Geologically, the Earth’s crust comprises of more than one type of mineral element, whose combination makes up the Earth’s crust. There exist three main categories of solid aggregate mineral deposits (rocks) that make the Earth’s crust namely Sedimentary, Metamorphic, and Igneous rocks. Classification of these rocks into these three groups depends on a number of factors that include there mineral content, method of formation, texture, and the dimension of their grains or particles. For example, igneous rocks owe their existence to volcanic activities, because of magma cooling either on or within the Earth’s crust. Contrary to this, the formation of metamorphic rocks depends on the prevailing pressure and temperatures whereas, sedimentary rocks originate from accumulated sand, rock and organic matter (Murck, Skinner, & Mackenzie, 2009, pp.
Sedimentary Rocks and Plate Tectonic
The formation of sedimentary rocks depends on the nature of the land terrain, because for sedimentation to take place there has to be the washing of soil, dead organic matter, or rock particle to low lying areas. One primary characteristic of the sedimentation process is the accumulation of rock particles carried mostly in a dissolved form by transport agents.
The movements of plate boundaries, which can either result in different plates sliding over each other or colliding with each other, cause an irregular Earth’s surface, comprising of lowlands and highlands. For example, because of sliding of surfaces in convergent plate boundaries, most particles will deposit themselves between the two plates. Because of the extreme nature of pressure between them, the deposited sediments may undergo melting hence; forming igneous rocks when they cool back to their original form. In addition, because of the irregularity in the land’s terrain, gravitation force will make water to flow from high to lowlands hence, leading to accumulation of eroded particles in low-lying areas, which in most cases are water bodies.
On the other hand, sometimes the movement of plate can cause weaknesses in the Earth’s crust that can trigger a volcanic eruption that can lead to accumulation of volcanic products, for example, ash falls and lahars in water bodies (Murck, Skinner, & Mackenzie, 2009, pp. 248-251). Continuation accumulation of particles will lead to increased lithostatic pressure on the sediments, forming several sediment layers through a process called lithification and diagenesis. It is important to note that, unlike igneous and metamorphic rocks’ compositions that vary from their original forming components, because of the extremity nature of temperature and pressure conditions that these types of rocks form under; sedimentary rocks maintain their forming rocks’ characteristics, because they form under moderate temperature and pressure conditions (Murck, Skinner, & Mackenzie, 2009, pp. 226-228). Because of the varying nature of particle deposited in water bodies, the diagenesis process can result in three categories of sedimentary rocks.
The first category of sedimentary rocks is clastic sedimentary rocks. Clastic sedimentary rocks form because of the buildup of pre-existing rocks in bedding layers. Through the diagenesis process, pieces of clasts, which vary in size, accumulate together forming a rock that maintains some properties of its forming material. Examples of mineral elements that make clasts for example, sandstones, and siltstones include feldspar, clay minerals, lutile, and arenite.
As compared to sandstones that have coarser although bigger particles, sandstones and conglomerates have smaller and rougher particles. Classification of clastic rocks depends on the nature of particles forming the rocks in terms of size, mineral composition, and the matrix of the rock (Murck, Skinner, & Mackenzie, 2009, pp. 228-230). Organic or biological sedimentary rocks make up the second class of sedimentary rocks.
unlike clastic rocks that result from the accumulation and diagenesis of pre-existing rocks, this rocks primary components are remnants of living things, accumulated in low-lying areas and have undergone the lithification process. Most minerals that form organic sedimentary rocks are rich in carbonates minerals, which in most cases will eventually form limestone deposits. Examples of organic sedimentary rocks are flint and strotolites (Murck, Skinner, & Mackenzie, 2009, pp.
230). The third class of sedimentary rocks is chemical sedimentary rocks; rocks that form because of super-saturation and precipitation of dissolved minerals. Most chemical sedimentary rocks form in marine environments or in water bodies characterized by high evaporation rates; a process that increases water concentration hence, facilitating precipitation. Examples of chemical sedimentary rocks are halite and gypsum (Murck, Skinner, & Dana, 2010, pp.
230-235). One primary characteristic that differentiates sedimentary rocks the other two classes of rocks is there chemical composition hence, the need to understand their origin and methods of formation. For example, igneous rocks are products of volcanic activities hence, the tendency of most of them to be rich in minerals found in magma, for example, biotite. Metamorphic minerals being products formed under intense pressures and temperatures, most of them are unique from the common minerals. On the other hand, because sedimentary rocks are products of mineral deposits, most of them will have precipitated minerals, for example, quartz (Murck, Skinner, & Mackenzie, 2009, pp.
228-263). A second main method of differentiating these rocks is by use of their texture. Most sedimentary rocks have grains that differ in sizes, shape, and roundness whereas most grains that make igneous rocks are crystalline in nature and they intertwine in varying orientations. On the other hand, metamorphic rocks have share the crystalline and interlocking property with igneous rocks, although their grains are organized and have specific patterns. It is important to note that, sometimes identifying these different types of mineral is hard using the naked eye hence, the need to use geologist’s services with help of magnifying lenses (Murck, Skinner, & Mackenzie, 2009, pp. 228-263). In conclusion, there is a close correlation between rocks mores o sedimentary rocks and plate movements, such movements being the primary determinants of the Earth’s terrain.
Murck, B., Skinner, B., Mackenzie, D.
(2009). Visualizing geology. New Jersey: Wiley, John & Sons.