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The crust is one of the three main concentric layers which make up the Earth’s interior. It is a very thin layer of solid rock which forms the outermost shell of the planet that supports living organisms as well as natural surface features such as rivers, lakes, and mountains. The Crust is significantly thinner than both the core and the mantle (the other two main layers making up the Earth’s interior). As a matter of fact, the crust accounts for less than 1 % of the earth’s total volume.
Properties and composition The crust not only varies in thickness with the other concentric layers but also differs due to its properties and composition. The solid crust is not only significantly thinner when compared to the other layers but is also less dense and less hot. Due to its solid nature, relative thinness and low density, the crust is brittle and prone to cracking. Moreover, the crust is also not uniform in its thickness with some of its regions being less thick 1 km thick and other regions is more than 80 km thick. The crust consists of a mixture of chemical elements, minerals, and rock types. The most abundant elements present in the crust are oxygen, silicon, aluminium, and iron. Other elements such as calcium, sodium, potassium, and magnesium are also present, however, these are present in more minute quantities.
The elements in the crust are often found combined with one another to form various compounds. Such compounds give rise to minerals. Minerals are the building blocks of rock. By definition, minerals are naturally occurring inorganic solids with definite chemical compositions and well-ordered internal structures. Minerals are generally composed of two or more elements. The crust is made up of over 2000 different minerals. However, many of these are present in very small quantities. As a matter of fact, the crust is mainly composed of six minerals which are feldspar, quartz, pyroxene amphibole, mica, and olivine. Feldspar is the most abundant mineral present in the crust. It consists of silicon, oxygen and other metallic elements such as sodium, potassium, calcium, and aluminum. Feldspar may exist in different varieties according to which metal element is present.
Two main types of feldspar exist which are known as plagioclase and alkali feldspar. This mineral has a light cream to salmon pink color. The second most abundant mineral is quartz. Quartz is one of the primary components of granite and sand. It is a hard, water-insoluble mineral which mainly consists of silica (SiO2). Quartz is usually colorless or white. Other minerals such as pyroxene, amphibole, mica, and olivine are also present however in smaller abundances. The minerals present in the crust (mainly feldspar and quartz) mix together to form different rock types. Rocks can be classified into three groups which are igneous rocks, sedimentary rocks, and metamorphic rocks. Igneous rock is the most abundant rock type found in the Earth’s crust. Rocks of this type form when molten rock such as magma or lava cools and solidifies. Igneous rocks are sometimes called the ‘primary rocks’ or the ‘parents of all rocks’ since they formed the Earth’s first crust and gave rise to all other types of rock. Igneous rocks can be classified as being either intrusive or extrusive based on the mode of formation and occurrence. Intrusive rocks are those rocks which form when magma solidifies beneath the earth’s surface. Some examples of this type of rock include granite, diorite, and gabbro.
Extrusive rocks, on the other hand, refer to rocks which form when lava cools on the earth’s surface. Such rocks include basalt, andesite, and rhyolite. Sedimentary rock forms from the accumulation of sediment and organic matter. Sediments come from weathered or eroded previously existing igneous or metamorphic rock. When sediments accumulate, the increased pressure causes the sediments to compress and form sedimentary rocks. This process is known as Lithification. Sedimentary rocks are found mainly in the upper parts of the crust since such types of rock are not very stable under high temperatures and high pressure. Some examples of sedimentary rock are shale, sandstone, and limestone.
Metamorphic rock refers to a rock which forms when igneous or sedimentary rocks undergo changes in their structure due to high pressure and high temperatures. The influence of heat and pressure causes recrystallization and reorganization of molecules within original rocks which results in overall changes in the rocks’ hardness and color. This process is known as metamorphism. This explains why sedimentary rock is not very stable in lower parts of the crust. Examples of metamorphic rock include marble (originating from limestone), quartzite (originating from sandstone) and blueschist (originating from basalt). Different types of crust The crust can be divided into two types – continental crust and oceanic crust. In general continental crust is the part of the crust which gives rise to continents whereas oceanic crust is the part of the crust which underlies the earth’s oceans. These types of crust differ from one another in terms of thickness and density and composition. Continental crust covers approximately 40% of the planet. This type of crust is mostly exposed to the air. It is older and thicker than oceanic crust. In fact, the continental crust is around 2 billion years old and is on average 35-40 km thick. The rocks of continental crust are sometimes referred to as ‘Sial’ by some geologists. This is due to the fact that the continental crust is mainly composed of granite which has silica (SiO2) and alumina (AlO3) as its most abundant chemicals.
