IndexTypes of magmaChemical composition of magmasInitial composition of magmaMagmatic differentiationCrystal fractionationTypes of magmaBasaltic magma - SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, NaAndesitic magma - SiO2 55-65% by weight, intermediate. in Fe, Mg, Ca, Na, KRheolitic magma - SiO2 65-75%, low in Fe, Mg, Ca, high in K, NaThe temperature of magmas is difficult to measure (due to the danger involved), but measurement in the laboratory is limited, field observations indicate that the eruption temperature of various magmas is as follows: Basaltic magma - from 1,000 to 1,200 oCAndesitic magma - from 800 to 1,000 oCRiolitic magma - from 650 to 800oCHow magmas are formed in the EarthAs 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 the magmas because it doesn't have the right chemical composition. The outer core is made mostly of iron, but the magmas are silicate liquids. So the magmas say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an Original Essay Since the rest of the earth is solid, for magmas to form, some parts of the earth must become 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 residual heat from the original accretion process, heat released from the sinking of materials to form the core, and heat released from 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. Therefore, magmas form only under special circumstances and, therefore, volcanoes are only found on the Earth's surface in areas above where these special circumstances occur. (Volcanoes don't occur everywhere, as we'll soon see.) To understand this we must first look at the way rocks and minerals dissolve. To understand this, we first need to look at how minerals and rocks dissolve. As the pressure on Earth increases, 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 process that occurs during partial melting and transport. Initial composition of magma The initial composition of 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) gives rise to mafic/basaltic magmas. The fusion of crustal sources produces more siliceous magmas. In general, the most siliceous magmas form with low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. Thus, the melting of a mafic source then produces a felsic or intermediate magma. The melting of the ultramafic (source of peridotite) produces a basaltic magma. Magmatic differentiation But processes that operate during transport to the surface or during storage in the crust can alter the chemical composition of the magma. These processes are called 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 surrounding rock and incorporate this melting into the magma. Since small amounts of partial melt result in siliceous liquid compositions, adding this melt to magma will make it more siliceous. Mixing.