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76 CHAPTER 5

point will be reached where the magma becomes Freundt, A. & Rosi, M. (Eds) (2001) From Magma to
supersaturated and gas starts to exsolve Tephra. Elsevier, 334 pp. ISBN: 0444507086.
forming bubbles within the magma. The most Gerlach, T.M. (1986) Exsolution of H O, CO , and S
2
2
common gases within magmas are H O, CO , SO during eruptive episodes at Kilauea Volcano,
2 2 2
and H S. Hawai’I. J. Geophys Res. 91, 12,177–85.
2
• The solubility of gases in magmas is dependent Kaminski, E. & Jaupart, C. (1997) Expansion and
on both the type of gas and the composition of quenching of vesicular pumice fragments in Plinian
the magma. The solubility of H O in rhyolite is, for eruptions. J. Geophys. Res. 102, 12,187–203.
2
instance, considerably greater than that in basalt, Klug, C. & Cashman, K.V. (1996) Permeability devel-
opment in vesiculating magmas: implications for
whereas the solubility of CO is considerably
2
less than the solubility of H O in either a basalt fragmentation. Bull. Volcanol. 58, 87–100.
2 Mader, H.H., Zhang, Y., Phillips, J.C., Sparks, R.S.J.,
or a rhyolite. The solubility of volatiles is more
Sturtevant, B. & Stolper, E. (1994) Experimental
complex when more than one species is present
simulations of explosive degassing of magma.
because they interact with one another.
Nature 372, 85–8.
• When bubbles first form in a magma they are
Mangan, M.T. & Cashman, K.V. (1996) The structure
typically extremely small. Bubbles can grow from
of basaltic scoria and reticulite and inferences for
their initial size as the result of three processes:
vesiculation, foam formation, and fragmentation
diffusion, decompression and coalescence. The
in lava fountains. J. Volcanol. Geotherm. Res. 73,
relative importance of each process depends 1–18.
on the amount of gas present in the magma, the Sparks, R.S.J. (1978) The dynamics of bubble forma-
magma composition and the magma rise speed. tion and growth in magmas: a review and analysis.
• Bubble coalescence due to large bubbles over- J. Volcanol. Geotherm. Res. 3, 1–37.
taking smaller ones is important in low-viscosity Sugioka, I. & Bursik, M. (1995) Explosive fragmenta-
magmas at low magma rise speeds. The rise speed, tion of erupting magma. Nature 373, 689–92.
and its control on whether significant bubble
coalescence does or does not occur, determines
the fundamental character of the resulting erup-
5.9 Questions to think about
tion. If the magma rise speed is low and thus bub-
ble coalescence is important, the gas segregates
from the magma and rises through it, causing the 1 Why are magmas stored in magma reservoirs

kinds of discrete transient explosions described at depths of a few kilometers below the surface
in Chapter 7. If the rise speed is higher then sig- more likely to contain bubbles of carbon dioxide
nificant coalescence will not occur. In this case than bubbles of water vapor?
the magma and gas stay locked together and this 2 What is the main factor controlling whether
leads to continuous, steady explosive eruptions volatiles exsolve in a way close to being in equi-
in a wide range of magma compositions as dis- librium with the decreasing pressure in a rising
cussed in Chapter 6. magma or out of equilibrium with the pressure?
3 What are the three processes by which the aver-
age size of the gas bubbles in a rising magma
5.8 Further reading increases?
4 What property of the magma allows bubbles
Cashman, K.V., Sturtevant, B., Papale, P. & Navon, O. to coalesce more easily in basaltic magmas than
(2000) Magma fragmentation. In Encyclopedia more evolved magmas?
of Volcanoes (Ed. H. Sigurdsson), pp. 421–30. 5 What is the main control on whether an explo-
Academic Press, San Diego, CA. sive eruption involves the discharge of a steady
Dixon, J.E. (1997) Degassing of alkali basalts. Amer. or unsteady stream of gas and pyroclasts through
Mineral. 82, 368–78. the vent?