the bottom layers. Sandstone, limestone, shale and gypsum are all examples of sedimentary rocks. Sedimentary rocks often form in deltas and rivers because the constant flow of the river drags sediments from the river bed and deposits them in a confined area.

Igneous rocks are formed when molten rocks cool. Magma and lava spewing from volcanoes are the two most common source for igneous rocks. Granite, pumice, obsidian and scoria are all examples of igneous rocks.

Metamorphic rocks begin as either sedimentary or igneous rocks. Sometimes such rocks end up being moved into the deep parts of the Earth’s crust, where temperatures are high and pressures are extreme. These unusual conditions serve to heat and change the rocks. Marble, slate and gneiss are examples of metamorphic rocks.

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