Diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation, at temperatures and pressures less than that required for the formation of metamorphic rocks. It does not include changes from weathering, it is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification. This process excludes surface metamorphism; these changes happen at low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures. Hydrothermal solutions, meteoric groundwater, permeability and time are all influential factors. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis.
Porosity decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization. The study of diagenesis in rocks is used to understand the geologic history they have undergone and the nature and type of fluids that have circulated through them. From a commercial standpoint, such studies aid in assessing the likelihood of finding various economically viable mineral and hydrocarbon deposits; the process of diagenesis is important in the decomposition of bone tissue. The term diagenesis meaning "across generation", is extensively used in geology. However, this term has filtered into the field of anthropology and paleontology to describe the changes and alterations that take place on skeletal material. Diagenesis "is the cumulative physical and biological environment. In order to assess the potential impact of diagenesis on archaeological or fossil bones, many factors need to be assessed, beginning with elemental and mineralogical composition of bone and enveloping soil, as well as the local burial environment.
The composite nature of bone, comprising one-third organic and two thirds mineral renders its diagenesis more complex. Alteration occurs at all scales from molecular loss and substitution, through crystallite reorganization and microstructural changes, in many cases, to disintegration of the complete unit. Three general pathways of the diagenesis of bone have been identified: chemical deterioration of the organic phase. Chemical deterioration of the mineral phase. Biological attack of the composite, they are as follows: The dissolution of collagen depends on time and environmental pH. At high temperatures, the rate of collagen loss will be accelerated and extreme pH can cause collagen swelling and accelerated hydrolysis. Due to the increase in porosity of bones through collagen loss, the bone becomes susceptible to hydrolytic infiltration where the hydroxyapatite, with its affinity for amino acids, permits charged species of endogenous and exogenous origin to take up residence; the hydrolytic activity plays a key role in the mineral phase transformations that exposes the collagen to accelerated chemical- and bio-degradation.
Chemical changes affect crystallinity. Mechanisms of chemical change, such as the uptake of F− or CO3− may cause recrystallization where hydroxyapatite is dissolved and re-precipitated allowing for the incorporation of substitution of exogenous material. Once an individual has been interred, microbial attack, the most common mechanism of bone deterioration, occurs rapidly. During this phase, most bone collagen is lost and porosity is increased; the dissolution of the mineral phase caused by low pH permits access to the collagen by extracellular microbial enzymes thus microbial attack. When animal or plant matter is buried during sedimentation, the constituent organic molecules break down due to the increase in temperature and pressure; this transformation occurs in the first few hundred meters of burial and results in the creation of two primary products: kerogens and bitumens. It is accepted that hydrocarbons are formed by the thermal alteration of these kerogens. In this way, given certain conditions kerogens will break down to form hydrocarbons through a chemical process known as cracking, or catagenesis.
A kinetic model based on experimental data can capture most of the essential transformation in diagenesis, a mathematical model in a compacting porous medium to model the dissolution-precipitation mechanism. These models have been intensively applied in real geological applications. Diagenesis has been divided, based on hydrocarbon and coal genesis into: eodiagenesis and telodiagenesis. During the early or eodiagenesis stage shales lose pore water, little to no hydrocarbons are formed and coal varies between lignite and sub-bituminous. During mesodiagenesis, dehydration of clay minerals occurs, the main development of oil genesis occurs and high to low volatile bituminous coals are formed. During telodiagenesis, organic matter undergoes cracking and dry gas is produced. Early diagenesis in newly formed aquatic sediments is mediated by microorganisms using different electron acceptors as part of their metabolism. O
Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way sand becomes "sandstone", gravel becomes "conglomerate" or "breccia". Cementation occurs as part of the lithification of sediments. Cementation occurs below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are required to complete the cementation process. Common mineral cements include calcite, quartz or silica phases like cristobalite, iron oxides, clay minerals, but other mineral cements occur. Cementation is continuous in the groundwater zone, so much so that the term "zone of cementation" is sometimes used interchangeably. Cementation occurs in fissures or other openings of existing rocks and is a dynamic process more or less in equilibrium with a dissolution or dissolving process.
