Geologists classify rocks according to the processes by which they were formed. Igneous, metamorphic and sedimentary rocks have unique histories, and each category is further subdivided into more specific classes that place a rock firmly in the overall geological classification scheme.
Igneous rocks are those that formed in the undercrust and later extruded to the surface. Some igneous rocks flow to the surface as molten lava, while others erupt in less viscid masses called tephra. Other igneous rocks, called plutons, rise near the surface strata but stop and gradually cool before they are exposed by erosion. Obsidian is an example of an igneous rock that emerges as lava and cools, and granite is typically formed as a pluton under the crust.
Sedimentary rock is formed after rocks of other types erode away from their beds and are transported to a common dump site. The character of sedimentary rocks is largely determined by the grains that go into it. Sandstone is formed by large, angular grains that have been compacted over time, while mudstone is usually more finely grained and forms from silt deposits.
Metamorphic rock, such as marble, usually begins as a sedimentary deposit, but its minerals undergo transformation as heat and pressure force the crystals to reform inside the rock.