Essay: The internal composition and structure of the earth

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  • The internal composition and structure of the earth
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The internal composition and structure of the earth can be described in two ways. For example, by looking at the divided layers of the earth (core, mantle and crust) shown through research which is based on seismic waves and their velocity when travelling through each individual layer or by looking at the ability of which the layers can flow (concerning the lithosphere and asthenosphere). In order to examine both the properties of the internal composition and the evidence of which it is based upon, the order of each layer will be looked at individually following the same descending structure of Figure 1 (from crust to inner core).

The first outermost layer of the Earth is the crust. This is often described metaphorically as an egg shell. There are two classifications of crust, the continental crust, ranging from ’35 to 40km thick’ and the oceanic crust, ranging from ‘7 to 10km thick.’ (Marshak, 2005). Both types of crust have been found to be configured of a variety of rocks. With the continental crust forming granite rich rock and the oceanic crust forming basalt rich rock, (Smithson et al, 2008) therefore the density differs. By measuring the earths density, geologists were able to find that rocks at the earths surface (both granite and basalt) have a low density, of only ‘2.2-2.5 g/cm3’. (Marshak,2005) thus exceeding the average density of the earth. Alongside, the density of surface rocks evidence using geothermal gradient is able to show the way in which the upper part of the crust averages 15-50C per km. (Marshak, 2005) . We are able to see that the crust forms the outer layer of the earth due to the geothermal gradient and density of the earth increasing when getting closer to the core, for example ‘35km below the surface of a continent the temperature reaches 400-700C’. (Marshak, 2005) Even though 35km below the surface still forms part of the continental crust the evidence helps piece together, alongside following evidence concerning the mantle and core what the Earth is made up of and how it is structured.

Below the layer of less dense crust is the mantle. This layer is ‘2,885km thick’ (Marshak, 2005), this is a total of 84% (Encyclopaedia Entry Mantle’, n.d.) of the earth leading to this layer being the largest layer in terms of volume. However, the mantle can be split into three different sub-layers; the upper mantle, the transition zone and the lower mantle, each of which in turn have their own thickness and to some extent properties, as shown in Figure 2. This diagram shows that there are changes in terms of composition within the structure of the mantle itself. It also allows a visual representation of the way in which the volume of the mantle differs to the crust in addition to the difference in density between oceanic and continental crusts. Although the layers have some different properties, the mantle as a whole is made up of predominantly Peridotite, which is a dense, ultramafic rock. Due to the density of this rock and the minerals of which it consists, e.g. olivine the majority of the mantle is solid, however due to the temperature the magma flows very slowly, which eventually causes the plates of the crust above to move marginally. The temperature of the mantle has the ability to change significantly within different regions. The inconsistency of temperature, result in geologists assuming that mantle convect, with warm magma gradually flowing upwards whilst the cooler magma which also happens to be denser, sinks. Due to the eventual plate movement within the crust layer (caused by the convection currents in the magma) studies of the seismic waves travelling through the earth when earthquakes occur can be used to pin point the different layers within the internal structure of the earth. This can be done due to the the waves travelling at different velocities when in different materials. Therefore, by detecting the depths by which the velocities change, researchers and geologists can see where the the boundaries of each layer are. This applies to both the internal layers of the mantle as well as the boundaries between the three main layers. As a result of the mantle sharing a boundary with the core, much of the lower mantle is composed of minerals which are transitional between iron (from the core) and the lighter oxides of silicon and aluminium (Smithson et al, 2008)

The core like the mantle is split into different layers, the outer core and inner core. Similarly to the mantle, the layers of the core have many dissimilar properties to each others composition and structure. Both layers are made up of iron alloy. This was found out via comparing properties of metallic meteorites to the core. Although being made up of the same thing the outer core and inner core each can be described as its own layer in its own right within the structure of the earth. This conclusion came about through the study of earthquakes and the way in which certain types of seismic waves cannot pass through the outer part of the core. The outer core forms between ‘2,900 and 5,155km’(Marshak, 2005) and is liquid iron alloy. This is due to the high temperature and pressure to which is down to the overlaying rocks. With the rate of temperature being so great the ability of the liquid iron alloy to lock atoms into a solid framework is minimal even despite the pressure. In contrast, the inner core forms between ‘5,155km-6,371km’ (Marshak, 2005) and is formed of solid iron nickel alloy. Although the the inner core is hotter than the outer core it is a solid. This is due to the even greater pressure it is under along with the depth. The immense amount of pressure that the inner core is subjected to, approximately ‘3,600,000 atm’, results in atoms packing together tightly in the dense materials surrounding, thus resulting in a solid centre. However, to an extent there still is a small amount of ambiguity when studying the core and the centre of the earth. This is due to the inability of accessing this area of the earths internal structure. Therefore as of 2005, calculations have shown that it may reach 4,700C. This is approximately 800C less than the surface of the sun (Marshak,2005).

In conclusion, geologists and researchers are able to investigate the internal structure of the earth alongside the composition to which it is made up of via evidence such as the above discussed; the earths density, pressure and temperature inside the earth, study of earthquakes as well as other factors such as the earths shape. However there still lies great complexity around much of the layers within the earth due to the inaccessible areas mainly due to temperature and pressure but also due to the advances in technology. Until the quaternary sector and research innovations become vastly advance the likelihood of truly understanding what is going on within the interior of the earth is still left open to question.

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