13.4: What Are Metamorphic Grades?
What Are Metamorphic Grades?
Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade (Figure 13.7).
Figure 13.7: Metamorphic grades are categorized by the amount of heat and pressure. (CC-BY 4.0; Chloe Branciforte)
Very Low and Low-grade metamorphism takes place at approximately 150–300°C and relatively low pressure. This is not far beyond the conditions in which sediments are lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look similar to its protolith. Low-grade metamorphic rocks tend to be characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Examples of low-grade hydrous minerals include clay, serpentine, and chlorite. Under low-grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope.
Intermediate-grade metamorphism takes place at approximately 300–450°C and at moderate pressures. Low-grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may now grow. Garnet is an example of a mineral which may form porphyroblasts Links to an external site., metamorphic mineral grains that are larger in size and more equant in shape (about the same diameter in all directions), thus standing out among the smaller, flatter, or more elongate minerals.
High-grade metamorphism takes place at temperatures above about 450°C. Micas tend to break down. New minerals such as hornblende will form, which is stable at higher temperatures. As metamorphic grade increases to even higher grade, however, all hydrous minerals, including hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.