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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
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
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.