All rocks in ~ the surface ar of the planet experience boost in pressure because of the weight of the overlying sediment and rock layersand with increasing depth, there is a matching increase in pressure. This raised pressure does no necessarily reason a rock to become metamorphic, because this details pressure is generally equal in all directions and also is well-known as lithostatic pressure. Lithostatic press is comparable to hydrostatic pressure, such together the pressure on the eardrums a swimmer will experience as the or she dives deep in the water. Lithostatic press on rocks below the earth’s surface may have a change in all at once rock volume, yet will not reason a readjust in the shape. An instance of decreasing volume as result of lithostatic pressure would be a closer packing of clasts and also reduction the pore room within a clastic sedimentary rock. But what if the push on a absent is unequal, and also the rocks become squeezed in one direction an ext than an additional direction? This is recognized as differential pressure, and also it can result in a far-ranging change in the appearance of a rock. Number 11.1 demonstrates how a mineral can adjust shape due to differential pressure, in this case with the biggest pressures from the top and bottom (as prove by the huge gray arrows). Two at first rounded mineral grains (Figure 11.1A) within a sedimentary rock room experiencing the biggest amount of push at the contact in between the seed (see red arrows in the figure), and the binding linking the atom in this grain will certainly break. The atoms will certainly migrate right into the area the lesser pressure and reform a shortcut with various other atoms in the mineral serial (Figure 11.1B). Together a result, the grains have actually a flattened shape that is perpendicular come the direction of biggest pressure (Figure 11.1C).

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Figure 11.1 just shows the deformation of 2 grains; imagine the this is happening to every one of the grains in the sedimentary rock, or to all of the phenocrysts (crystals) in an igneous rock. In the case, friend will end up with the whole rock having actually minerals to adjust in a specific direction, all by the breaking of bonds in between atoms in a mineral, and also reforming (recrystallizing) in the reduced pressure areas amongst the seed or crystals in the rock. The end result is a rock v a metamorphic pattern dubbed a foliation. Metamorphic foliations space the fads seen in a absent that has actually experienced differential pressure; this foliations may be fairly flator have a wavy appearance possibly due to much more than one direction of greatest pressure. Some rocks may additionally develop what is called a lineation, which have the right to be developed by one elongation of minerals that kind a linear function through the rock. To understand the difference in between a foliation and a lineation, let us use some food analogies: a ridge of pancakes demonstrates a foliation in your breakfast food, through each pancake great representing planarization minerals. If you look at the height of the pancakes, you will certainly not view a pattern, however if you check out the stack of pancakes indigenous the side, or reduced through the stack through your knife in any type of orientation other than parallel to the pancake layers, friend will view the layering or foliation. However, if within that pancake stack, there existed a part of bacon (yum), the bacon would be the lineation in her breakfast “rock”, and also you might or may not watch it as soon as you reduced through the pancake stack. We will comment on foliations, and the different types of foliations, in a later on section of this chapter.


Probably the most common reason of metamorphism is a readjust in temperature. Regularly times metamorphism requires both boost in temperature in addition to the pressure alters as described in the over section. Greater temperatures space often connected with metamorphism due to chemically reactive fluids (which us will talk about in the next section). The vast classification for metamorphism right into low, medium and also high grades of metamorphic adjust exists mainly due to temperature conditions; this will likewise 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 the the minerals into new crystal shapes and also sometimes the development of foliations and lineations as described in the ahead section. However, recrystallization can be as result of just temperature changes without any differential press conditions, and when temperatures room increased, there have the right to be a corresponding increase in mineral sizes as initially tiny minerals come to be fused into larger crystals. This fusing of plenty of smaller mineral sizes right into fewer and also larger mineral size is well-known as annealing in metallurgy.However,for metamorphic rocks, it is still referred to as recrystallization.