Added pressure and temperature theory

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         <h1 class="subHeadings">Theory</h1>
         <h2 class="theoryHeadings" id="pressureandtemp">Pressure and Temperature</h2>
         <p>
-	</p>
+        The heat contained within the Earth is generated by two main sources: the formation of the Earth,
+        and the decay of radioactive isotopes. The Earth was formed by the accretion of a large number of
+        planitesimals as it cleared its orbit. The impact of those planitesimals generated a large amount
+        of heat which is still being lost from the Earth’s core today. The other source of heat comes
+        from the radioactive decay of elements within the crust and mantle of the Earth.
+        The primary radioactive isotopes in the Earth are uranium-235, uranium-238, thorium-232,
+        and potassium-40. Radioactive decay is the dominant form of heat flow at the surface of the Earth,
+        providing approximately 80% of the heat budget.
+        <br /><br />
+        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.
+        <br /><br />
+        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.
+        <br /><br />
+        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.
+        <br /><br />
+        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.
+        <br /><br />
+        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.
+	    </p>
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