Clastic rocks are composed of fragments, or clasts, of pre-existing
minerals
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed ...
and rock. A clast is a fragment of
geological detritus
Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephe ...
,
[Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3] chunks, and smaller grains of rock broken off other rocks by
physical weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
.
[Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-5] Geologists use the term clastic to refer to
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s and particles in
sediment transport
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural system ...
, whether in
suspension
Suspension or suspended may refer to:
Science and engineering
* Suspension (topology), in mathematics
* Suspension (dynamical systems), in mathematics
* Suspension of a ring, in mathematics
* Suspension (chemistry), small solid particles suspende ...
or as
bed load
The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load.
Bed load moves by rolling, sliding, and/or saltating (hop ...
, and in
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
deposits.
Sedimentary clastic rocks
Clastic
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s are rocks composed predominantly of broken pieces or ''clasts'' of older
weathered
''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...
and
eroded
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...
rocks. Clastic sediments or sedimentary rocks are classified based on
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...
, clast and cementing material (
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
) composition, and texture. The classification factors are often 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
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...
from solid rock upstream.
Grain size varies from
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
in
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
s and
claystone
Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...
s; through
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
in
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...
s;
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
in
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s; and
gravel
Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone.
Gravel is classifi ...
,
cobble, to
boulder
In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive.
In ...
sized fragments in
conglomerates and
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s. The
Krumbein phi (φ) scale numerically orders these terms in a logarithmic size scale.
Siliciclastic sedimentary rocks
''Siliciclastic'' rocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms of
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...
or as silicates.
Composition
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, accessory minerals, rock fragments, and chemical sediments.
[Boggs, Jr., Sam. Principles of Sedimentology and Stratigraphy. Pearson Prentice Hall: Upper Saddle River, New Jersey, 2006]
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 (SiO
2).
Quartz makes up approximately 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
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
s, including both potassium and plagioclase feldspars.
Feldspars comprise a considerably lesser portion of framework grains and minerals. They only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. Clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels.
Accessory minerals are associated with those whose presence in the rock are not directly important to the classification of the specimen. These generally occur in smaller amounts in comparison to the quartz, and feldspars. Furthermore, those that do occur are generally heavy minerals or coarse grained micas (both
muscovite
Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...
and
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more ...
).
Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of the composition of sandstone. They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of
mudrocks. Though they sometimes are, rock fragments are not always sedimentary in origin. They can also be
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
or
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
.
Chemical cements vary in abundance but are predominantly found in sandstones. The two major types are silicate based and carbonate based. The majority of silica cements are composed of quartz, but can include
chert
Chert () 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 ...
,
opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
, feldspars and
zeolites
Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
.
Composition includes the
chemical
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...
and
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
ogic make-up of the single or varied fragments and the cementing material (
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
) holding the clasts together as a rock. These differences are most commonly used in the framework grains of sandstones. Sandstones rich in quartz are called
quartz arenite
A quartz arenite or quartzarenite is a sandstone composed of greater than 90% detrital quartz. Quartz arenites are the most mature sedimentary rocks possible, and are often referred to as ultra- or super-mature, and are usually cemented by sili ...
s, those rich in feldspar are called
arkoses, and those rich in
lithics
Lithic may refer to:
*Relating to stone tools
**Lithic analysis, the analysis of stone tools and other chipped stone artifacts
**Lithic core, the part of a stone which has had flakes removed from it
**Lithic flake, the portion of a rock removed to ...
are called
lithic sandstones.
Classification
Siliciclastic sedimentary rocks are composed of mainly silicate particles derived from the weathering of older rocks and pyroclastic volcanism. While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories:
conglomerates,
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s, and
mudrocks. The term ''clay'' is used to classify particles smaller than .0039 millimeters. However, the term can also be used to refer to a family of sheet silicate minerals.
''Silt'' refers to particles that have a diameter between .062 and .0039 millimeters. The term ''mud'' is used when clay and silt particles are mixed in the sediment; ''mudrock'' is the name of the rock created with these sediments. 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 that is larger than two millimeters is considered gravel. This category includes
pebbles, cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates.
= Conglomerates and breccias
=
![Breccia mcr1](https://upload.wikimedia.org/wikipedia/commons/4/4a/Breccia_mcr1.jpg)
Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix.
[Neuendorf, Klaus; Mehl, James; Jackson, Julia Glossary of Geology, Fifth Edition. American Geological Institute: Alexandria, VA; 2005.] These rocks are often 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 of origin and depositional mechanisms they are very similar to sandstones. As a result, the two categories often contain the same sedimentary structures.
