Definition of Igneous Rocks - Term Paper Example

IGNEOUS ROCKS (by The of the The of the school The and the where it is located The date Igneous Rocks Rocks are divided into three categories namely sedimentary, metamorphic, and igneous rocks. Igneous rocks are created by the cooling and subsequent solidification of liquid magma. Igneous rocks usually form with or without crystallization below the surface of the earth as intrusive or plutonic rocks or on the earth’s crust as extrusive volcanic rocks (Krebs, 2003, p. 96). These two forms of rock formations lead to dissimilar types of igneous rocks. This paper aims to expand on the igneous rock phenomenon by focusing on the descriptions and explanations of the petrography and petrochemistry of the rock types in reference to maps and diagrams of the locations where these rocks are found. Igneous rocks are made when molten rock material called magma cools and solidifies. Magma may be thought of as a molten rock but it is not regularly a simple molten fluid. Nonetheless, it is rather a multifaceted blend of molten solid materials, for example, crystals, rocks, as well as dissolved vapor. Magma’s chemical composition and physical properties such as viscosity and density are all important factors that control magma’s overall behavior, for instance, how fast or slow it will move, how explosive it will be, what minerals will form in it, and their grain sizes and physical interrelationships. In terms of chemical composition, all magmas except rare carbonites are rich in carbonate component. They are also silicate magmas in which the dominant component is silica (SiO2), which generally comprises 45 % or more by weight. Alumina (Al2O3), with its abundance in common igneous rocks anywhere between 13 and 18 %, is at a distant second. Igneous rocks are categorized according to location, chemical composition, surface consistency, and mineralogy. Two significant factors used for the taxonomy of igneous rocks are the rock grain size, which largely depends on the cooling history, determines the texture and the mineral or chemical composition of the rock (Krebs, 2003 p. 95). Classification of Igneous Rocks The identification of these rocks is majorly done by texture. The grain consistency of volcanic rocks comprising the size, distribution of mineral grains, shape, orientation, and the intergranular relationships will decide if the rock is designated as simple or pyroclastic lava, or a tuff. Subsequently, it also determines what minerals are found in what kind of igneous rock. Phaneritic These are large rocks with mineral grains that are big enough to be identified by the eye. They are formed when magma slowly cools to form intrusive mafic igneous rocks. The well documented types of these rocks are Granite and Gabbro rocks, which have low silica of about 45 to 52% and typically high in iron magnesium content. In order to understand more about this type of igneous rock, a sample of gabbro rock was got from Chennai, India and an examination was done (Young, 2003. p 142). Gabbro is an intrusive rough-grained, murky-colored igneous rock. It is typically black or dark green in color and comprising mostly of the minerals clinopyroxene, plagioclase, and augite. Gabbro is comprised majorly of plagioclase rich in calcium, feldspar regularly labradorite or bytownite, and clinopyroxene augite. Small amounts of olivine and orthopyroxene have been found in these rocks and these mineral compositions are the cause of the gabbro rocks slight amount of light colour. Dissimilar to many other igneous rocks, gabbro typically contains small amounts of quartz (Young, 2003. p. 143). Figure 1: image of a gabbro rock (The specimen shown below is about five centimeters across) Aphanitic: These are rocks with grains that are too small to be seen by eye. Rapidly solidified extruded magma and marginal facies of shallow intrusions, for example, Rhyolites and Basalt are classified as felsic igneous rocks comprising of a high silica ratio usually greater than 63%. For further comprehension of the features of these kinds of rocks, a sample of basalt rock was used in this study (Arribas, Critelli, & Johnsson, 2007, p. 142). Basalt is a murky-colored, fine-particle, igneous rock comprised of essentially plagioclase and pyroxene chemical compositions. It has a composition similar to gabbro but the dissimilarity between basalt and gabbro is that basalt is fine-grained while gabbro is coarse-grained. Basalt mostly holds a chemical composition of 65 wt.% SiO2, 4 wt.% total alkalis, 1.50 wt.% TiO2, 12 wt.% FeO and 13 wt.% Al2O3. Particles of CaO are usually near 9 wt. %, and 12% wt. MgO (Arribas, Critelli, & Johnsson, 2007, p.143). Olympus Mons Volcano: This shield volcano is composed of basalt and has enormous calderas at the summit. Olympus Mons is the highest topographic feature on Mars and is the largest known volcano in Earth’s solar system. It is about 600 kilometers in diameter and 25 kilometers high. Porphyritic; These rocks are characterized by fragmental texture formed by volatile volcanic explosions and regularly appear more alike to sedimentary rocks owing to their bimodal particle size dispersal, for example, Porphyritic Granite (Mccaffrey, Lonergan, & Wilkinson. 1999, p. 170). Porphyritic Granite is a bright-colored igneous rock with particles regarded as small, but large enough to be observable by the necked eye. The rock is formed due to slow crystallization of magma below Earth’s crust. Granite is majorly comprised of quartz and feldspar with small amounts of mica and amphiboles. It is attributable to these mineral compositions that granite has a white, red, pink, brown, or gray color, which allows dark mineral grains to be visible throughout the rock (Mccaffrey, Lonergan, & Wilkinson, 1999, p. 171) .The specimen below, is about five centimeters across. Molds of giant spodumene crystals at the Etta Mines, Black Hills, and Pennington County, South Dakota Glassy- These rocks have a superficial glassy texture, which is evidence of extreme cooling hence no crystals could form in the process. Nonetheless, mineral composition is also crucially vital and studies have shown that these rocks have high silica (SiO2) concentrations such as rhyolite (Mattern, 2006, p.16). Obsidian is a dark glass-colored volcanic igneous rock that is formed when magma cools so quickly that atoms are incapable to position themselves into a crystalline arrangement. The rock is an unstructured material identified as a "mineraloid." The outcome is a volcanic glass like look with a smooth even texture. Most obsidians rocks have a configuration like rhyolite and granite. Studies have shown that granites and rhyolites can be created from the same molten material as obsidian and are regularly geologically related with the obsidian rock locations. Not often do volcanic glasses occur with a configuration related to basalt and gabbro (Mattern, 2006, p.17). The specimen shown above is about five centimeters across. Obsidian along the edge of a lava flow in central Oregon Bibliography Arribas, J., Critelli, S., & Johnsson, M. J. (2007). Sedimentary provenance and petrogenesis perspectives from petrography and geochemistry. Boulder, Colo, Geological Society of America. Krebs, R. E. (2003). The basics of earth science. Westport, Conn, Greenwood Press. Mattern, J. (2006). Igneous rocks and the rock cycle. New York, PowerKids Press. Mccaffrey, K., Lonergan, L., & Wilkinson, J. (1999). Fractures, fluid flow and mineralization. London, The Geological Society. Young, D. A. (2003). Mind over magma: the story of igneous petrology. Princeton, N.J., Princeton University Press. Read More