The Rocks of the Andes Mountains - Coursework Example

The Rocks Of The Andes Mountains Question 1 The six most common minerals or mineral groups in sedimentary rocks include: a. Quartz. Examples include flint, chert, and chalcedony. b. Olivine. Example include chrysotile. c. Amphibole. Examples include hornblende and glaucophane. d. Mica. Examples are muscovite and biotite. e. Pyroxene. Example is antigorite. f. K-feldspar. Example is the plagioclase. These minerals are commonly found in sedimentary rocks for the reason that when they are exposed to weathering, they tend to disintegrate or dissolve completely or partially into smaller particles as a result of the mineral weathering (Biju-Duval 45-48). Question 2 The rocks of the Andes Mountains are predominantly andesite for the reason that they resulted from the type of magma known as andesite that resulted from the orogenic volcanism. Subsequently, the rocks are predominantly andesite given that they are a feldspar of the series of plagioclase; hence, higher in feldspar (Andesite 1). The andesite are not equivalently comprised of the diorite or plutonic for the reason that these two are not intermediate in their composition as is an andesite. Additionally, diorites are intrusive igneous rocks that mostly comprise of plagioclase feldspar, but to a lower degree. As such, there is no equivalence in the rocks since the character of andesite is that it results from magma melting and assimilation to the surface, which is not the case for the diorites. Thus, it tends to be higher in silicon than the diorites (Cull 74-78). The formation of andesite in the Andes Mountain region is often defined by the melting and assimilation of the rock fragments by the rising magma to the surface. This crustal extension and magmatic activity that occurs during the melting and assimilation process of the rocks makes the rocks in the region to predominantly change to andesitic nature due to a change in their mineral composition. Largely, this process can be affiliated to the main geologic event referred to as the Andean Orogeny, which was mostly characterized by the subduction of the ocean crust (McCann 125-129). Question 3 The process of magma differentiation may be attributed to the creation of various compositions of rocks within a single magma chamber. By definition, magma differentiation may be referred to as the process of causing the composition of magma to change i.e. the process by which igneous rocks that are chemically different forming from the initial magma.. A case example of such a process is known to have resulted in the development of Cascade Mountains in Western Oregon that were largely formed out of a single batholith (large pluton) during the intrusive cooling process. The formation of magmas usually takes place as a result of the partial melting of silicate rocks found either in the continental crust or earth’s mantle. During this process, magma is subjected to cooling, which primarily can be attributed to the change in its composition as a result of migrating from its partial melting point to an area of low stress and cooler crust volume. As such, the magma differentiation process that leads to the creation of rocks of varying composition begins with the cooling stage of the magma causing it to be crystallized into various minerals. The crystallization takes into consideration the liquid or melt portion of the magma. Subsequently, the magma differentiation may be caused by contamination caused by assimilation of the rocks on the walls; thus, mixing two or more magmas and replenishes the magma in the chamber by fresh and hotter magma (Cull 121-123). This whole process of magma differentiation that results in the crystallization of the rocks into various minerals may be generalized into four main stages, commonly denoted as FARM process. This FARM process contains four processes, commonly known as: Fractional crystallization. Assimilation. Replenishment, and Mixing of the magma. These processes are dependent on the magma temperature, which is also a critical feature in the differentiation process. Varying composition of rocks shall be caused when the minerals that have high-temperatures crystallizes and settles out. This causes the molten material to have high concentrations of components that may later on form low-temperature minerals enriched rocks e.g. granite. In this process, those rocks that are crystallized last in the magmatic intrusion will be highly rich in minerals that are of low temperatures; examples being mica, quartz, and potassium. Considering that magma chamber is associated with great temperature enough to melt the rocks, and low pressure to allow for the expansion and existence in liquid state of the magma materials, the magma differentiation would take place based on two main aspects: texture and composition of rocks. With regards to texture, variations in the resultant rocks results from the length of time taken by a magma to cool; hence, defining the grain size of the minerals. Based on the FARM process, variation in the rocks in a particular magma chamber results in the manner that with the fractional crystallization, the rocks are removed and segregated from the melt of minerals that are being precipitated; thus, leading to the change in the melt’s composition. In the magmas or silicate melts, this fractional crystallization process becomes complex given that it is influenced by several other phenomenal characteristics such as the pressure, composition and temperature of the magma during the cooling process. Subsequently, this process is defined by the composition of the magma, which is the primary control upon which the rock minerals are crystallized due to the cooling of the magma (McCann 136-143). With the assimilation process, the main concept is in relation to the falsification of the mafic and ultramafic magmas as they rise along the crust. This process by assuming that the hot primitive magma will intrude into the cooler, felsic crust, causes the crust to melt; hence, mixing with the magma, and altering the rocks’ compositions. As for the replenishment stage, the variation in the rocks’ composition results during the cooling process undergone by the magma along the liquid line of descent; thus, resulting in the formation of a limited homogenous intrusive rock body. This formed intrusive rock is often characterized by uniformity in its composition and mineralogy, and partially differentiated cumulate mass. This condition often results in the materialization of an ideal rock combination similar to that of the Bowen’s reaction series. However, in most instances, the magmatic systems will be polyphase in nature having numerous magmatic pulses. Therefore, the liquid line of magma descent will be interrupted by the fresh batch of injected hot and undifferentiated magma that causes the fractional crystallization process