Figure 4.8 Variations in the volatile compositions of magmas as a function of silica content ĭifferences in viscosity and volatile level have significant implications for the nature of volcanic eruptions. Although there are many exceptions to this trend, mafic magmas typically have 1% to 3% volatiles, intermediate magmas have 3% to 4% volatiles, and felsic magmas have 4% to 7% volatiles. The general relationship between the SiO 2 content of magma and the amount of volatiles is shown in Figure 4.8. The most abundant volatile in magma is water (H 2O), followed typically by carbon dioxide (CO 2), and then by sulphur dioxide (SO 2). Second, felsic magmas tend to have higher levels of volatiles that is, components that behave as gases during volcanic eruptions. First, as we’ve already discussed, felsic magmas tend to be more viscous because they have more silica, and hence more polymerization. įrom the perspective of volcanism there are some important differences between felsic and mafic magmas. Figure 4.7 The important processes that lead to changes in the composition of magmas stored within magma chambers within relatively felsic rocks of the crust. Settling of ferromagnesian crystals from the upper part of the magma, and possible remelting of those crystals in the lower part can both contribute to the vertical zonation from relatively mafic at the bottom to more felsic at the top. Partial melting of country rock and country-rock xenoliths increases the overall felsic character of the magma first, because the country rocks tends to be more felsic than the magma, and second, because the more felsic components of the country rock melt preferentially. At subduction zones, where the magma ascends through significant thicknesses of crust, interaction between the magma and the crustal rock - some of which is quite felsic - leads to increases in the felsic character of the magma.Īs shown in Figure 4.7, several processes can make magma that is stored in a chamber within the crust more felsic, and can also contribute to development of vertical zonation from more mafic at the bottom to more felsic at the top. At divergent boundaries and oceanic mantle plumes, where there is little interaction with crustal materials and magma fractionation to create felsic melts does not take place, the magma tends to be consistently mafic. As noted in the previous section, the types of magma produced in the various volcanic settings can differ significantly.
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