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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. The layers are distinguished by a change in the velocity of seismic waves at their boundaries. 
	The crust is the upper most part of the earth, with depths ranging from an average of 7km in in the oceans, to an average of 38km in continental crust. 
	The crust thickens underneath mountain ranges, and can reach depths of 90km 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(~300Ma) due to subduction processes. 
	Continental crust is much more varied in structure and composition than oceanic, but has an overall average composition of granodiorite. 
	<br /><br />
	The boundary of the crust and mantle is defined by the Mohorovi?i? 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. The mantle is dominated by 
	silicate minerals that are rich in iron and magnesium, chiefly pyroxenes and polymorphs of olivine, forming peridotite. The mantle, while solid, behaves plastically, 
	allowing to flow at very slow rates.
	<br /><br />
	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 crystalizing minerals from the liquid part of the core as the Earth cools. 
	<br /><br />
	The crust and mantle are also further distinguished by material properties into the lithosphere, asthenosphere, and mesosphere. 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 ~100km under oceanic crust, and 200-300km under continental crust (Twiss & Moores, 2007). 
	The lithosphere-asthenosphere boundary is defined by the 1300K 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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		<figcaption>Image sourced from <a href="http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif">USGS</a>.</figcaption>
	</figure>
	</p>
	<h2 class="theoryHeadings" id="pressureandtemp">Pressure and temperature</h2>
        <p>
	</p>
        <h2 class="theoryHeadings" id="bowensreactionseries">Bowen's reaction series</h2>
        <p>
	Bowen's Reaction Series arranges the silicate minerals in the order that they crystallise from a magma. The minerals at the top of the series crystallise from the melt at higher temperature than those 
	lower down. It contains a continuous series, (right hand limb), discontinuous series (left hand limb), and the residual phases that describe the reaction pathway of different silicate minerals. 
	The discontinuous series crystallises different minerals, with abrupt changes separating the different minerals due to a mineral reacting with the melt to form a different mineral (eg. Olivine reacting to crystallise pyroxene). 
	The continuous series always crystallises plagioclase,  but the composition of the plagioclase varies from more calcic at higher temperatures and more sodic as the temperature decreases. 
	with the minerals at the bottom of the series being more stable, and less susceptible to weathering. 
	The residual phases are the minerals at the bottom and crystallise last. Bowen's reaction series also predicts the stability of minerals in the low pressure conditions at the Earth's surface, 
	<br /><br />
	It should be noted that all reactions do not start crystallising olivine/anorthite-rich plagioclase and continue through until they crystallise quartz. 
	The actual reactions depend on many factors, such as the chemical composition of the melt, temperature, pressure, and amount of fractional crystallisation. 
	For example, basalts form from the crystallisation of olivine, pyroxene and calcic plagioclase meaning that crystallisation stopped without the series progressing. 
	If more fractional crystallisation were to occur, more intermediate and felsic minerals can crystallise.
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