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STEADY EXPLOSIVE ERUPTIONS 85

Fig. 6.6 The eruption plume from
the March 22, 1915, eruption of
Lassen Peak volcano, showing the
classic umbrella shape of the upper
part of the plume. (Photograph
credit: R.I. Meyers, courtesy of the
National Oceanic and Atmospheric
Administration, National Geophysical
Data Center.)

clasts, a point will be reached where the bulk den- Table 6.2 Plume heights generated during selected
sity of the plume equals that of the surrounding air 20th century volcanic eruptions.
and the plume can rise no further through thermal
Eruption Plume height (km)
buoyancy alone. The height at which this occurs is
called the level of neutral buoyancy. However,
Santa Maria (Guatemala), 1902 27–48
the plume still has some inertia as it rises slowly Soufriere (St Vincent), 1902 14.5–16
through the neutral buoyancy level and so it over- Hekla (Iceland), 1947 24
shoots this level somewhat, spreading sideways Bezymianny (Kamchatka), 1956 34–45
and downwind as it does so. This part of the plume Hekla (Iceland), 1970 14
Fuego (Guatemala), 1971 10
is known as the umbrella region (Fig. 6.5) and an
Heimaey (Iceland), 1973 2–3
example can be seen in Fig. 6.6.
Ngauruhoe (New Zealand), 1974 1.5–3.7
Soufriere (St Vincent), 1979 18
6.5.2 Controls on plume height Mount St Helens (USA), 1980 25
El Chichón (Mexico), 1982 20–24
The heights of plumes in historic eruptions vary
widely (Table 6.2). What controls the height to Data taken from Wilson et al. (1978); Carey, S.N. and
Sigurdsson, H. (1982) Influence of particle aggregation on
which a given plume rises? The biggest factor is
deposition of distal tephra from the May 18, 1980, eruption
the thermal buoyancy of the plume, and this is
of Mount St Helens Volcano. J. Geophys. Res., 87,
controlled by the thermal energy available, so the
7061–7072; and Sparks, R.S.J., Moore, J.G. and Rice, C.J.
highest plumes are those with the most heat. The (1986) The initial giant umbrella cloud of the May 18th,
plumes with the greatest heat are those with the 1980, explosive eruption of Mount St Helens. J. Volcanol.
largest mass flux. Heat is provided to a rising Geotherm. Res., 28, 257–274.
plume by the clasts (and the volcanic gas) con-
tained within it. The mass flux is a measure of the
mass of magma erupted per unit time, so the higher dicted for pure gas plumes by Morton et al. (1956):
the mass flux the greater the amount of heat sup- the height of the plume is proportional to the
plied to the plume in a given time. Wilson et al. fourth root of the eruption rate. For a “standard
(1978) and Settle (1978) both showed that volcanic atmosphere” on Earth the relationship is
plumes follow very nearly the same relationship
between mass flux and plume height that was pre- H = 0.236 M 1/4 (6.7)
f