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146 CHAPTER 10

chemical composition has a strong influence on the Table 10.1 Typical total water contents (wt%) in
detailed nature of eruptions. In what way, then, various magmas.
does magma chemistry influence the physics of erup-
Basalts* 0.1–1.5
tions (and therefore the character and products
Andesites 1– 4
of eruptions)? Two properties, the viscosity of the
Dacites and rhyolites 3–7
magma and the magma gas content, are strongly
linked to magma composition and play an important *Excluding those in subduction settings where water
role in controlling the physics of specific eruptions. contents can be as large as 4–6 wt%.
The gas content ultimately determines whether an
eruption is explosive or effusive, but it also plays a The presence of the gas bubbles themselves also
role in determining the magma viscosity, and it is has an effect on the viscosity, one which depends
the viscosity which determines how fast, and hence on the rate at which the magma is sheared. The rate
with what volume flux, a magma rises through a of shear of a fluid is measured by the change in its
given sized fissure system toward the surface. flow speed over a given distance within the fluid. At
The viscosity of magma is in fact controlled by low shear rates (e.g., a slow-moving fluid in a wide
various interconnected factors in addition to the vol- channel or conduit) the presence of bubbles will
atile content: these are the silica content of the have the effect of increasing magma viscosity, but
magma, its temperature, its crystal content and the at higher shear rates (a fast-flowing fluid in a narrow
gas bubble content. High silica content and low tem- channel or conduit) the bubbles readily deform and
perature make a magma more viscous. As it happens, have the effect of reducing the viscosity. High-silica
high silica content magmas melt at lower tempera- magmas, with inherently high viscosities due to all
tures than those with lower silica contents such as the other factors mentioned, tend to fall into the
basalts and so are inherently cooler, so that these low shear-rate category, and so this effect only makes
two factors enhance one another. The presence of their viscosities even greater still. It is the inher-
crystals within a magma tends to increase the magma ently low-viscosity primitive magmas that benefit
viscosity. But crystals begin to form as a magma from this “shear-thinning” process and may have
cools (e.g., when it nears the surface or is stored for their viscosities reduced somewhat in this way.
long periods in a magma chamber) and so again this
factor tends to combine with the previous two. 10 10
Chapter 5 looked at the solubility of various

volatile species in different magmas. The two most 10 8 Rhyolite
important volatiles in magma are usually water
(H O) and carbon dioxide (CO ). The solubility of 10 6 Dacite
2 2
CO is not strongly affected by magma composi-
2
tion, but the solubility of H O is (Fig. 5.2). This Viscosity (Pa s) 10 4
2 Andesite
compositional influence on H O solubility means
2
that, in general, the amount of H O dissolved in 100
2 Basalt
magmas is greatest in the most evolved (i.e., the
most silica-rich) magmas (Table 10.1). 1 Komatiite
At first sight one might think that this effect
would make high-silica melts less viscous than 0.01
primitive melts, but in fact all it does is to partially 600 800 1000 1200 1400 1600 1800
offset the high viscosity caused by the lower tem- Temperature (°C)
perature and higher silica content. And as a high-
Fig. 10.2 Viscosity as a function of temperature for a range
silica melt nears the surface, encounters lower
of common magma compositions. (Based on fig. 4 in Spera,
pressures and exsolves water into gas bubbles, the
F.J. (2000) Physical properties of magmas. Encyclopedia of
reduction in the water content of the melt dramat- Volcanoes. Academic Press, pp. 171–190, copyright Elsevier
ically increases the liquid viscosity even further. (2002).)