All rocks are aggregates of minerals, and are classified as either of igneous, sedimentary or metamorphic origin. Sedimentary rocks are formed when minerals are deposited and then accumulate to form the rock. The type and characteristic of each sedimentary rock depends upon its mineral content and method of formation. Because sedimentary rocks are formed by deposition, they are often found in layers or strata, and are also known by the synonym ‘stratified rocks’. Although there are many specific kinds of sedimentary rocks, geologists classify them into three basic groups.
The first group are known as clastic sedimentary rocks. These are formed out of the physically weathered debris from pre-existing rocks. The actions of wind, water and ice physically erode mineral particles from existing igneous, sedimentary or metamorphic rocks. This mineral sediment is then transported by these same natural forces, will eventually be deposited and in turn form new sedimentary rock. Examples of clastic sedimentary rocks include shales, sandstones and breccia, which range from containing very fine particles to relatively large particles cemented together.
Sedimentary rocks can also be deposited through chemical processes, hence the term chemical sedimentary rocks for the second group. These rocks generally form when minerals precipitate out of solution. Halite, or rock salt, is one example of this process, with large deposits left behind when ancient seas evaporated. Some types of limestone are also deposited by chemical precipitation. Travertine is one such, formed by the direct and rapid deposition of calcite. The Colosseum in Rome is constructed mainly from travertine. One of the ways the iron ore hematite can be formed is by precipitation from mineral rich water.
The third group of sedimentary rocks are the organic sedimentary rocks. As the name suggests, living organisms are involved in their formation. The majority of limestones were formed by the deposition of the calcium rich shells and skeletons of marine organisms, living in shallow pre-historic seas. Limestone may be ‘pure’, containing predominately calcium carbonate, or may have varying degrees of impurity. Some limestones contain silica, again obtained from the bodies of microscopic marine life. Organic limestones often contain distinct shell fragments and micro-fossils. Coal is an organic sedimentary rock formed by the deposition of plant material in oxygen deficient conditions. The plant material cannot decay normally, so it gradually builds up into layers. Then covered by layers of mud or sand, it is compacted and over millions of years is formed into coal. The highest grades of coal consist almost entirely of carbon with some impurities. Coal is distinct from most sedimentary rocks in that it is combustible.
All rocks were originally igneous in origin; various mechanical and chemical processes formed the variety of sedimentary and metamorphic rocks seen today. All these processes are still ongoing, and rocks are still being eroded, transported and deposited in a continuous cycle.