4.2: Magma Composition and Eruption Style
- Page ID
- 33106
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The types of magma produced in the differing volcanic settings can differ quite significantly. At divergent boundaries and oceanic mantle plumes, where there is little interaction with crustal materials, the magma tends to be consistently mafic (~50% silica with significant metals). 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 (~70% silica with few metals)—results in magma that is relatively felsic.
In summary:
- Divergent boundaries - mafic
- Mantle plumes in ocean - mafic
- Convergent boundaries with subduction - felsic
From the perspective of volcanism there are some important differences between felsic and mafic magmas. First, felsic magmas tend to be more viscous and are therefore sticky because they have more silica, and more polymerization. Mafic magmas are less viscous and are therefore more "runny". Second, felsic magmas tend to have higher levels of volatiles—that is components that behave as gases during volcanic eruptions. The most abundant volatile in magma is water (H2O), followed, typically, by carbon dioxide (CO2) and then by sulphur dioxide (SO2). The general relationship between the SiO2 content of magma and the amount of volatiles is shown on Figure \(\PageIndex{1}\). 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.
Differences in viscosity and volatile level have significant implications for the nature of volcanic eruptions. When magma is deep beneath the surface and under high pressure from the surrounding rocks, the gases remain dissolved. As magma approaches the surface the pressure exerted on it decreases. Gas bubbles start to form, and the more gas there is in the magma the more bubbles will form.
As is typical in mafic magmas, if the gas content is low or the magma is runny enough for gases to rise up through it and escape to surface, the pressure will not become excessive. Assuming there is a way for it to get to surface, the magma will flow out relatively gently.
If the magma is felsic, and therefore too viscous for gases to escape easily, or if it has a particularly high gas content, it is likely to be under high pressure. Viscous magma doesn’t flow easily, so even if there is a way for it to get out, it may not be able to flow out readily. Under these circumstances pressure will continue to build as more magma moves up from beneath. Eventually some part of the volcano will break and then that pent up pressure will lead to an explosive eruption.
In summary:
- felsic magmas have high viscosity, so they don't flow easily; they also tend to have a high gas content which gets trapped by the sticky nature of the magma; when the magma/lava erupts it is explosive
- mafic magmas have low viscosity, so they are very fluid; they also tend to have a low gas content; even the gas that is there easily escapes the fluid magma; when the magma/lava erupts it is gentle
Mantle plume magmas in the middle of plates and spreading-ridge magmas at divergent boundaries tend to be consistently mafic and so gentle eruptions are the norm. At subduction zones the average magma composition ranges from felsic to intermediate even within the same magma body, so different eruptions can have very different magma compositions. Eruption styles can be correspondingly variable but are generally violent.
In summary:
- Divergent boundaries - gentle eruptions
- Mantle plumes in ocean - gentle eruptions
- Convergent boundaries with subduction - violent eruptions
Looking back at Figure \(\PageIndex{1}\), the eruptions on the far right and far left of the image created at subduction zones would be violent. Those occurring at the hotspot or at the divergent boundary in the ocean would be gentle.
Media Attributions
- Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0, after , H-U. (2004). Volcanism. Springer-Verlag, Heidelberg. https://link.springer.com/content/pd...42-18952-4.pdf