Page 130 - Fundamentals of Physical Volcanology

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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 107

April 5, 1982

April 11, 1982

April 21, 1982

April 25, 1982

Fig. 8.3 The progressive westward drift of the eruption cloud produced by the eruption of El Chichón volcano in Mexico
in 1982 was tracked by satellites. Note that dispersal of the cloud north and southward is limited. (Redrawn from fig. 1 in
Robock, A. and Matson, M. (1983) Circumglobal transport of the El Chichón volcanic dust cloud. Science, 221, 195–197.
Reprinted with permission from the American Association for the Advancement of Science.)

average upward speed is zero, there is still a great
8.2.2 Fallout from the umbrella region
deal of turbulence in the cloud, so some particles
The smaller clasts within the eruption column are are swept up in updrafts while others are being
carried up into the umbrella region, the region in helped to move down. As discussed below, the
which the column is neutrally buoyant and stops small particles carried up into umbrella clouds only

rising (Figs 6.5, 6.6 & 8.1). In this region the column fall very slowly when released, and may spread in
spreads laterally (Fig. 8.2) and becomes what might the atmosphere for days to weeks (Fig. 8.3). The
be better called an eruption cloud. This spread is very smallest particles are the aerosols formed
caused by the constant addition of material from when droplets of water containing dissolved gases
below. In the absence of any wind, the umbrella such as SO collect around tiny silicate grains.
2
cloud would spread out symmetrically so that if Turbulence in upper atmosphere winds can carry
viewed from above it would appear circular. In these particles around the planet for months to years.
most cases, however, the cloud is affected by the
wind and moves downwind as it spreads laterally.
8.2.3 Fall speeds of pyroclasts
This effect can be seen in images of active eruption
clouds (Fig. 1.2) and is evident in the distribution A clast released from an eruption column or um-
pattern of deposition of the deposited fallout clasts brella cloud accelerates downward until the air
(Fig. 8.2). drag retarding it is just balanced by its gravitational
In the laterally spreading umbrella region the weight, at which point it reaches a steady final
average upward velocity is essentially zero. Without speed, its terminal velocity. The terminal velocity
any net support from a rising gas stream, the clasts of a clast is dependent mainly on its radius and den-
that are still in this region will fall out progressively. sity and also, to some extent, its shape. In the early
They do not all do so at once because, although the 1970s experiments were carried out to investigate