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Petrology is the study of rocks, their composition, and the conditions that lead to their creation. Rocks are composed of minerals. There are 3 main types of rock, igneous, metamorphic, and sedimentary.
Igneous rocks are formed as melted material from the earth's interior cools to become rock. This cooling may have occurred within the earth. If this is the case then the crystal size will generally be large enough to see and the rock is called a plutonic rock. If the rock is forced out on to the surface of the earth then it will cool fairly quickly and there will not be time for minerals to form. This type of rock is called volcanic rock. Most igneous rocks fit into these two very general categories, but there are many types of igneous rocks as you will see below.
Sedimentary rocks are formed as layers of debris are deposited or precipitated and cemented to form rock. These rocks may be composed materials like sand, mud, pebbles, shells, or coral that has been cemented together or be created by precipatation of minerals from the sea. The sedimentary rocks that are composed of rock particles such as sand or mud are called clastic rocks. Those that form by precipatation are called nonclastic.
Metamorphic rocks are those that have been changed
as a result of heat or pressure. For example a sandstone (a sedimentary
rock composed of cemented sand grains) may become recrystallized under
increased pressure and temperature to become a quartzite. There are
two major types of metamorphic rock; foliated and nonfoliated. Foliations
are layers so if a metamorphic rocks shows layering or banding it is a
foliated rock, if not then it is a nonfoliated rock.
