How Are the Different Types of Rocks Formed?


Quick Answer

Sedimentary rocks are formed from sediment that is accumulated and compacted over time, while metamorphic rocks are the result of heat and pressure deep within the Earth's surface. Igneous rocks are created when magma reaches the surface and then cools and hardens.

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How Are the Different Types of Rocks Formed?
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Full Answer

Sedimentary rock is where fossils are found. Most sedimentary rock is formed around or in water. Along with sand, pebbles and other inorganic materials found in sediment, shells, leaves and dead animals also fall into the mix. The organic material, such as the flesh, deteriorates, but the skeletons remain. As the sediment is compacted, the fossils are encased, and the mixture is turned into multilayered rock.

The name metamorphic comes from the word metamorphose, which means to change. A metamorphic rock is one that changes from one type of rock to another when subjected to the pressures and heat beneath the Earth's surface. When tectonic plates rub together, sometimes causing earthquakes, the pressure is even greater. This plate shifting also causes metamorphic rocks to form. One example is limestone changing into marble.

One way igneous rocks are formed is when molten lava erupts from beneath the Earth's surface and it cools and solidifies. Obsidian, which looks much like black glass, is one example. The other way is when rock, water and trapped gases are propelled out of volcano in an explosive eruption. As the rocks rapidly cool, trapped gases escape the material, leaving behind holes and gas bubbles. Pumice is an example.

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