Metamorphism

The metamorphism ( gr μεταμόρφωσις Metamorphosis " Metamorphosis", "transformation" ) is the conversion of the mineralogical composition of a rock due to a change in temperature and / or pressure conditions. Here, from the parent rock ( protolith ) creates a metamorphic rock ( metamorphic ).

Upon metamorphosis occurs under the altered physical conditions to mineral reactions, ie for new or alteration of minerals, the rock remains in the solid state. Melts a rock by increasing the temperature on one speaks of anatexis.

Overview

A delineation of the metamorphism of diagenesis, the processes that lead to the formation of sedimentary rocks from sediments, can not be accurately drawn, as it can also occur in the diagenesis to Mineralneu and transformations. There are various definitions, according to which is to speak of a metamorphosis when certain minerals occur or are no longer present or certain pressure and temperature limits have been exceeded.

At metamorphosis, the chemical composition of the rock often remains unchanged, then one speaks of isochemischer metamorphosis. As can be involved in a metamorphosis always fluid phases, this condition is seldom strictly met. If the element constituents of a rock is changed significantly, there is a metasomatism. This does not apply when only H2O or CO2 are supplied or removed.

The metamorphosis of a rock is affected by pressure and temperature. One speaks of a prograde metamorphism, when pressure and temperature during metamorphism increase, and by a retrograde metamorphosis or Diaphthorese when pressure and temperature decrease during metamorphosis.

In principle, one observes two types of transformation of rocks:

By phase transformations ( mineral reactions ), new minerals from the existing ones. Minerals can coexist only under certain pressure and temperature conditions. If these conditions are not met, the minerals can react to form other minerals. Such mineral reactions are often very complex. Some of the newly formed minerals put in these reactions, other substances such as Free water or take them on, thus it comes to the above-mentioned phenomenon of metasomatism.

During the crystallization of minerals, there is structural transformations in the rock. By When adjusting, by pressure solution processes after Rieck ash principle or the oriented growth of minerals in the rock forms a foliation, which is more pronounced the more phyllosilicates ( mica) in the rock are present.

Types of metamorphism

The possible course of metamorphism depends on the case passed through pressure and temperature conditions. These can be very different and thus cause different types of metamorphosis. Rocks which have undergone a certain type of metamorphosis, often wear characteristics, for example, certain mineral assemblages, structural features thereof and others.

Regional metamorphism

The addition of " regional " states that this type of metamorphosis over large volumes (in some cases over several 1000 km ³) takes place, usually caused by tectonic subsidence of large parts of the earth's crust. These rocks fall by sinking about by folding or subduction at continental margins under high pressure and / or temperature that determine the transformation of minerals alike. Typical rocks are, for example, mica schist, gneiss, amphibolites.

Stressed pressure metamorphism

The pressure- stressed metamorphosis is a typical characteristic of subduction zones. In this relatively cold material oceanic crust is sunk. The case running metamorphosis is therefore determined by relatively low temperatures and high pressures. Rocks that have undergone the pressure stressed metamorphosis are characterized by typical minerals such as glaucophane schists in blue or omphacite in eclogites.

Contact metamorphism

The contact metamorphism is the temperature- stressed metamorphosis. Kontaktmetamorphe rocks are found mainly in the vicinity of igneous intrusions. The hot magma heats the surrounding rock and thus leads its metamorphosis brought about. The range of metamorphism is called contact zone. A typical feature kontaktmetamorpher rocks are caused by mineral reactions knots and often the lack of a foliation. The contact metamorphism can occur Horn clif, fruit and node shale.

Shock metamorphism

This very extreme type of metamorphism is caused by violent shock waves and can lead to the destruction of whole rock games and the destruction of crystal lattices. It is limited to meteorite crater ( and on the locations of underground nuclear tests ). Typical characteristics for shock metamorphism is the occurrence of high -pressure minerals such as coesite or, due to the collapse of crystal lattices of diaplectic glass. The shock metamorphism leads to the destruction of rock bodies, which is macroscopically visible (eg in the suevite of the Nordlinger Ries ).

Dislocation metamorphism

The dislocation metamorphism is also called Dynamo metamorphosis. In active fault zones, the rock against each other is greatly changed by the movement of two blocks. Reacts the rock while on mechanical stress brittle, meaning it breaks and is ground, so there arise cataclasites. If the rock reacts ductile mechanical stress, caused by Neukristallisation mylonites with a distinctive, embossed by the constant movement of the structure.

When earthquake occurs a brief and sudden movement of rock games. Thereby opening cavities, can it by the sudden pressure release Implosionsbrekzien arise that fill the cavity again. Due to the resulting in a sudden movement in the movement area frictional heat may result in transient melting of rock sections and to the formation of pseudotachylites. Also can be caused by the collapse of crystal lattices diaplectic glasses.

Classification of metamorphosis

There are different systems for describing the metamorphosis degree, the has reached a rock. In analogy to the facies of sediments metamorphic conditions (pressure, temperature) can be summarized by metamorphic Faziesgruppen. Another possibility is to determine the degree of metamorphosis using certain mineral reactions. The fact that certain minerals are the result of other shows that certain boundary temperatures or pressure limits have been exceeded.

Border areas of the metamorphosis

Opposite of diagenesis can not be clearly defined metamorphosis as similar processes in a rock in the diagenetic transformation of a sediment. Often an arbitrary demarcation is made when certain pressure and temperature conditions have been exceeded. The limit range of diagenesis is often referred to as Anchimetamorphose.

The anatexis, resulting in the partial or complete melting of rocks, is also a process to limit of metamorphism. Metamorphosis always takes place in the solid state, while formed during anatexis melting.

In metasomatism the general chemical composition of the relevant rock ( Gesteinschemismus ) is changed by mass transfer, during the actual metamorphosis isochemisch, i.e., the general chemical composition of the rock does not change.