=Sandstones
=
![Lower Antelope Canyon 478](https://upload.wikimedia.org/wikipedia/commons/3/32/Lower_Antelope_Canyon_478.jpg)
Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. These sand-size particles are often
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...
but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. Classification schemes vary widely, but most geologists have adopted the
Dott scheme,
[Dott, R. H., Wacke, graywacke and matrix – What Approach to Immature Sandstone Classification: Journal of Sedimentary Petrology, v. 34, pp. 625–32., 1996.] which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains.
=Mudrocks
=
Rocks that are classified as mudrocks are very fine grained. Silt and clay represent at least 50% of the material that mudrocks are composed of. Classification schemes for mudrocks tend to vary, but most are based on the grain size of the major constituents. In mudrocks, these are generally silt, and clay.
According to Blatt, Middleton and Murray
[Blatt, h., Middleton, G. V. & Murray, R. C. 1972. Origin of Sedimentary Rocks. Prentice Hall Inc., Englewood Cliffs, 634 pp.] mudrocks that are composed mainly of silt particles are classified as siltstones. In turn, rocks that possess clay as the majority particle are called claystones. In geology, a mixture of both silt and clay is called mud. Rocks that possess large amounts of both clay and silt are called mudstones. In some cases the term shale is also used to refer to mudrocks and is still widely accepted by most. However, others have used the term shale to further divide mudrocks based on the percentage of clay constituents. The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds. The more clay present in a given specimen, the more laminated a rock is. Shale, in this case, is reserved for mudrocks that are laminated, while mudstone refers those that are not.
Image:Red mudrock.JPG, Red mudrock
Image:MarcellusShaleCloseUp.jpg, Black Shale
Diagenesis of siliciclastic sedimentary rocks
Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. The process of turning loose sediment into hard sedimentary rocks is called
lithification
Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
. During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. The primary physical process in lithification is compaction. As sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. Burial continues and the weight of overlying sediments causes an increase in temperature and pressure. This increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. Porosity is further reduced by the precipitation of minerals into the remaining pore spaces.
The final stage in the process is
diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
and will be discussed in detail below.
Cementation
Cementation is the diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through the deposition or precipitation of minerals in the spaces between the individual grains of sediment.
Cementation can occur simultaneously with deposition or at another time. Furthermore, once a sediment is deposited, it becomes subject to cementation through the various stages of diagenesis discussed below.
Shallow burial (eogenesis)
Eogenesis refers to the early stages of diagenesis. This can take place at very shallow depths, ranging from a few meters to tens of meters below the surface. The changes that occur during this diagenetic phase mainly relate to the reworking of the sediments. Compaction and grain repacking,
bioturbation, as well as mineralogical changes all occur at varying degrees.
Due to the shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage. Organisms rework sediment near the depositional interface by burrowing, crawling, and in some cases sediment ingestion. This process can destroy sedimentary structures that were present upon deposition of the sediment. Structures such as lamination will give way to new structures associated with the activity of organisms. Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur. This mainly involves the precipitation of new minerals.
=Mineralogical changes during eogenesis
=
Mineralogical changes that occur during eogenesis are dependent on the environment in which that sediment has been deposited. For example, the formation of
pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
is characteristic of reducing conditions in marine environments.
Pyrite can form as cement, or replace organic materials, such as wood fragments. Other important reactions include the formation of
chlorite,
glauconite
Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance.
It crystallizes with a monoclinic geometry. Its name is derived from the Gre ...
,
illite
Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...
and
iron oxide (if oxygenated pore water is present). The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions. In non marine environments oxidizing conditions are almost always prevalent, meaning iron oxides are commonly produced along with
kaolin
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
group clay minerals. The precipitation of quartz and calcite cements may also occur in non marine conditions.
Deep burial (mesogenesis)
=Compaction
=
As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning. This causes increased pressure between grains thus increasing the solubility of grains. As a result, the partial dissolution of silicate grains occurs. This is called pressure solutions. Chemically speaking, increases in temperature can also cause chemical reaction rates to increase. This increases the solubility of most common minerals (aside from evaporites).
Furthermore, beds thin and porosity decreases allowing cementation to occur by the precipitation of silica or carbonate cements into remaining pore space.
