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48 CHAPTER 4

Fig. 4.6 Contours of the percentage
Caldera of melt present in the region beneath
the summit caldera of Axial Volcano,
NW 5–25% SE a basaltic shield volcano on the Juan
Depth below sea floor (km) 2 3 4 5 3–15% 1–5% completely molten central region of
1
de Fuca ridge on the floor of the
Pacific ocean. The hottest, almost
the magma chamber is surrounded by
progressively cooler “mushy” zones
0.4–2%
in which an ever larger proportion of
solid crystals is present. (Adapted by
permission from Macmillan Publishers
6
Ltd: Nature, West, M., Menke, W.,
Tolstoy, M., Webb, S. and Sohn, R.,
–15 –10 –5 0 5 10 15 Magma storage beneath Axial Volcano
Distance (km) on the Juan de Fuca mid-ocean ridge.
413, 833–836, copyright (2001).)

or eruptive event (causing deflation and subsidence). model was developed by a scientist named Mogi
The magma chamber can thus be pictured simplis- which relates the changes in volume of a sphere
tically as being like a balloon which is repeatedly buried at a depth, h, beneath the surface to the
blown up and let down again. In 1958 a theoretical changes in the elevation of the ground surface, ∆h,

Table 4.2 Magma chamber depths and sizes inferred from geophysical observations.
Volcano Depth of magma Chamber Caldera Geophysical
3
chamber (km) volume (km ) diameter (km) data used
Axial, Juan de Fuca Ridge 2.25–6 250 3 × 8 Seismic
Hekla, Iceland ∼9? (center) 145 – Deformation
Kilauea, Hawai’I 2–7 14–65 3.5 × 5 Deformation and seismic
Krafla, Iceland 2.5–7 28–56 8 × 9 Deformation and seismic
Long Valley, USA 5–25 17 × 32 Seismic
Mauna Loa 3–8 65 2.5 × 5 Deformation and seismic
Mono Craters, USA 8–10 (top) 200–600 – Seismic
Mount St Helens, USA 7–11, 9–14 – Deformation and seismic
Yellowstone, USA 6–12 40 × 70 Magnetic
Data from Koyanagi, R.Y., Unger, J.D., Endo, E.T. and Okamura, A.T. (1976) Shallow earthquakes associated with inflation
episodes at the summit of Kilauea Volcano, Hawai’I. Bull. Volcanol., 39, 621–631; Einarsson, P. (1978) S-wave shadows in
the Krafla caldera in NE-Iceland, evidence for a magma chamber in the crust. Bull. Volcanol., 41, 1–9; Iyer, H.M. (1984)
Geophysical evidence for the locations, shapes and sizes, and internal structures of magma chambers beneath regions of
Quaternary volcanism. Philos. Trans. R. Soc. Lond., Ser. A, 310, 473–510; Achauer, U., Greene, L., Evans, J.R. and Iyer, H.M.
(1986) Nature of the magma chamber underlying the Mono Craters area, Eastern California, as determined from teleseismic
travel time residuals. J. Geophys. Res., 91, 13,873–13,891; Ryan, M.P. (1987) Neutral bouyancy and the mechanical evolution
of magmatic systems. In Magmatic Processes: Physiochemical Principles, pp. 259–287. Special Publication No. 1, The
Geochemical Society; Sigurdsson, H. (1987) Dyke injection in Iceland: a review. In Mafic Dyke Swarms, pp. 55–64. Special
Paper 34, Geological Association of Canada; Barker, S.E. and Malone, S.D. (1991) Magmatic system geometry at Mount St
Helens modelled from the stress-field associated with posteruptive earthquakes. J. Geophys. Res., 96, 11,883–11,894; Lees,
J.M. (1992) The magma system of Mount St Helens: non-linear high-resolution P-wave tomography. J. Volcanol. Geotherm.
Res., 53, 103–116; Rutherford, M.J. and Gardner, J.E. (2000) Rates of magma ascent. In Encyclopedia of Volcanoes,
pp. 207–217. Academic Press; and West et al. (2001).