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Basaltic magma – SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, NaAndesitic magma – SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, KRhyolitic magma – SiO2 65-75%, low in Fe, Mg, Ca, high in K, NaTemperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows: Basaltic magma – 1000 to 1200 oCAndesitic magma – 800 to 1000 oCRhyolitic magma – 650 to 800oCHow Magmas Form in the EarthAs we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids. Thus, magmas
Since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present. We know that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances, and thus, volcanoes are only found on the Earth’s surface in areas above where these special circumstances occur. (Volcanoes don’t just occur anywhere, as we shall soon see). To understand this, we must first look at how rocks and mineral melt. To understand this, we must first look at how minerals and rocks melt. As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
The chemical composition of magma can vary depending on the rock that initially melts (the source rock), and process that occur during partial melting and transport.
The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting. In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas. Melting of crustal sources yields more siliceous magmas.
In general, more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma.
But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma. These processes are referred to as magmatic differentiation and include assimilation, mixing, and fractional crystallization.
Assimilation – As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous.
Mixing – If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks.
When magma solidifies to form a rock, it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. Depending on how many minerals are lost in this fashion, a wide range of compositions can be made. The processes are called magmatic differentiation by crystal fractionation. Crystals can be removed by a variety of processes. If the crystals are denser than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case. Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO2. Initially the composition of this magma is expressed as 50% SiO2 and 50% MgO.
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