Contact metamorphism

Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Metamorphism occurring at increasing pressure and temperature conditions is known as ''prograde metamorphism'', while decreasing temperature and pressure characterize ''retrograde metamorphism''.

Metamorphic petrology is the study of metamorphism. Metamorphic petrologists rely heavily on statistical mechanics and experimental petrology to understand metamorphic processes.

Metamorphic processes

Metamorphism is the set of processes by which existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree. The importance of heating in the formation of metamorphic rock was first recognized by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology. Hutton wrote in 1795 that some rock beds of the Scottish Highlands had originally been sedimentary rock, but had been transformed by great heat.

Hutton also speculated that pressure was important in metamorphism. This hypothesis was tested by his friend, James Hall, who sealed chalk into a makeshift pressure vessel constructed from a cannon barrel and heated it in an iron foundry furnace. Hall found that this produced a material strongly resembling marble, rather than the usual quicklime produced by heating of chalk in the open air. French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place without metasomatism (isochemical metamorphism) or at depths of just a few hundred meters where pressures are relatively low (for example, in contact metamorphism).

Rock can be transformed without melting because heat causes atomic bonds to break, freeing the atoms to move and form new bonds with other atoms. Pore fluid present between mineral grains is an important medium through which atoms are exchanged. This permits recrystallization of existing minerals or crystallization of new minerals with different crystalline structures or chemical compositions (neocrystallization). The transformation converts the minerals in the protolith into forms that are more stable (closer to chemical equilibrium) under the conditions of pressure and temperature at which metamorphism takes place.

Metamorphism is generally regarded to begin at temperatures of . This excludes diagenetic changes due to compaction and lithification, which result in the formation of sedimentary rocks. The upper boundary of metamorphic conditions lies at the solidus of the rock, which is the temperature at which the rock begins to melt. At this point, the process becomes an igneous process. The solidus temperature depends on the composition of the rock, the pressure, and whether the rock is saturated with water. Typical solidus temperatures range from for wet granite at a few hundred megapascals (Mpa) of pressure to about for wet basalt at atmospheric pressure. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not fully segregated from the refractory residue.

The metamorphic process can occur at almost any pressure, from near surface pressure (for contact metamorphism) to pressures in excess of 16 kbar (1500 Mpa).

Recrystallization

The change in the grain size and orientation in the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rocks limestone and chalk change into larger crystals in the metamorphic rock marble. In metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite, also known as metaquartzite, in which the often larger quartz crystals are interlocked. Both high temperatures and pressures contribute to recrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their points of contact (''pressure solution'') and redeposition in pore space.

During recrystallization, the identity of the mineral does not change, only its texture. Recrystallization generally begins when temperatures reach above half the melting point of the mineral on the Kelvin scale.

Pressure solution begins during diagenesis (the process of lithification of sediments into sedimentary rock) but is completed during early stages of metamorphism. For a sandstone protolith, the dividing line between diagenesis and metamorphism can be placed at the point where strained quartz grains begin to be replaced by new, unstrained, small quartz grains, producing a ''mortar texture'' that can be identified in thin sections under a polarizing microscope. With increasing grade of metamorphism, further recrystallization produces ''foam texture'', characterized by polygonal grains meeting at triple junctions, and then ''porphyroblastic texture'', characterized by coarse, irregular grains, including some larger grains (porphyroblasts.)

Metamorphic rocks are typically more coarsely crystalline than the protolith from which they formed. Atoms in the interior of a crystal are surrounded by a stable arrangement of neighboring atoms. This is partially missing at the surface of the crystal, producing a ''surface energy'' that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.

Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminate strain energy by recrystallizing as a fine-grained rock called ''mylonite''. Certain kinds of rock, such as those rich in quartz, carbonate minerals, or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.

Phase change

Phase change metamorphism is the creating of a new mineral with the same chemical formula as a mineral of the protolith. This involves a rearrangement of the atoms in the crystals. An example is provided by the aluminium silicate minerals, kyanite, andalusite, and sillimanite. All three have the identical composition, . Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to andalusite at a temperature of about . Andalusite, in turn, transforms to sillimanite when the temperature reaches about . At pressures above about 4 kbar (400 Mpa), kyanite transforms directly to sillimanite as the temperature increases. A similar phase change is sometimes seen between calcite and aragonite, with calcite transforming to aragonite at elevated pressure and relatively low temperature.

Neocrystallization

Neocrystallization involves the creation of new mineral crystals different from the protolith. Chemical reactions digest the minerals of the protolith which yields new minerals. This is a very slow process as it can also involve the diffusion of atoms through solid crystals.

An example of a neocrystallization reaction is the reaction of fayalite with plagioclase at elevated pressure and temperature to form garnet. The reaction is:

Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism. These reactions are possible because of rapid diffusion of atoms at elevated temperature. Pore fluid between mineral grains can be an important medium through which atoms are exchanged.

