Factors That Control Metamorphic Processes

Factors that control metamorphic processes

The parent rock or protolith is the rock that exists before metamorphism starts. It can be any of the three rock types: sedimentary, igneous, or metamorphic.  The critical feature of the parent rock is its mineral composition.  This is because the stability of minerals (how influenced they are by changing conditions) is what counts when metamorphism takes place. In other words, when a rock is subjected to increased temperatures and pressures, certain minerals will undergo chemical reactions and turn into new minerals, while others might just change their shape.

Metamorphic facies and types of metamorphism shown in the context of depth and temperature. The metamorphic rocks formed from a mudrock protolith under regional metamorphism with a typical geothermal gradient are listed. Source: Karla Panchuk

Because some metamorphic rocks form as part of a continuous series as pressures and temperatures increase progressively, some people use the term parent rock to apply to the very first rock that metamorphism happened to, rather than referring to each stage of metamorphic rock as the parent rock to the next stage. 

The problem is that we won’t always know whether metamorphism happened in an uninterrupted sequence or whether metamorphism stopped and started again for different reasons at different times.  If the former is the case it would be no problem to use this system, but if the latter is the case, then it is important to not skip over the intervening steps by saying they are part of a smooth series.  For that reason we will use the term parent rock to apply to the direct precursor of the metamorphic rock we’re interested in.

The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place. As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). All minerals are stable over a specific range of temperatures. For example, quartz is stable from surface temperatures (whatever the weather can throw at it) all the way up to about 1800°C. If the pressure is higher, that upper limit will be higher. If there is water present, it will be lower. On the other hand, most clay minerals are only stable up to about 150° or 200°C.  Above that, they transform into micas. Most other common minerals have upper limits between 150°C and 1000°C.

Some minerals will crystallize into different polymorphs (same composition but different crystalline structure) depending on the temperature and pressure. Quartz is a good example because slightly different forms are stable between 0°C and 1800°C. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al₂SiO₅. They are stable at different pressures and temperatures, and, as we will see later, they are important indicators of pressures and temperatures in metamorphic rocks.

Pressure is important in metamorphic processes for two main reasons. First, it has implications for mineral stability. Second, it has implications for the texture of metamorphic rocks. Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together, and because they may contain mineral polymorphs in which the atoms are more closely packed. 

Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. In areas of plate convergence, the pressure in one direction (perpendicular to the direction of convergence) is typically greater than in the other directions. In situations where different blocks of the crust are being pushed in different directions, the rocks will be subjected to sheer stress.

One of the results of directed pressure and sheer stress is that rocks become foliated — meaning that minerals within them become aligned.

Water is the main fluid present within rocks of the crust, and the only one that we’ll consider here. The presence of water is important for two main reasons. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. So, while the water doesn’t necessarily change the outcome of a metamorphic process, it speeds the process up so metamorphism might take place over a shorter time period, or metamorphic processes that might not otherwise have had time to be completed are completed.

Secondly, water, especially hot water, can have elevated concentrations of dissolved substances, and therefore it is an important medium for moving certain elements around within the crust. So not only does water facilitate metamorphic reactions on a grain-to-grain basis, it also allows for the transportation of ions from one place to another. This is very important in hydrothermal processes, which are discussed toward the end of this chapter, and in the formation of mineral deposits.

Most metamorphic reactions take place at very slow rates. For example, the growth of new minerals within a rock during metamorphism has been estimated to be about 1 mm per million years. For this reason, it is very difficult to study metamorphic processes in a lab.

While the rate of metamorphism is slow, the tectonic processes that lead to metamorphism are also very slow, so in most cases, the chance for metamorphic reactions to be completed is high. For example, one important metamorphic setting is many kilometres deep within the roots of mountain ranges. A mountain range takes tens of millions of years to form, and tens of millions of years more to be eroded to the extent that we can see the rocks that were metamorphosed deep beneath it.