Diagenesis refers to the processes that sediments undergo to become rock, including chemical and physical changes. It can also involve the formation of hydrocarbons and fossilization. Cementation and compaction are important steps in diagenesis, and the process can continue even after lithification. Diagenesis can take thousands or millions of years, and studying rocks can provide information about the Earth’s history.
Diagenesis a term used in two ways, the first of which refers to the process of rearranging the components of a substance into a new or different substance. The second, and more common usage, refers to all the processes to which sediments are subjected or undergo in the period of time that begins with their deposition and continues until their transformation into rock. It also refers to the additional chemical and physical processes that can change those rocks, up to and including metamorphism. In geology metamorphism is the alteration of rock through geological processes involving extremes of temperature and pressure.
Geologists classify rocks into three categories, based on the circumstances of their formation. Sedimentary rocks are formed by converting layers of sediment into rock, a process that takes a lot 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 under the earth’s surface and lava refers to molten rock after it has emerged on the surface. Metamorphic rocks are igneous or sedimentary rocks that have been transformed by extreme pressure, angular forces, or temperature, but without completely melting the rock and absorbing it into the magma layer.
All the processes, both chemical and physical, which sediments undergo when they transform into rock, as well as a number of processes which influence the characteristics of the rock are grouped under the term diagenesis. These processes are mainly chemical in nature, but also involve physical processes, such as stratification. Diagenesis, however, does not include erosion, which belongs to another class of geological processes.
Diagenetic processes can be very complex and too numerous to list in an article of this size, but they fall into several categories. One of the most important types of diagenetic processes is the conversion of biological material in sediment layers into hydrocarbons, which is the initiation of the formation of crude oil and other fossil fuels. Fossilization is a diagenetic process that occurs at the molecular level as the individual cells of an organism, particularly in its bones, have certain compounds replaced by calcite and other minerals, which dissolve in water and settle as the water seeps through the sediment layers.
Cementation is an important step in diagenesis that results in the bonding of individual sediment grains to each other. This is a chemical process involving a dissolved mineral, such as calcite or silica, which is precipitated by water as it seeps through the sediment. The pressure of the overlying sediment layers causes a physical diagenetic process called compaction. This compaction coupled with the filtering of the mineral laden water causes the sediment grains to bind to the dissolved minerals. As the sediment dries, the minerals harden, forming a natural cement. Sandstone is a common form of rock that forms this way.
Many more complex steps of diagenesis are also possible, including altering the composition of sediment layers by filtering the water carrying dissolved minerals. New minerals can be formed by this process and sometimes some minerals or compounds can be leached from the sediments and replaced by others. Lithification, which occurs during diagenesis, is the actual conversion of sediment into rock. Diagenesis can, however, continue after lithification.
Many of the diagenesis processes 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 a great deal about the past, including information about tectonic movements in the Earth’s crust, environmental data, and other information about rock formation and the Earth’s history.
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