Table of contentsTypes of magmaChemical composition of magmasInitial composition of magmaMagmatic differentiationCrystal fractionationTypes of magmaBasaltic magma - SiO2 45-55% by weight, high in Fe, Mg, Ca, low in K , Andesitic NaMagma - SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, KRhyolitic magma - SiO2 65-75%, low in Fe, Mg, Ca, rich in K, NaThe temperature of magmas is difficult to measure (due to the danger involved), but measurements in Laboratory and limited field observations indicate that the eruption temperature of various magmas is as follows: Basaltic magma - 1000 to 1200 oC Andesitic magma - 800 to 1000 oCR Hyolitic magma - 650 to 800 oC How magmas form in the Earth As we have seen, the only part of the earth that is liquid is the outer core. But the core is unlikely to be the source of magmas because it doesn't have the right chemical composition. The outer core is mostly iron, but the magmas are silicate liquids. So, magmasSay no to plagiarism. Get a custom essay on “Why Violent Video Games Should Not Be Banned”?Get the original essaySince the rest of the earth is solid, for magmas to form, some part of the earth must become hot enough to make melt rocks. here. We know that temperature increases with depth in the earth, along the geothermal gradient. The earth is hot internally due to residual heat from the initial accretion process, heat released by the sinking of materials to form the core, and 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 so, except for the outer core, most of the Earth is solid. Thus, magmas only form under particular circumstances and volcanoes are only found on the Earth's surface in areas above which those particular circumstances occur. (Volcanoes don't appear just anywhere, as we'll soon see). To understand this, we must first observe how rocks and minerals melt. To understand this, we must first observe how minerals and rocks melt. As pressure increases in the Earth, the melting temperature also changes. For pure minerals, there are two general cases. Chemical Composition of Magmas The chemical composition of magma can vary depending on the rock that initially melts (the source rock) and the processes that occur during partial melting and transport. Initial magma composition The initial composition of the magma is dictated by the composition of the parent rock and the degree of partial melting. In general, the melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas. The melting of crustal sources produces more siliceous magmas. In general, more siliceous magmas form through low degrees of partial melting. As the degree of partial melting increases, fewer siliceous compositions can be generated. Thus, the melting of a mafic source gives a felsic or intermediate magma. The fusion of the ultramafic (source of peridotite) gives a basaltic magma. Magmatic differentiation But the processes that take place during transport to the surface or during storage in the crust can modify the chemical composition of the magma. These processes are called magmatic differentiation and include assimilation, mixing, and fractional crystallization. Assimilation – When magma passes through cooler rock before reaching the surface, it can partially melt the surrounding rock and incorporate.