In this process minerals crystallize from watery solutions that percolate through the pores between grain of sediment. The cement that is produced may or may not have the same chemical composition as the sediment. In sandstones, framework grains are often cemented by silica or carbonate. The extent of cementation is dependent on the composition of the sediment. For example, in lithic sandstones, cementation is less extensive because pore space between framework grains is filled with a muddy matrix that leaves little space for precipitation to occur. This is often the case for mudrocks as well. As a result of compaction, the clayey sediments comprising mudrocks are relatively impermeable.
=Dissolution
=
Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial. Conditions that encourage this are essentially opposite of those required for cementation. Rock fragments and silicate minerals of low stability, such as
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
feldspar,
pyroxenes, and
amphiboles, may dissolve as a result of increasing burial temperatures and the presence of organic acids in pore waters. The dissolution of frame work grains and cements increases porosity particularly in sandstones.
=Mineral replacement
=
This refers to the process whereby one mineral is dissolved and a new mineral fills the space via precipitation. Replacement can be partial or complete. Complete replacement destroys the identity of the original minerals or rock fragments giving a biased view of the original mineralogy of the rock.
Porosity can also be affected by this process. For example, clay minerals tend to fill up pore space and thereby reducing porosity.
Telogenesis
In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event or
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
.
When uplift occurs, it exposes buried deposits to a radically new environment. Because the process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water. Under these conditions, framework grains and cement are again subjected to dissolution and in turn increasing porosity. On the other hand, telogenesis can also change framework grains to clays, thus reducing porosity. These changes are dependent on the specific conditions that the rock is exposed as well as the composition of the rock and pore waters. Specific pore waters, can cause the further precipitation of carbonate or silica cements. This process can also encourage the process of oxidation on a variety of iron bearing minerals.
Sedimentary breccias
Sedimentary
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either:
# In submarine
debris flow
Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally ...
s,
avalanches
An avalanche is a rapid flow of snow down a slope, such as a hill or mountain.
Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earth ...
, mud flow or mass flow in an aqueous medium. Technically,
turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites wer ...
s are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.
# As angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified
colluvium
Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combinati ...
. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in
grabens.
In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from
drilling information. Sedimentary breccias are an integral host rock for many
sedimentary exhalative deposits
Sedimentary exhalative deposits (SEDEX or SedEx deposits) are zinc-lead deposits originally interpreted to have been formed by discharge of metal-bearing basinal fluids onto the seafloor resulting in the precipitation of mainly stratiform ore, ...
.
Igneous clastic rocks
Clastic
igneous rocks include
pyroclastic volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
s such as
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
,
agglomerate and intrusive
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s, as well as some marginal
eutaxitic and
taxitic intrusive morphologies. Igneous clastic rocks are broken by flow, injection or explosive disruption of solid or semi-solid igneous rocks or
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
s.
Igneous clastic rocks can be divided into two classes:
# Broken, fragmental rocks produced by
intrusive processes, usually associated with
plutons
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
or porphyry stocks
# Broken, fragmental rocks associated with volcanic eruptions, both of lava and pyroclastic type
Metamorphic clastic rocks
Clastic
metamorphic rocks
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
include
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s formed in
faults, as well as some
protomylonite and
pseudotachylite
Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veinsTrouw, R.A.J., C.W. Passchier, and D.J. Wiersma (2010) ''Atlas of Mylonites- and related microstructures.'' Sprin ...
. Occasionally, metamorphic rocks can be brecciated via
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
fluids, forming a
hydrofracture breccia.
Hydrothermal clastic rocks
Hydrothermal clastic rocks are generally restricted to those formed by
hydrofracture, the process by which
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
circulation cracks and brecciates the wall rocks and fills it in with veins. This is particularly prominent in
epithermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
deposits and is associated with
alteration zones around many intrusive rocks, especially
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...
s. Many
skarn
Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate mineral ...
and
greisen
Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic-hy ...
deposits are associated with hydrothermal breccias.
Impact breccias
A fairly rare form of clastic rock may form during
meteorite impact. This is composed primarily of ejecta; clasts of
country rock, melted rock fragments,
tektite
Tektites (from grc, τηκτός , meaning 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz ...
s (glass ejected from the impact crater) and exotic fragments, including fragments derived from the impactor itself.
Identifying a clastic rock as an impact breccia requires recognising
shatter cone
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the rang ...
s, tektites,
spherulite
In petrology, spherulites () are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice grain, with a dul ...
s, and the morphology of an impact
crater, as well as potentially recognizing particular chemical and trace element signatures, especially
osmiridium.
References
{{DEFAULTSORT:Clastic Rock
Sedimentology
Metamorphic rocks
Igneous rocks
he:סלע משקע#סלעים קלאסטיים