Metamorphic Rocks

Metamorphic Rocks

Axia College of the University of Phoenix

Metamorphic rock is the transformation of an existing rock type in a process called metamorphism, which means change in form. Metamorphic rocks make up a large part of the Earth 's crust and are classified by texture and by chemical and mineral assemblage (Murck, Skinner, & Mackenzie, 2008). Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. These rocks were once igneous or sedimentary rocks. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors (Murck, Skinner, & Mackenzie, 2008). The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.

There are two categories of metamorphic rocks foliated and non-foliated. In foliated metamorphic rocks, the composition of the original rock is not apparent. Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below the Earth 's surface (Murck, Skinner, & Mackenzie, 2008). Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. Rocks exhibiting foliation include the typical sequence formed by the prograde metamorphism of mud rocks; slate, phyllite, schist and gneiss. Foliated rock is also known as S-tectonite in sheared rock masses.

The crystals in non-foliated rocks combine, change or rearrange, but they don 't form parallel bands, because they are composed of mostly one mineral. Non-foliated metamorphic rocks are formed around igneous intrusions where the temperatures are high but the pressures are relatively low and equal in all directions (Murck, Skinner, & Mackenzie, 2008). The original minerals within the rock recrystallize into larger sizes and the atoms become more tightly packed together, increasing the density of the rock. Non-foliated metamorphic rocks are identified on the basis of their composition. Non-foliated metamorphic rocks are most often derived from single mineral sedimentary rocks like limestone and quartz sandstone.

References

Murck, B. W., Skinner, B. J., & Mackenzie, D. (2008). Visualizing geology. Hoboken, NJ: John Wiley & Sons.

References: Murck, B. W., Skinner, B. J., & Mackenzie, D. (2008). Visualizing geology. Hoboken, NJ: John Wiley & Sons.