The crust is one of the three main concentric layers that make up the interior of the Earth. It is a very thin layer of solid rock that forms the outermost shell of the planet that supports living organisms and 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 that make up the Earth's interior). The crust, in fact, represents less than 1% of the total volume of the earth. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Properties and composition The crust not only varies in thickness with the other concentric layers, but also differs in its properties and composition. The solid crust is not only significantly thinner than the other layers, but it 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. Furthermore, the crust is also not of uniform thickness: some of its regions are less thick (1 km), and other regions are more than 80 km thick. The crust is made up of a mixture of chemical elements, minerals and rock types. The most abundant elements present in the crust are oxygen, silicon, aluminum and iron. Other elements such as calcium, sodium, potassium and magnesium are also present, but present in smaller quantities. The elements of the crust are often found combining with each other to form various compounds. These compounds give rise to minerals. Minerals are the building blocks of rock. By definition, minerals are naturally occurring inorganic solids with defined 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, amphibole pyroxene, mica and olivine. Feldspar is the most abundant mineral present in the crust. It is made up of silicon, oxygen and other metallic elements such as sodium, potassium, calcium and aluminium. Feldspar can exist in different varieties depending on the metallic element present. There are two main types of feldspar known as plagioclase and alkali feldspar. This mineral has a color from light cream to salmon pink. 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, consisting mainly of silica (SiO2). Quartz is usually colorless or white. However, other minerals such as pyroxene, amphibole, mica and olivine are also present in smaller quantities. The minerals present in the crust (mainly feldspar and quartz) mix together to form different types of rock. Rocks can be classified into three groups: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rock is the most abundant type of rock present 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 “primary rocks” or “parents of all rocks” because they formed the Earth's first crust and gave rise to all other types of rocks. Igneous rocks can be classified as intrusive or extrusive based on how they are formed and occur. Intrusive rocks are those rocks that form when magma solidifies beneath the Earth's surface. Some examples of this rock type include granite, diorite, and gabbro. Extrusive rocks, on the other hand, refer to rocks that form when lava cools on the surfaceterrestrial. Such rocks include basalt, andesite and rhyolite. Sedimentary rocks are formed from the accumulation of sediment and organic matter. The sediments come from previously existing, weathered, or eroded igneous or metamorphic rocks. As sediment accumulates, the increase in pressure causes the sediment to compress and form sedimentary rocks. This process is known as lithification. Sedimentary rocks are mainly found in the upper parts of the crust since such types of rocks are not very stable at high temperatures and high pressure. Some examples of sedimentary rock are shale, sandstone, and limestone. Metamorphic rock refers to a rock that 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 the recrystallization and reorganization of molecules within the original rocks, resulting in overall changes in the hardness and color of the rocks. This process is known as metamorphism. This explains why sedimentary rock is not very stable in the lower parts of the crust. Examples of metamorphic rocks include marble (originated from limestone), quartzite (originated from sandstone), and blueschist (originated from basalt). Different Types of Crust The crust can be divided into two types: continental crust and oceanic crust. In general, the continental crust is the part of the crust that gives rise to the continents while the oceanic crust is the part of the crust that underlies the Earth's oceans. These types of crust differ from each other in thickness, density and composition. Continental crust covers about 40% of the planet. This type of crust is mostly exposed to the air. It is older and thicker than oceanic crust. The continental crust, in fact, is about 2 billion years old and is on average 35-40 km thick. Rocks of the continental crust are sometimes called "Sial" by some geologists. This is because the continental crust is mainly composed of granite which has silica (SiO2) and alumina (AlO3) as the most abundant chemicals. Continental crust has a greater amount of oxygen than oceanic crust. This is due to the continental crust being more exposed to the atmosphere. Due to the chemical composition of granitic rocks, continental crust has a relatively low density compared to oceanic crust. The continental crust, in fact, has an average density of 2.7-3.0 g/cm3. In terms of minerals, the continental crust can be described as felsic since the most abundant minerals in granite are feldspar and quartz, while minerals such as amphibole, pyroxene and olivine are present only in trace amounts. Oceanic crust covers about 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 7-10 km thick. Furthermore, it is no more than 180 million years old (approximately). Old oceanic crust is destroyed in subduction zones. Oceanic crust forms on mid-ocean ridges following the process of seafloor spreading by which plates are pulled apart. This causes pressure to release in the mantle below. This pressure causes the melting of part of the peridotite (igneous rock present in the mantle). The molten peridotite gives rise to basaltic lava, which rises, cools, solidifies and forms new oceanic crust. Oceanic crust is denser than continental crust because it is mostly 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 a mafic rock since feldspar, amphibole and pyroxene are the mineralsmore abundant present. The Moho discontinuity There is a boundary between the crust and the upper part of the mantle. This boundary is called the Moho discontinuity and is named after Andrija Mohorovicic, the seismologist who discovered it. Mohorovicic discovered a sharp discontinuity whereby the speed of P waves and S waves increased suddenly. He understood that there is a relationship between the speed of seismic waves and the density of the material that the waves move through and therefore interpreted this discontinuity as a change in composition within our planet. He concluded that this sharp increase in seismic wave speed is due to the presence of a low-density crust above a high-density mantle. The lithosphere The combination of the crust and the rigid upper part of the mantle constitutes the so-called lithosphere. The lithosphere is divided into numerous plates. The lithosphere is made up of 7 major plates and several minor plates. These plates sit above the asthenosphere (the softest and least rigid layer of the mantle). The plates move due to convective currents generated when magma rises and falls in this part of the mantle. The movement of plates is responsible for both the creation and destruction of the crust as well as numerous volcanic and seismic activities. 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 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 underlying magma rises, cools and solidifies to give rise to the new crust. This type of boundary is found on 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 in this type of boundary. As continental plates move apart, the tensile forces generated when the plates pull apart cause cracks and faults to appear within the crust. As the plates continue to separate from each other, the rock between the faults sinks downward, creating what is known as a Rift Valley. An example of a Rift Valley is the East African Rift Valley in Kenya, Africa. Converging boundaries involve the movement of two plates against each other. When oceanic crust converges with continental crust, the thicker, denser oceanic crust sinks downward beneath the thinner, less dense continental crust into the mantle via subduction to form a subduction zone. When the crust subducts, heat, pressure and friction cause the crust to melt, turning it into magma. This magma rises through cracks in the crust and can give rise to composite volcanoes. Volcanic and seismic activity is common in 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 each other, neither plate sinks downward because both plates have similar densities. As a result, when plates push against each other, they push overlying sediments upward to form fold mountains. Magma can also rise through the cracks between the plates, giving rise to volcanoes. Examples of folded mountains include the Andes mountain range and the Himalayas. Transform faults are a type of plate boundary where the crust.