Factors that Control Metamorphism

Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism.

Temperature


Temperature increases with depth in the Earth along the Geothermal Gradient. Thus higher temperature can occur by burial of rock.


Temperature can also increase due to igneous intrusion.


Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.

rounded grains can become flattened in the direction of maximum stress.

minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and serpentine).

These sheet silicates will grow with their sheets orientated perpendicular to the direction of maximum stress. Preferred orientation of sheet silicates causes rocks to be easily broken along approximately parallel sheets. Such a structure is called a foliation.

Fluid Phase - Any existing open space between mineral grains in a rocks can potentially contain a fluid. This fluid is mostly H2O, but contains dissolved mineral matter. The fluid phase is important because chemical reactions that involve one solid mineral changing into another solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. Within increasing pressure of metamorphism, the pore spaces in which the fluid resides is reduced, and thus the fluid is driven off. Thus, no fluid will be present when pressure and temperature decrease and, as discussed earlier, retrograde metamorphism will be inhibited.


Time - The chemical reactions involved in metamorphism, along with recrystallization, and growth of new minerals are extremely slow processes. Laboratory experiments suggest that the longer the time available for metamorphism, the larger are the sizes of the mineral grains produced. Thus, coarse grained metamorphic rocks involve long times of metamorphism. Experiments suggest that the time involved is millions of years.


Types of Metamorphism

Cataclastic Metamorphism - This type of metamorphism is due to mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the sliding occurred.


Burial Metamorphism - When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300oC may develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed. The main minerals produced are the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.


Contact Metamorphism - Occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion. Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to a zone surrounding the intrusion, called a metamorphic aureole. Outside of the contact aureole, the rocks are unmetamorphosed. The grade of metamorphism increases in all directions toward the intrusion. Because temperature differences between the surrounding rock and the intruded magma are larger at shallow levels in the crust, contact metamorphism is usually referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.