Continental crust has a higher amount of oxygen when compared to oceanic crust. This is due to continental crust being more exposed to the atmosphere. Due to the chemical composition of granitic rocks, the continental crust has a relatively low density when compared to oceanic crust. In fact, the continental crust has an average density of 2.7-3.0 g/cm3. In terms of its minerals continental crust can be described as being felsic since the most abundant minerals in granite are feldspar and quartz whereas minerals such as amphibole, pyroxene and olivine are only present in trace amounts. Oceanic crust covers approximately 60% of the planet. This type of crust is thin and relatively young. It is no more than about 20 km thick and is on average 7-10km thick. Moreover, it no older than 180 million years (approximately).
The old oceanic crust is destroyed at subduction zones. Oceanic crust is formed at mid-ocean ridges as a result of the process of seafloor spreading whereby plates are pulled apart. This causes pressure in the underlying mantle to be released. Such pressure causes part of the peridotite (igneous rock present in the mantle) to melt. The melted peridotite gives rise to basaltic lava, which rises, cools, solidifies and forms a new oceanic crust. Oceanic crust is denser than continental crust since it is mainly composed of basalt. This type of crust has an average density of 3.0-3.3 g/cm3. Basaltic rocks are sometimes called ‘Sima’ by geologists due to the presence of silica and magnesium. In mineral terms, basalt is considered as being a mafic rock due to feldspar, amphibole, and pyroxene being the most abundant minerals present. The Moho-discontinuity There exists a boundary between the crust and the upper part of the mantle. This boundary is called the Moho-discontinuity and it was named after Andrija Mohorovicic, the seismologist who discovered it. Mohorovicic discovered a sharp discontinuity whereby the velocities of P-waves and S-waves increased in an abrupt fashion. He understood that there was a relationship between the velocity of seismic waves and the density of the material the waves are moving through and thus he interpreted this discontinuity as a change in composition within our planet. He concluded that this sharp increase in seismic wave velocity is due to a low-density crust being present over a high-density mantle. The Lithosphere The crust and the rigid top part of the mantle together form what is known as the lithosphere. The lithosphere is subdivided into a number of plates.
The lithosphere is made up of 7 major plates and several minor plates. These plates lie on top of the asthenosphere (the mantle’s softer and less rigid layer). The plates move as a result of convection currents generated when magma rises and sinks in this part of the mantle. The movement of plates is responsible for both the creation and destruction of crust in addition to numerous volcanic and seismic activity. Plate Boundaries Adjacent plates can interact with each other in different ways giving rise to different plate boundaries. Plate boundaries are classified into three types; divergent boundaries, convergent boundaries or transform faults. The type of boundary depends on the direction in which the adjacent plates are moving. With divergent boundaries, two plates move in opposite directions away from each other. When two adjacent oceanic plates move apart through the process of seafloor spreading, the magma underneath the plates upwells, cools and solidifies to give rise to the new crust. This type of boundary is found at mid-ocean ridges such as the Mid-Atlantic Ridge (the boundary between the North American plate and the Eurasian plate)
The formation of shield volcanoes or volcanic islands such as Iceland is common at this type of boundary. When continental plates move apart, the tensional forces generated when the plates get pulled apart causes cracks and faults to appear within the crust. As the plates continue to pull apart from each other, rock present between faults sinks downwards to give what is known as a rift valley. An example of a rift valley is the East African rift valley in Kenya, Africa. Convergent boundaries involve the movement of two plates against each other. When oceanic crust converges with continental crust, the thicker, denser oceanic crust sinks downwards beneath the thinner, less dense continental crust into the mantle via subduction to form a subduction zone. As the crust subducts, heat, pressure, and friction cause the crust to melt into magma. This magma rises up through cracks in the crust and can give rise to composite volcanoes.
Volcanic and earthquake activity is common at this type of boundary. An example of this type of boundary is the subduction of the Pacific plate as it converges with the Eurasian plate. When two continental plates converge into one another, neither plate sinks downwards due to both plates having similar densities. As a result, when the plates push into each other, they push overlying sediment upwards to form fold mountains. Magma can also rise through cracks present between the plates to give rise to volcanoes. Examples of fold mountains include the Andes mountain range and the Himalayas. Transform faults are a type of plate boundary whereby crust is neither created nor destroyed. With this type of boundary, two plates slide past each other in opposite directions. When two plates slide past each other, friction causes the two plates to get stuck. When this occurs, pressure builds up between the plates.
Eventually, the pressure is released in the form of an earthquake. The San Andreas Fault is an example of this type of boundary. In spite of the crust occupying less than 1% of the earth’s total volume, it serves an important role in supporting all living organisms. The crust exists in two different types, both of which have different chemical compositions and properties. The crust also forms part of the lithosphere and gives rise to plates and tectonic activity such as earthquakes and volcanoes.
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