Cement found on the sea floor is aragonite and can take different textural forms. These textural forms include pendant cement, meniscus cement, isopachous cement, needle cement, botryoidal cement, blocky cement, syntaxial rim cement, coarse mosaic cement; the environment in which each of the cements is found depends on the pore space available. Cements that are found in phreatic zones include: isopachous and syntaxial rim cements; as for calcite cementation, which occurs in meteoric realms, the cement is produced by the dissolution of less stable aragonite and high-Mg calcite. Classifying rocks while using the Folk classification depends on the matrix, either sparry or micritic. Beachrock is a type of carbonate beach sand, cemented together by a process called synsedimentary cementation. Beachrock may contain meniscus cements or pendant cements; as the water between the narrow spaces of grains drains from the beachrock, a small portion of it is held back by capillary forces, where meniscus cement will form.
Pendant cements form on the bottom of grains. Hardgrounds are hard crusts of carbonate material that form on the bottom of the ocean floor, below the lowest tide level. Isopachous cement forms in subaqueous conditions where the grains are surrounded by water. Carbonate cements can be formed by biological organisms such as Sporosarcina pasteurii, which binds sand together given organic compounds and a calcium source. Boggs, Sam Jr. 2006, Principles of Sedimentology and Stratigraphy, 4th ed. New Jersey, Pearson Education Inc. Boggs, Sam, Jr. 2011, "Principles of Sedimentology and Stratigraphy", 5th ed. New Jersey, Pearson Education Inc. Chiung-Wen Chou, Eric Seagren, Ahmet Aydilek, Timothy Maugel. "Bacterially-Induced Calcite Precipitation via Ureolysis", American Society for Microbiology 11 November 2008 Retrieved 20 February 2010
In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, ice, air, plants and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind erosion, zoogenic erosion, anthropogenic erosion; the particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Natural rates of erosion are controlled by the action of geological weathering geomorphic drivers, such as rainfall; the rates at which such processes act control. Physical erosion proceeds fastest on steeply sloping surfaces, rates may be sensitive to some climatically-controlled properties including amounts of water supplied, wind speed, wave fetch, or atmospheric temperature.
Feedbacks are possible between rates of erosion and the amount of eroded material, carried by, for example, a river or glacier. Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, which control the arrival and emplacement of material at a new location. While erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At well-known agriculture sites such as the Appalachian Mountains, intensive farming practices have caused erosion up to 100x the speed of the natural rate of erosion in the region. Excessive erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses.
Water and wind erosion are the two primary causes of land degradation. Intensive agriculture, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall, the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, gully erosion. Splash erosion is seen as the first and least severe stage in the soil erosion process, followed by sheet erosion rill erosion and gully erosion. In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles; the distance these soil particles travel can be as much as 0.6 m vertically and 1.5 m horizontally on level ground. If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs.
If the runoff has sufficient flow energy, it will transport loosened soil particles down the slope. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are of the order of a few centimetres or less and along-channel slopes may be quite steep; this means that rills exhibit hydraulic physics different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and flows in narrow channels during or after heavy rains or melting snow, removing soil to a considerable depth. Valley or stream erosion occurs with continued water flow along a linear feature; the erosion is both downward, deepening the valley, headward, extending the valley into the hillside, creating head cuts and steep banks.
In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain; the stream gradient becomes nearly flat, lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone
Lithic fragment (geology)
Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named by Bill Dickinson in 1970. Lithic fragments can be derived from igneous or metamorphic rocks. A lithic fragment is defined using the Gazzi-Dickinson point-counting method and being in the sand-size fraction. Sand grains in sedimentary rocks that are fragments of larger rocks that are not identified using the Gazzi-Dickinson method are called rock fragments instead of lithic fragments. Sandstones rich in lithic fragments are called lithic sandstones; these can include granular, microlitic and vitric. These correlations between composition and volcanic lithic fragment type are approximate, at best. By definition, intrusive igneous rock fragments can not be considered lithic fragments; these can include shale siltstone fragments, chert. These can include fine-grained schist and phyllite fragments, among others
Weathering is the breaking down of rocks and minerals as well as wood and artificial materials through contact with the Earth's atmosphere and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, snow, wind and gravity and being transported and deposited in other locations. Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water and pressure; the second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals known as biological weathering in the breakdown of rocks and minerals. While physical weathering is accentuated in cold or dry environments, chemical reactions are most intense where the climate is wet and hot.
