P and S waves travel at different speeds and these speeds depend upon the density of the material as well as the elasticity. The velocity of P and S waves slows when the material they are travelling through become more and more dense and denseness generally increases with depth. However they travel faster through materials with greater elasticity (Monroe, J.S., Wicander, R., Hazlett, R.W. 2006) and as result of this, the waves are then refracted or curved as seen in Figure 2. From studying these waves, we can see that the P and S waves change velocity as they travel through the Earth and encounter different materials and densities. By timing and understanding the properties of these waves, scientists are able to gain a greater understanding of what the interior structure of the Earth is.
The Earth’s crust is the thinnest layer of the planet’s internal structure. There are two types of crust, the thick continental crust beneath the continents and the oceanic crust, which is beneath the ocean. The average thickness of continental crust is 45km and the average for oceanic is 8km (Merali, Z., Skinner, B.J. 2009). The crust is made up of rocky material and both P and S waves are able to pass through. However this information only tells us that the crust is solid, and not its actual composition. The deepest hole in the world is currently in the Kola Peninsula of Russia. This drill hole has reached a depth just over 12km into the Earth’s crust (Coffey 2010). This hole has enabled scientists to gather rock samples from the crust and are hence able to discover what composes the crust of the Earth. The oceanic crust is mainly composed of basalts whilst the continental crust is mainly composed of granite.
The mantle is the thickest layer inside the Earth. It accounts for about two-thirds of the Earth’s mass and is made up of a thick layer of solid, hot rock. We know the mantle exists due to Mohorovičić discontinuity (moho). This discontinuity locates the edge between the mantle and the crust. The moho showed a large difference in the velocity of P and S waves between the crust and the mantle, showing that there is a rapid change in material (Chaklader, A.C.D). The other discontinuity is known as the transition zone. The transition zone marks the difference between the upper and lower mantle as a result of sudden seismic-velocity changes between 410km and 660km (Gupta, H.K. 2011). This is a result of a change in chemical composition of the olivine.
We know that the mantle is solid as both P and S waves are able to travel through. However the S-waves that pass through the upper mantle slow down and are partially absorbed. This is due to the low velocity zone. The low velocity zone is closely related to the asthenosphere. This is where the rocks in the mantle are close to melting point and are not as elastic, relating to the observed decrease in seismic wave velocity (Monroe, J.S., Wicander, R., Hazlett, R.W. 2006). In oceanic continental areas the low velocity zone is shallower and thicker than in continental shield areas. This then shows that the mantle is a solid, as well as containing a small amount of liquid. Scientists have also known that lava or magma, when erupted from volcanoes, comes from the mantle. By studying the molten rock, such as xenoliths and ophiolites we are able to understand the composition of the mantle, and from this study, it is now thought that the mantle mainly consists of olivine (Kamber. B 2009).
The core is the central sphere within our Earth. It is where the planet’s magnetic field is produced and is a major source of Earth’s internal heat. There are two parts, the inner and outer core. The outer core is known to be metal liquid, with no elastic rigidity and a relatively low viscosity. Scientists know this due to extensive studies on the behaviour of P and S-waves. Also the presence of our magnetic field provides strong evidence that the core of the Earth is metal. It is impossible for a magnet to exist, as the temperatures are too high for permanent magnetism. It is thought that the core is mostly made of iron and due to the outer core being liquid; the movement of molten metal could generate the Earth’s magnetic field, as iron is a substantial conductor. As the inner core is solid, it is unable to move and therefore produce the magnetic field. The outer core blocks the seismic S-waves so we therefore know that the outer core is molten.
The inner core is a sphere of solid metal, more specifically; iron, in the centre of the Earth. Scientists know this due to the passing of s-waves through this centre and also through figuring out the mass of the Earth by using gravity. We are able to work out the mass of Earth by knowing the velocity at which objects fall to Earth. Because scientists know the density of the crust and the mantle they are able to determine the weight of each segment. And compared this with the total weight of the Earth. After at first reaching a number that was too small, they concluded that there must be something heavier and denser in the middle of the mantle. The continuing pressure of the Earth when it first formed caused its original material to heat. It became so hot that these materials began to melt and this is what caused the layering of the Earth. The heavier materials condensed into the middle of the Earth and the lighter materials floated up to the surface of the planet.
From all this we can see that the current model for the internal structure of the Earth was created using all different types of scientific discoveries including both indirect and direct methods. It is important that we see the difference between fact and theory in order to produce an accurate representation of our world today.
Bibliography
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