Three agents of metamorphism are temperature, pressure and chemically active fluids. Metamorphic rocks form by recrystallization in the solid state because of changes in temperature, pressure, or the composition of pore fluids.
Agents of Metamorphism
The process of metamorphism of rocks happens due to the effects of high temperature, pressure and chemically active fluids. These three are acknowledged as agent of metamorphism. Usually, all these three acts together to metamorphose the rock units. In some cases, any one or two of these may dominate and play an active role in metamorphism. The following are a few pertinent details about metamorphic agents.
Temperature
Heat is one of the most important factors in metamorphism. The cause of high temperature which is responsible for metamorphism of the rock mass is either due to depth or due to the contact with magma body (i.e., magma chamber or magmatic intrusion). Temperature increases about 15° to 30°C for each kilometer of depth in the crust. The metamorphic changes mainly take place in the temperature range of 350-850°C. The temperature rise also increases the chemical fluid activity in rocks and enables reactions during metamorphism.
Pressure
The pressure which causes metamorphism is of two different kinds- uniform pressure and directed pressure. Uniform pressure increases with increasing overburden. It acts vertically downwards and affects the volume of both liquids and solids. It is significant only at great depths. This also means that high temperatures will also be associated (due to geothermal gradient) with high uniform pressure. So, both pressure and temperature act together for metamorphism of rocks.
The directed pressure that is also called stress, is due to tectonic forces. Such pressure acts in any direction, i.e., upwards, downwards or sidewards. It is effective in the upper layers of the crust and increases with depth to some extent. With further increase in depth, it decreases and finally disappears. This is so because at very great depths, due to the combined effect of high temperature and pressure, the rocks will not be rigid and become plastic. In this way, the regions of influence of uniform pressure and directed pressure are reciprocal. The application of stress gives rise to shearing movements in the rock. During metamorphism, new platy mineral grains grow in the direction of least stress, producing a planar texture called foliation.
Chemically Active Fluids
Chemically active fluids play a key role in different ways in causing metamorphism. Metamorphism of any type cannot take place for solid minerals in a perfectly dry state, the presence of a liquid medium of some kind is essential. Thus, fluids act as carriers of chemical components that actually take part in chemical reactions and in mineralogical changes. Fluids provide the necessary medium and facilitate the reactions to take place. The most common fluid which plays such a role is water. It occurs within the rock in numerous interstitial pores, fractures, cracks, shear zones, etc.
The huge quantities of volatiles that are associated with magmatic bodies ultimately pervade through the surrounding country rocks by means of diffusion and cause compositional changes even in rocks far off from the magma.
The magma or the hot juvenile hydrothermal solutions (containing various chemicals) may react directly with those rocks with which they come in contact. The heat that is generally associated with the prior contexts (either due to the depth factor or due to conduction of heat from magma) plays a positive role in accelerating the reactions.
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