However, both types of weathering occur together, each tends to accelerate the other. For example, physical abrasion decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions; the various agents act in concert to convert primary minerals to secondary minerals and release plant nutrient elements in soluble forms. The materials left over after the rock breaks down combined with organic material creates soil; the mineral content of the soil is determined by the parent material. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition. Physical weathering called mechanical weathering or disaggregation, is the class of processes that causes the disintegration of rocks without chemical change; the primary process in physical weathering is abrasion. However and physical weathering go hand in hand. Physical weathering can occur due to temperature, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.
Abrasion by water and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material. Plant roots pry them apart, resulting in some disintegration. However, such biotic influences are of little importance in producing parent material when compared to the drastic physical effects of water, ice and temperature change. Thermal stress weathering, sometimes called insolation weathering, results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals; as some minerals expand more than others, temperature changes set up differential stresses that cause the rock to crack apart. Because the outer surface of a rock is warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers.
This process may be accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments. Thermal stress weathering comprises thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night; the repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is called exfoliation. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized heat can expand a boulder; the thermal heat from wildfire can cause significant weathering of rocks and boulders, heat can expand a boulder and thermal shock can occur.
The differential expansion of a thermal gradient can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material. If nothing stops this crack from propagating through the material, it will result in the object's structure to fail. Frost weathering called ice wedging or cryofracturing, is the collective name for several processes where ice is present; these processes include frost frost-wedging and freeze -- thaw weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses nea
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, in sediment deposits. Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size and cementing material composition, texture; the classification factors are useful in determining a sample's environment of deposition. An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream. Grain size varies from clay in claystones; the Krumbein phi scale numerically orders these terms in a logarithmic size scale.
Siliciclastic rocks are clastic noncarbonate rocks that are composed exclusively of silicon, either as forms of quartz or as silicates. The composition of siliciclastic sedimentary rocks includes the chemical and mineralogical components of the framework as well as the cementing material that make up these rocks. Boggs divides them into four categories. Major minerals can be categorized into subdivisions based on their resistance to chemical decomposition; those that possess a great resistance to decomposition are categorized as stable, while those that do not are considered less stable. The most common stable mineral in siliciclastic sedimentary rocks is quartz. Quartz makes up 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. Less stable minerals present in this type of rocks are feldspars, including both potassium and plagioclase feldspars. Feldspars comprise a lesser portion of framework grains and minerals, they only make up about 15 percent of framework grains in 5 % of minerals in shales.
Clay mineral groups are present in mudrocks but can be found in other siliciclastic sedimentary rocks at lower levels. Accessory minerals are associated with those whose presence in the rock are not directly important to the classification of the specimen; these occur in smaller amounts in comparison to the quartz, feldspars. Furthermore, those that do occur are heavy minerals or coarse grained micas. Rock fragments occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of the composition of sandstone, they make up most of the gravel size particles in conglomerates but contribute only a small amount to the composition of mudrocks. Though they sometimes are, rock fragments are not always sedimentary in origin, they can be metamorphic or igneous. Chemical cements are predominantly found in sandstones; the two major types, are silicate carbonate based. The majority of silica cements are composed of quartz but can include, opal and zeolites. Composition includes the chemical and mineralogic make-up of the single or varied fragments and the cementing material holding the clasts together as a rock.
These differences are most used in the framework grains of sandstones. Sandstones rich in quartz are called quartz arenites, those rich in feldspar are called arkoses, those rich in lithics are called lithic sandstones. Siliciclastic sedimentary rocks are composed of silicate particles derived by the weathering of older rocks and pyroclastic volcanism. While grain size and cementing material composition, texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories; the term clay is used to classify particles smaller than.0039 millimeters. However, term can be used to refer to a family of sheet silicate minerals. Silt refers to particles that have a diameter between.0039 millimeters. The term mud is used when silt particles are mixed in the sediment. Furthermore, particles that reach diameters between.062 and 2 millimeters fall into the category of sand. When sand is cemented together and lithified it becomes known as sandstone.
Any particle, larger than two millimeters is considered gravel. This category includes pebbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates. Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are held together by a finer grained matrix; these rocks are subdivided into conglomerates and breccias. The major characteristic that divides these two categories is the amount of rounding; the gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. Conglomerates are common in stratigraphic successions of most, if not all ages but only make up one percent or less, by weight of the total sedimentary rock mass. In terms or origin and depositional mechanisms they are similar to sandstones; as a result, the two categories contain the same sedimentary structures. Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often