A lava flow cools to form igneous rock.
All Earth's myriad rocks form by means of three main processes. Depending on the parameters, material may form into igneous, metamorphic or sedimentary rocks, which each in turn have their subcategories. This does not mean they will always remain in these forms. Rocks are in fact part of a vast cycle in which any formation of rock can be reformed into one of the other two given the right circumstances.
Igneous Rock
Granite is a common igneous rock.
Igneous rock forms from cooled magma or lava. If it forms in underground pockets, then it cools to form intrusive igneous rock, such as granite. If it forms above ground after a volcanic eruption, it becomes extrusive igneous rock, such as obsidian.
Sedimentary Rock
Coal forms from animal remains compressed into rock over thousands of years.
Erosion from water and weather carries material and deposits it downstream. These deposits form layers and as the pressure accumulates over thousands of years, the lower levels are pressed into sedimentary rock. There are three main types, depending on the type of debris form which they formed. Clastic sedimentary rocks, such as sandstone, are formed from inorganic debris. Organic sedimentary rocks, like limestone or coal, are formed from plant or animal remains. And chemical sedimentary rock, such as rock salt, is formed when dissolved materials come out of solution.
Metamorphic Rock
The surface of marble shows the irregular pattern of this metamorphic rock.
Metamorphic rocks form when other types of rocks undergo extreme heat, pressure and chemical processes deep in Earth's crust. Foliated metamorphic rocks, such as slate, form with visible layers or banded striations from pressure. Non-foliated metamorphic rocks are more irregular, such as marble.
The Rock Cycle
Each form of rock can turn into the other under the right circumstances, thereby continuing the ever-shifting rock cycle. Igneous and sedimentary rock become metamorphic rock through high pressure and heat. Sedimentary and metamorphic rock become igneous rock by melting back into magma to be cooled. And igneous and metamorphic rock become sedimentary rock through erosion.
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