Geologists classify rocks based on the materials from which they are made.
Basically, a rock is any naturally occurring consolidated material made up of one or more minerals. Geologists classify rocks based on composition and the conditions of formation. There are three basic types of rocks on Earth -- igneous, metamorphic and sedimentary. Sedimentary rocks are then further divided into three types -- clastic, chemical and organic -- based on sediment type. Does this Spark an idea?
Identification
Exposed to weathering over time, igneous rocks, formed from magma, break apart into bits and pieces. These pieces, which vary in size from grains to large chunks, are called clasts. Through time, these clasts break down into smaller and smaller particles. When the fragments accumulate together and go through the process of diagenesis, sedimentary rock is the result. Diagenetic processes include the transport of materials from one location to another, weathering, cementation as the minerals move into spaces between clasts, and lithification -- the compacting of material into solid rock.
Clastic
Clastic sedimentary rocks consist of consolidated sediments created by weathering and transport. Geologists classify the different kinds of clastic sedimentary rocks by the sizes of the clasts. For example, sandstones are made up of grains that are minutely small, while conglomerates may consist of clasts up to 2 meters or 6 1/2 feet in diameter. Examples of clastic sedimentary rocks in addition to sandstones and conglomerates include arkose (a sandstone with grains of feldspar), breccia, limestone, mudstone (a clay-silt mixture), shale and siltstone (silt particles).
Chemical
Chemical sedimentary rocks form as a result of the evaporation of water, especially from lake beds and seas, when dissolved minerals are left to dry. This process, known as chemical precipitation, occurs as a result of repeated cycles of flooding and evaporation. Over time, as increasing deposits of minerals form and through diagenetic processes, chemical sedimentary rocks are lithified. Gypsum and halite are two examples of chemical sedimentary rocks. Humans use halite, a mineral from dried up lake beds, as table salt.
Organic
Organic sedimentary rock is formed from debris left over from organic processes such as decomposition of once living organisms. Coal, formed as a result of the decomposition of plant material, is an example of organic sedimentary rock. Limestone, the most common organic sedimentary rock, is formed from skeletal material remains and shells of marine animals, including corals, that have undergone the lithification process. The main element in limestone is calcium carbonate, left behind by the decomposed organisms.
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