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        <h1 class="subHeadings">Theory</h1>
        <h2 class="theoryHeadings" id="pressureandtemp">Pressure and Temperature</h2>
        <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.
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