7.1 Controls over Metamorphic Processes

The main factors that regulate metamorphic procedures are:

The mineral ingredient of the parental rockThe temperature in ~ which metamorphism bring away placeThe lot and form of pressure during metamorphismThe species of fluids (mostly water) the are existing during metamorphismThe amount of time easily accessible for metamorphismParent Rock

The parent rock is the rock the exists before metamorphism starts. In many cases, this is sedimentary or igneous rock, yet metamorphic rock the reaches the surface and is climate reburied can also be considered a parental rock. Top top the other hand, if, because that example, a mudstone is metamorphosed to slate and also then hidden deeper wherein it is metamorphosed to schist, the parent rock the the schist is mudstone, not slate. The an essential feature of the parental rock is that mineral composition because it is the security of minerals the counts as soon as metamorphism bring away place. In various other words, once a rock is subjected to boosted temperatures, details minerals may end up being unstable and start to recrystallize into new minerals.

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Temperature

The temperature that the rock is subjected to is a crucial variable in controlling the form of metamorphism the takes place. Together we learned in the paper definition of igneous rocks, mineral stability is a function of temperature, pressure, and also the presence of fluids (especially water). Every minerals room stable end a details range the temperatures. Because that example, quartz is steady from ecological temperatures (whatever the weather have the right to throw at it) all the method up to about 1800°C. If the push is higher, that top limit will be higher. If over there is water present, it will certainly be lower. ~ above the other hand, most clay mineral are just stable up to about 150° or 200°C; above that, lock transform into micas. Most other common minerals have upper limits between 150°C and also 1000°C.

Some minerals will certainly crystallize into different polymorphs (same composition, yet different crystalline structure) depending upon the temperature and also pressure. Quartz is a good example as slightly various forms are stable in between 0°C and also 1800°C. The mineral kyanite, andalusite, and also sillimanite are polymorphs v the composition Al2SiO5. They room stable at different pressures and temperatures, and, together we will watch later, castle are important indicators of pressures and temperatures in metamorphic rocks (Figure 7.3).

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Figure 7.3 The temperature and also pressure stability areas of the three polymorphs that Al2SiO5 (Pressure is identical to depth. Kyanite is stable at low to moderate temperatures and also low come high pressures, andalusite at moderate temperatures and low pressures, and also sillimanite at greater temperatures.) Pressure

Pressure is essential in metamorphic procedures for two main reasons. First, it has actually implications for mineral stability (Figure 7.3). Second, it has implications for the texture of metamorphic rocks. Rocks that space subjected to really high confining pressures are commonly denser than others because the mineral grains are squeezed with each other (Figure 7.4a), and because they might contain mineral polymorphs in which the atoms are much more closely packed. Since of bowl tectonics, pressures in ~ the late are frequently not used equally in all directions. In areas of bowl convergence, the push in one direction (perpendicular come the direction the convergence) is typically greater than in the other directions (Figure 7.4b). In situations where various blocks the the crust space being moved in different directions, the rocks will be subjected to sheer tension (Figure 7.4c).

One the the results of command pressure and sheer stress and anxiety is that rocks become foliated — an interpretation that they’ll have actually a directional fabric. Foliation is defined in an ext detail later on in this chapter.

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Figure 7.4 an illustration the different varieties of pressure on rocks. (a) confining pressure, wherein the push is basically equal in all directions, (b) command pressure, where the pressure form the political parties is higher than that from the top and bottom, and (c) sheer stress resulted in by different blocks of absent being pushed in various directions. (In a and b over there is additionally pressure in and also out that the page.) Fluids

Water is the key fluid current within rocks of the crust, and the just one the we’ll consider here. The visibility of water is necessary for two key reasons. First, water facilitates the transfer of ions in between minerals and also within minerals, and also therefore boosts the rates at i m sorry metamorphic reactions take it place. So, when the water doesn’t necessarily readjust the result of a metamorphic process, it speeds the process up so metamorphism can take ar over a much shorter time period, or metamorphic processes that can not otherwise have had actually time to it is in completed room completed.

Secondly, water, especially hot water, have the right to have elevated concentrations of dissolved substances, and therefore the is an essential medium because that moving certain elements around within the crust. Therefore not just does water facilitate metamorphic reaction on a grain-to-grain basis, that also permits for the transport of ion from one location to another. This is really important in hydrothermal processes, which are disputed toward the end of this chapter, and in the development of mineral deposits.

Time

Most metamorphic reactions take ar at very slow rates. Because that example, the expansion of brand-new minerals within a rock throughout metamorphism has been estimated to be about 1 mm per million years. For this reason, it is very an overwhelming to examine metamorphic procedures in a lab.

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While the price of metamorphism is slow, the tectonic procedures that cause metamorphism room also very slow, therefore in most cases, the chance for metamorphic reaction to be completed is high. Because that example, one essential metamorphic setting is countless kilometres deep in ~ the root of hill ranges. A mountain range takes tens of numerous years to form, and also tens of numerous years an ext to be eroded to the degree that we have the right to see the rocks that were metamorphosed deep in ~ it.