A particularly important group of neocrystallization reactions are those that release volatiles such as water and carbon dioxide. During metamorphism of basalt to eclogite in subduction zones, hydrous minerals break down, producing copious quantities of water. The water rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating magma via flux melting. The mantle-derived magmas can ultimately reach the Earth's surface, resulting in volcanic eruptions. The resulting arc volcanoes tend to produce dangerous eruptions, because their high water content makes them extremely explosive.

Examples of ''dehydration reactions'' that release water include:

An example of a decarbonation reaction is:

Plastic deformation

In plastic deformation pressure is applied to the protolith, which causes it to shear or bend, but not break. In order for this to happen temperatures must be high enough that brittle fractures do not occur, but not so high that diffusion of crystals takes place.
As with pressure solution, the early stages of plastic deformation begin during diagenesis.

Types

Regional

''Regional metamorphism'' is a general term for metamorphism that affects entire regions of the Earth's crust. It most often refers to ''dynamothermal metamorphism'', which takes place in ''orogenic belts'' (regions where mountain building is taking place), but also includes ''burial metamorphism'', which results simply from rock being buried to great depths below the Earth's surface in a subsiding basin.

Dynamothermal

To many geologists, regional metamorphism is practically synonymous with dynamothermal metamorphism. This form of metamorphism takes place at convergent plate boundaries, where two continental plates or a continental plate and an island arc collide. The collision zone becomes a belt of mountain formation called an ''orogeny''. The orogenic belt is characterized by thickening of the Earth's crust, during which the deeply buried crustal rock is subjected to high temperatures and pressures and is intensely deformed. Subsequent erosion of the mountains exposes the roots of the orogenic belt as extensive outcrops of metamorphic rock, characteristic of mountain chains.

Metamorphic rock formed in these settings tends to shown well-developed foliation. Foliation develops when a rock is being shortened along one axis during metamorphism. This causes crystals of platy minerals, such as mica and chlorite, to become rotated such that their short axes are parallel to the direction of shortening. This results in a banded, or foliated, rock, with the bands showing the colors of the minerals that formed them. Foliated rock often develops planes of cleavage. Slate is an example of a foliated metamorphic rock, originating from shale, and it typically shows well-developed cleavage that allows slate to be split into thin plates.

The type of foliation that develops depends on the metamorphic grade. For instance, starting with a mudstone, the following sequence develops with increasing temperature: The mudstone is first converted to slate, which is a very fine-grained, foliated metamorphic rock, characteristic of very low grade metamorphism. Slate in turn is converted to phyllite, which is fine-grained and found in areas of low grade metamorphism. Schist is medium to coarse-grained and found in areas of medium grade metamorphism. High-grade metamorphism transforms the rock to gneiss, which is coarse to very coarse-grained.

Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. Marble lacks platy minerals and is generally not foliated, which allows its use as a material for sculpture and architecture.

Collisional orogenies are preceded by subduction of oceanic crust. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone produce their own distinctive regional metamorphic effects, characterized by paired metamorphic belts.

The pioneering work of George Barrow on regional metamorphism in the Scottish Highlands showed that some regional metamorphism produces well-defined, mappable zones of increasing metamorphic grade. This ''Barrovian metamorphism'' is the most recognized metamorphic series in the world. However, Barrovian metamorphism is specific to pelitic rock, formed from mudstone or siltstone, and it is not unique even in pelitic rock. A different sequence in the northeast of Scotland defines '' Buchan metamorphism'', which took place at lower pressure than the Barrovian.

Burial

Burial metamorphism takes place simply through rock being buried to great depths below the Earth's surface in a subsiding basin. Here the rock subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Burial metamorphism tends to produced low-grade metamorphic rock. This shows none of the effects of deformation and folding so characteristic of dynamothermal metamorphism.

Examples of metamorphic rocks formed by burial metamorphism include some of the rocks of the Midcontinent Rift System of North America, such as the Sioux Quartzite, and in the Hamersley Basin of Australia.

Contact (thermal)

Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole, the contact aureole, or simply the aureole. Contact metamorphic rocks are usually known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained and extremely tough.

Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusion, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism, extending not more than one or two dike thicknesses into the surrounding rock, whereas the aureoles around batholiths can be up to several kilometers wide.

The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of pelitic or aluminosilicate rocks and the minerals they form. The metamorphic grades of aureoles at shallow depth are albite-epidote hornfels, hornblende hornfels, pyroxene hornfels, and sillimanite hornfels, in increasing order of temperature of formation. However, the albite-epidote hornfels is often not formed, even though it is the lowest temperature grade.

Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. An extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore