Introduction
Metamorphic rocks are formed when pre-formed and old rocks undergo metamorphism, which is a process whereby the rocks are exposed to high temperatures and pressure. The rocks are initially under intense pressure due to the outer layers exerting tons and tons of pressure on the inner layers. The pressure leads to temperature changes due to the build-up of heat, which finally causes a change in the shape of the parent rock. This comprises the mechanisms behind the structural changes of the rocks. The rocks may also undergo chemical changes due to intrusion of magma or through tectonic subduction, while pressure is due to the weight of overlying materials. Metamorphism is also possible due to the mixing of carbon dioxide and water.
Origin and formation of metamorphic rocks
Metamorphic rocks originate from other rocks such as sedimentary, igneous, and other metamorphic rocks. These rocks change their composition due to intense pressure and temperature. These changes include structural and textural changes. The changes in temperature and pressure occur above the point of diagenesis but do not melt the rock. When a parent rock is exposed to different temperatures and pressures, different metamorphic rocks may result.
As a result, metamorphic rocks are classified into facies depending on the range of temperature and pressure involved in their formation (Mason, 1990, par.13). However, since it is not possible to measure the temperature and pressure involved in the formation of these rocks, geologists classify the rocks depending on the extent of metamorphism. There are three different types of metamorphism involved in the formation of metamorphic rocks. These include; thermal, dynamic, and metasomatic metamorphism.
Categories of metamorphic rocks
There are two main categories of metamorphic rocks. These include; foliated and non-foliated rocks. Non-foliated rocks are composed of one mineral and as a result, their crystals combine undergoing rearranges but do not form parallel bands. Non-foliated rocks include marble that forms from limestone composed of calcite, Quartzite (that is formed from sandstone and quartz mineral), and meta-conglomerate formed from conglomerate whereby the grains are flattened (Mason, 1990, par.4). Foliated metamorphic rocks consist of many minerals. These minerals may be rotated and rearranged while still inside the rock. Examples of foliated rocks include rocks are schist, slate, and gneiss. Formation of slate occurs when shale undergoes metamorphism, while schist and gneiss originate from higher degree metamorphism than slate.
Distinguishing characteristics between foliated and non-foliated metamorphic rocks
Foliated rocks consist of color bands as opposed to non-foliated rocks. This is so because foliated rocks have minerals whose axis is in one direction, which are absent in non-foliated rocks (Mason, 1990, p.7).
Foliated rocks have layers and therefore crumble easily such as mica while non-foliated rocks do not have such layers and hence do not crumble easily. Additionally, foliated rocks have a planar fabric as opposed to non-foliated rocks, which do not consist of planar fabrics. Moreover, foliated rocks have a sheet-like structure compared to non-foliated, which lack a platy or sheet-like structure (Mason, 1990, par.16).
Another distinguishing characteristic between foliated and non-foliated rocks is determined by their mode of formation. For instance, foliated rocks are formed when high pressure and temperature force the minerals within the rocks to re-align or grow resulting in the formation of bands that are parallel. On the other hand, non-foliated rocks are produced in various ways. They can be formed when a parent rock is baked by the heat, changing the structure of the mineral usually in the absence of pressure changes. Alternatively, they are formed from minerals that are not flat and no matter how much pressure is applied, they are never aligned (Mason, 1990, par.17).
Conclusion
The essay provides an account of the process of metamorphism, which is a process by which new rocks are produced from old or pre-formed rocks. The essay also highlights the different processes underlying metamorphism. Additionally, the essay provides examples of different categories of metamorphic rocks and their distinguishing features in general.
References
Mason, R. (1990). Metamorphic Rocks.Australia:Allen & Unwin publishers. Top of Form