Diagenesis a term used in two ways, the first of which refers to the process of reordering the components of a substance into a new or different substance. The second, and more common usage, refers to all of the processes that sediments are subjected to or undergo during the period of time beginning with their deposition and continuing up to their conversion into rock. It also refers to the additional chemical and physical processes which can change those rocks, up to the point of metamorphism. In geology metamorphism is the alteration of rock through geologic processes involving extremes in temperature and pressure.
Geologists classify rocks into three categories, based on the circumstances of their formation. Sedimentary rocks are formed by the conversion of layers of sediment into rock, a process which requires a great deal of time and pressure. Igneous rocks are formed by the cooling of lava or magma. Magma and lava are two terms for the same type of material, but magma refers to molten rock when it is still beneath the Earth's surface, and lava refers to the molten rock after it emerges onto the surface. Metamorphic rocks are igneous or sedimentary rocks that have been transformed by extremes in pressure, angular forces, or temperature, but without completely melting the rock and absorbing it into the magma layer.
All of the processes, both chemical and physical, that sediments undergo as they are transformed into rock, as well as a number of processes that influence the characteristics of the rock are grouped together under the term diagenesis. These processes are primarily chemical in nature but also involve physical processes, such as stratification. Diagenesis does not, however, include weathering, which belongs to another class of geologic process.
Diagenetic processes can be very complex and are too numerous to list in an article of this size, but fall into several categories. One of the most important types of diagenetic processes is the conversion of biological material in the sediment layers into hydrocarbons, which is the beginning of the formation of crude oil and other fossil fuels. Fossilization is a diagenetic process that occurs on a molecular level as individual cells of an organism, particularly in its bones, have certain compounds replaced by calcite and other minerals, which are dissolved in water and deposited as the water filters through the layers of sediment.
Cementation is an important phase of diagenesis that results in the bonding of individual grains of sediment to each other. This is a chemical process which involves a dissolved mineral, such as calcite or silica, being precipitated from water as it filters through the sediment. The pressure of overlying layers of sediment results in a physical diagenetic process called compaction. This compaction coupled with the filtering of mineral laden water causes the grains of sediment to bond to the dissolved minerals. As the sediment dries out, the minerals harden, forming a natural cement. Sandstone is a common form of rock that is formed in this way.
Many more complex phases of diagenesis are possible as well, including the altering of the composition of the sediment layers by the filtering of water carrying dissolved minerals. New minerals can be formed by this process, and sometimes certain minerals or compounds may be leached out of sediments and replaced by others. Lithification, which occurs during diagenesis, is the actual conversion of the sediments to rock. Diagenesis can, however, continue after lithification.
Many of the process of diagenesis take thousands or millions of years. Geologists, paleontologists, anthropologists, and archaeologists, among others, analyze rocks to deduce the diagenetic processes that created them. In this way, they learn much about the past, including information about tectonic movements in the Earth's crust, environmental data, and other information about the formation of rocks and Earth's history.