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Theory
Pressure and Temperature
+ The temperature of the Earth changes from around 0°C at the surface to over 5000°C in the core.
+ The geothermal gradient in the crust is approximately 30°C per kilometre in the crust on average,
+ however there is significant local variation due to things such as the conductive abilities of the
+ rocks at particular locations, as well as the presence of nearby magmatic intrusions.
+ The geothermal gradient in the mantle reduces significantly, down to approximately 0.3°C/km.
+ The base of the lithosphere is defined by the 1000°C isotherm. The base of the mantle is at
+ approximately 2800°C.
+
+ The method of heat transfer changes throughout the Earth. There are three main mechanisms for
+ heat transfer in the Earth: conduction, convection, and radiation. Starting in the inner core,
+ the main method of heat transfer is by conduction through the solid material. In the liquid,
+ outer core heat transfer is by both conduction and convection. The mantle is dominated by convection,
+ which is the driver of plate tectonics. The crust is again dominated by conduction, and finally
+ energy escapes the Earth to the atmosphere by radiation.
+
+ Pressure in the Earth continually increases with depth, according to the formula P = gρz,
+ where g is the gravitation field strength, ρ is the density and z is the depth. The main
+ difference in the pressure gradient is cause by the different lithologies in the layers
+ of the Earth. The average density of continental crust is approximately 2.7g/cm3 and
+ is ~3.0 g/cm3 in oceanic crust, and increases to about 3.3 g/cm3 in the mantle.
+ The increase of pressure with depth in the Earth affects the dominant mineralogy, as
+ well as the increase of the melting point of different minerals.
+
+ The structure of minerals, such as olivine becomes unstable as pressure increases.
+ Below about 410km olivine becomes unstable and transforms into wadsleyite, which has the
+ same chemical composition as olivine, but has a different crystal structure. As depth
+ increases, wadsleyite transforms to ringwoodite, which subsequently transforms into
+ perovskite at about 600km.
+
+ At higher pressure, minerals have a higher melting point. The effect is this phenomenon
+ is that the dominant method of melting in the mantle is not by increasing the temperature
+ past its melting point, but by decompression. Due to convective processes in the mantle,
+ as parts of the mantle rise the decrease in pressure lowers the melting temperature and
+ can cause the mantle to melt.
+