to be enhanced further (Biju-Duval 212-217). This second phase of fractional crystallization that further results in the variation of the rocks can be explained by factors such as: Presence of additional energy from the additional heat that allows for more convection and resorption of the mineral phases that previously existed into the magma; thus, causing the precipitation of the high-temperature minerals. Change in the composition of the magma due to the introduction of the fresh magma due to a variation in the chemical phases of the rocks being precipitated. Destabilization of the already precipitating minerals by the fresh magma. In this, the change in the temperature and composition of rocks can cause rapid crystallization of the mineral phases that were going through a eutectic crystallization process. Finally, the variation in the composition may be attributed to the mixing of the magma, as a process that results from the meeting and commingling of two magmas to form a magma composition with dynamic features borrowed from the two states. Question 4 Shale is considered the best parent material for the recording differences in metamorphic grade, better than the sandstone and limestone for a number of reasons as highlighted herein. First, through the application of the superposition concept, shale in its layer C can be considered as the oldest than the limestone; hence, making it more preferred. Second, shale is more preferred than the other types of rocks given that it has a finer grain size than the sandstone or limestone, besides being an elastic sedimentary rock (McCann 79-83). Third, shale is best considered as the parent material given that it dominates the composition of the sedimentary rocks. This is because shale, being a component of the feldspar mineral that is mostly abundant in the earth’s crust, the weathering process would easily render the shale into clay as opposed to the case of other rocks such as sandstone and limestone. This aspect takes into consideration the composition of consolidated sedimentary particles of shale in its conversion processes. Subsequently, shales are more preferred over the limestone and sandstone as they are the sedimentary rock types that are most abundant (accounting for about 60% of the sediments). The shales are also easily metamorphosed as compared to the limestone or sandstones, and that it is mostly characterized by the production of chemical reactions that occurs in the minerals within this rock. Additionally, this aspect makes shale to be more preferred considering that the chemical processes that defines its being entirely take place when it is in the solid state. This implies that the shale would not require to undergo any melting process as commonly identified in the igneous processes. Subsequently, shale is preferred as the parent material because it would slowly undergo the weathering process as compared to limestone or sandstone. As well, shale is a parent rock because it can become schist, and the schist can become a gneiss. The arrangement of the rocks from the lowest to the highest grade can be in the manner below: Chlorite, mica schist, garnet-staurolite, phyllite, migmatite, kyanite schist, gneiss, and slate. This order makes references to the order of index minerals that often starts from the chlorite and ends with sillimanite. Considering the metamorphic zones, the differences in the minerals that are found in these zones is often a reflection of the metamorphic zones to which they belong (McCann 85-89). Question 5 The formation of metamorphic rocks can be from any type of rocks whether sedimentary, igneous or already existing metamorphic rocks. This process, however, entails the recrystallization of the rocks in the solid state with little change in the rocks general chemical composition. The process of metamorphism alters or changes the mineralogy or texture of the pre-existing rocks through a sequence of biochemical reactions that takes place between the rock minerals. The main driving forces of metamorphism are: a. Temperature. b. High pressure. c. Pore/Chemical fluids. Given that metamorphism will take place largely in the solid state without encountering any form of melting, its chemical reaction processes take place deeply in the subduction zone under subjection to higher temperatures and pressures. The result of this is the formation of new minerals and textures of rocks. During the metamorphism process, the platy minerals grow in the direction where they’ll encounter the least stress in their production of foliation. In this, those rocks that have only one mineral, which is also non-platy will result in the production of non-foliated rocks such as marble. With regard to temperature, usually the rocks are maintained at temperatures below the 2000C where they remain unchanged. However, as temperature rises, the crystal lattices get broken down resulting in the reformation into various atomic combinations and minerals. However, the formation of temperature is dependent on the mineral composition of a particular rock. The high temperatures come from the hot magma that has a temperature ranging from 700-12000C, resulting in contact metamorphism (Biju-Duval 174-178). However, with every deep burial of the rocks, the temperatures would rise by about 15-300C per kilometer resulting in the regional metamorphism sue to the continental and subduction collision of rocks. With regard to pressure, metamorphism results from the hydrostatic pressure that is produced as a result of the weight derived from the overlying rocks. When this pressure is directed to the rocks, it produces foliation in rocks that are regionally metamorphosed. With regard to the chemical fluids, metamorphism results during metasomatism in which the transportation of the fluids results in the gaining or loss of atoms. For instance, during this process, de-watering of the clay occurs as it changes to minerals such as garnet or kyanite. Additionally, the chemical fluids results in hydrothermal alteration in which hot water injected into the rocks due to metasomatism results in the production of metallic ores such as zinc and lead (McCann 120-126). Ultimately, with seafloor metamorphism, the chemical fluids results in the conversion of olivine into hydrated silicates such as chlorite, talc, and serpentine. Question 6 (Scanned Calculations) Question 7 (Scanned Graph) Based on the percentage distributions of the SiO2, the igneous rock can be classified as Basaltic Andesite. Works Cited Andesite. World of Earth Science. 2003. Encyclopedia.com. 4 Dec. 2014  Biju-Duval, Bernard. Sedimentary Geology: Sedimentary Basins, Depositional Environments, Petroleum Formation. Paris: Éd. TECHNIP, 2002. Print. Cull, Selby. Rocks and Minerals. New York: Chelsea House, 2008. Print. McCann, T. The Geology of Central Europe. London: The Geological Society, 2008. Print. Read More