Pressure

All rocks in ~ the surface of the earth experience rise in pressure as result of the weight of the overlying sediment and rock layersand with enhancing depth, over there is a matching increase in pressure. This enhanced pressure does no necessarily cause a absent to become metamorphic, due to the fact that this details pressure is generally equal in every directions and also is known as lithostatic pressure. Lithostatic press is similar to hydrostatic pressure, such together the press on the eardrums a swimmer will endure as he or she dives deep in the water. Lithostatic push on rocks listed below the earth’s surface ar may have a readjust in in its entirety rock volume, yet will not reason a change in the shape. An example of diminish volume due to lithostatic pressure would be a closer pack of clasts and reduction of pore room within a clastic sedimentary rock. But what if the press on a rock is unequal, and the rocks end up being squeezed in one direction much more than an additional direction? This is known as differential pressure, and also it can result in a far-reaching change in the figure of a rock. Number 11.1 demonstrates just how a mineral can readjust shape because of differential pressure, in this instance with the greatest pressures native the top and also bottom (as demonstrated by the large gray arrows). Two originally rounded mineral seed (Figure 11.1A) in ~ a sedimentary rock are experiencing the biggest amount of pressure at the contact between the grains (see red arrows in the figure), and also the bonds linking the atom in this grain will certainly break. The atoms will certainly migrate right into the area of lesser pressure and reform a bond with other atoms in the mineral serial (Figure 11.1B). Together a result, the grains have a flattened shape that is perpendicular to the direction of best pressure (Figure 11.1C).

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Figure 11.1 just shows the deformation of two grains; imagine that this is happening to every one of the seed in the sedimentary rock, or to all of the phenocrysts (crystals) in one igneous rock. In that case, you will end up v the whole rock having minerals set in a details direction, every by the breaking of bonds in between atoms in a mineral, and also reforming (recrystallizing) in the lower pressure areas amongst the seed or crystals in the rock. The end result is a rock with a metamorphic pattern referred to as a foliation. Metamorphic foliations room the patterns seen in a rock that has experienced differential pressure; these foliations might be fairly flator have actually a wavy appearance possibly due to much more than one direction of greatest pressure. Some rocks may likewise develop what is dubbed a lineation, which have the right to be formed by an elongation of mineral that form a linear feature through the rock. To recognize the difference in between a foliation and also a lineation, let united state use part food analogies: a stack of pancakes demonstrates a foliation in your breakfast food, with each pancake class representing flattened minerals. If friend look at the height of the pancakes, you will certainly not check out a pattern, yet if you check out the stack of pancakes indigenous the side, or cut through the stack v your knife in any type of orientation other than parallel to the pancake layers, girlfriend will check out the layering or foliation. However, if within the pancake stack, over there existed a part of bacon (yum), the bacon would be the lineation in her breakfast “rock”, and you may or may not watch it when you cut through the pancake stack. We will talk about foliations, and also the different varieties of foliations, in a later on section the this chapter.


Temperature

Probably the most common reason of metamorphism is a change in temperature. Frequently times metamorphism requires both rise in temperature together with the pressure changes as defined in the above section. Higher temperatures are often connected with metamorphism as result of chemically reactive fluids (which us will comment on in the next section). The wide classification because that metamorphism into low, medium and also high grades of metamorphic readjust exists mainly as result of temperature conditions; this will additionally be questioned in a later on section.

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Higher temperatures boost the vibrational energy in between the bonds linking atoms in the mineral structure, making it easier for bonds come be damaged in orderfor the recrystallization that the minerals into new crystal shapes and sometimes the advance of foliations and also lineations as explained in the vault section. However, recrystallization deserve to be as result of just temperature transforms without any type of differential press conditions, and when temperatures are increased, there have the right to be a equivalent increase in mineral sizes together initially tiny minerals end up being fused into larger crystals. This fusing of many smaller mineral sizes right into fewer and larger mineral sizes is recognized as annealing in metallurgy.However,for metamorphic rocks, the is still described as recrystallization.