Diagenesis in the context of "Sedimentology"

In geology, a rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere. The study of rocks involves multiple subdisciplines of geology, including petrology and mineralogy. It may be limited to rocks found on Earth, or it may include planetary geology that studies the rocks of other celestial objects.

Rocks are usually grouped into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed when magma cools in the Earth's crust, or lava cools on the ground surface or the seabed. Sedimentary rocks are formed by diagenesis and lithification of sediments, which in turn are formed by the weathering, transport, and deposition of existing rocks. Metamorphic rocks are formed when existing rocks are subjected to such high pressures and temperatures that they are transformed without significant melting.

Deposition is the geological process in which sediments, soil and rocks are added to a landform or landmass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.

This occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and friction, creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes. For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton, the deposition of which induced chemical processes (diagenesis) to deposit further calcium carbonate. Similarly, the formation of coal begins with the deposition of organic material, mainly from plants, in anaerobic conditions.

A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of plants or animals. In context of molecular biology, biogenic substances are referred to as biomolecules. They are generally isolated and measured through the use of chromatography and mass spectrometry techniques. Additionally, the transformation and exchange of biogenic substances can by modelled in the environment, particularly their transport in waterways.

The observation and measurement of biogenic substances is notably important in the fields of geology and biochemistry. A large proportion of isoprenoids and fatty acids in geological sediments are derived from plants and chlorophyll, and can be found in samples extending back to the Precambrian. These biogenic substances are capable of withstanding the diagenesis process in sediment, but may also be transformed into other materials. This makes them useful as biomarkers for geologists to verify the age, origin and degradation processes of different rocks.

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO₃), and dolomite rock (also known as dolostone), which is composed of dolomite (CaMg(CO₃)₂). They are usually classified on the basis of texture and grain size. Importantly, carbonate rocks can exist as metamorphic and igneous rocks, too. When recrystallized carbonate rocks are metamorphosed, marble is created. Rare igneous carbonate rocks even exist as intrusive carbonatites and, even rarer, there exists volcanic carbonate lava.

Carbonate rocks are also crucial components to understanding geologic history due to processes such as diagenesis in which carbonates undergo compositional changes based on kinetic effects. The correlation between this compositional change and temperature can be exploited to reconstruct past climate as is done in paleoclimatology. Carbonate rocks can also be used for understanding various other systems as described below.

Chert (/tʃɜːrt/) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO₂). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood. Where chert occurs in chalk or marl, it is usually called flint.

Chert is typically composed of the petrified remains of siliceous ooze, the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of diatoms, silicoflagellates, and radiolarians. Precambrian cherts are notable for the presence of fossil cyanobacteria. In addition to microfossils, chert occasionally contains macrofossils. However, some chert is devoid of any fossils.

Pyroclastic rocks are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroclasts. Pyroclastic rocks are a type of volcaniclastic deposit, which are deposits made predominantly of volcanic particles. Phreatic pyroclastic deposits are a variety of pyroclastic rock formed from volcanic steam explosions and are entirely composed of accidental clasts. Phreatomagmatic pyroclastic deposits are formed from explosive interaction of magma with groundwater. The word pyroclastic is derived from the Ancient Greek words πῦρ (pûr), meaning 'fire', and κλαστός (klastós), meaning 'broken'.

Unconsolidated accumulations of pyroclasts are described as tephra. Tephra may become lithified to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases (fumarolic alteration) or groundwater (e.g. hydrothermal alteration and diagenesis) and burial, or if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts and deform. This process is called welding.