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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 planetesimals as it cleared its orbit. 
        The impact of those planetesimals 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 />
        </p>
        <figure>
        <a href='{% static "three_d_viewer/images/seismic earth.png" %}'><img height=300 width=300 src='{% static "three_d_viewer/images/seismic earth.png" %}'></a>
        <figcaption>The seismic profile of the earth</figcaption>
        </figure>
        <p>
        <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/cm<sup>3</sup> and is ~3.0 g/cm<sup>3</sup> in oceanic crust, 
        and increases to about 3.3 g/cm<sup>3</sup> 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 <a href={% url url_extender|add:olivine.url olivine.id %}>olivine</a> becomes unstable as pressure increases. 
        Below about 410 km olivine (Mg<sub>2</sub>SiO<sub>4</sub>) becomes unstable and transforms into wadsleyite (Mg<sub>2</sub>SiO<sub>4</sub>), 
        which has the same chemical composition as olivine, but has a different crystal structure. As depth increases, 
        wadsleyite transforms to ringwoodite (Mg<sub>2</sub>SiO<sub>4</sub>) at ~520 km, which subsequently transforms into silicate perovskite 
        (MgSiO<sub>3</sub>) and magnesiowuestite (MgO) at about 660km depth. The increase in density is observed in an increase in the 
        velocity of P- and S-waves.
        <br /><br />
        </p>
        <figure>
        <a href='{% static "three_d_viewer/images/depth profile.png" %}' align="right"><img height=300 width=270 src='{% static "three_d_viewer/images/depth profile.png" %}'></a>
        <figcaption>The depth profile of the earth</figcaption>
        </figure>
        <p>
        <br />
        At higher pressure, minerals have a higher melting point. The effect of 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. Such process is observed at oceanic spreading centres, where partial decompression melting of the 
        mantle produces juvenile oceanic crust.
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