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        <h1 class="subHeadings">Theory</h1>
        <h2 class="theoryHeadings" id="structureofearth">Structure of Earth</h2>
        <p>
	The Earth's structure is differentiated in three distinct layers: the core, mantle, and crust (? Earth profile). The layers are distinguished 
    by a change in the velocity of seismic waves at their boundaries (? Seismic profile). The crust is the upper most part of the earth, with 
    depths ranging from an average of 7 km in the oceans, to an average of 38 km in continental crust. The crust thickens underneath mountain 
    ranges, and can reach depths of 90 km underneath the Himalayas. The composition of the crust also differs between oceanic and continental 
    environments. Ocean crust is young mafic crust dominated by basalts and gabbros that is recycled regularly (~300 Ma) due to subduction 
    processes (? Subduction). Continental crust is much more varied in structure and composition than oceanic, but has an overall average 
    composition of granodiorite. 
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    <img src="{% static "three_d_viewer/images/element abundance.png" %}" align="right" width="412" height="324">
	The boundary of the crust and mantle is defined by the Mohorovicic discontinuity, commonly referred to as the Moho. It is defined by a 
    sharp increase in seismic wave velocity, due to a change in material properties between crustal rocks and mantle rocks (? Seismic profile). 
    The mantle is dominated by silicate minerals that are rich in iron and magnesium, chiefly pyroxenes and polymorphs (? Polymorphs) of olivine, 
    forming peridotite. The mantle, while solid, behaves plastically, allowing to flow at very slow rates. 
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	The core is distinguished by the absence of S waves, leading to the inference that the core is liquid. 
    The core is separated into the outer core and the inner core. The outer core is liquid dominated by iron and nickel. 
    The inner core is solid, as determined by the strong refraction of P waves at the inner core-outer core boundary, and was formed by 
    crystallizing minerals from the liquid part of the core as the Earth cools. 
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    The crust and mantle are also further distinguished by material properties into the lithosphere, asthenosphere, and 
    mesosphere (? Earth profile). The chemical composition is uniform throughout the mantle though, but changes in pressure and temperature 
    determine which polymorphs will exist at different depths. The lithosphere contains the crust, and the upper part of the mantle down 
    to ~100 km under oceanic crust, and 200-300 km under continental crust (Twiss & Moores, 2007). The lithosphere-asthenosphere boundary 
    is defined by the 1300 K isotherm, which is the temperature where olivine starts to behave viscously. The rocks in the mesosphere are 
    under more pressure than those in the asthenosphere, so no longer behave viscously.
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		<img src="{% static "three_d_viewer/images/structure - usgs.gif" %}" style="padding-top:20px;">
		<figcaption><p>Image sourced from <a href="http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif">USGS</a>.</p></figcaption>
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