The basic building blocks of all rocks, minerals, liquids, and gases are chemical elements, which are made up of various combinations of atoms. For example, when lead atoms combine with sulfur atoms, the mineral galena is formed.
A mineral is a naturally occurring solid with a definite chemical composition and crystal structure. Rocks are naturally occurring solids composed of one or more minerals. Rocks are identified by the minerals they contain and are grouped according to their origin into three major classes: sedimentary, igneous, and metamorphic. Each group is subdivided on the basis of texture and mineral composition.
Most of the rocks found in Kentucky are sedimentary. Sedimentary rocks are formed from (1) the weathering and transport of pre-existing rocks and (2) the chemical precipitation of sediments. Examples of sedimentary rocks are limestones, sandstones, and shales. Igneous rocks result from the cooling of molten rock or magma to create rocks like granites, basalts, and rhyolites. Metamorphic rocks have been physically and mineralogically changed by heat and pressure to form another type of rock; for example, the sedimentary rock limestone will become the metamorphic rock marble; the sedimentary rock shale will become the metamorphic rock slate; and the igneous rock granite will become the metamorphic rock gneiss (pronounced nice). Igneous and metamorphic rocks are not common in Kentucky but have been observed in glacial drift in northern Kentucky, and have been found as constituents in sandstones in eastern Kentucky and in very deep wells drilled throughout the State.