Page 107 - Fundamentals of Physical Volcanology

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84 CHAPTER 6

Wind direction

Umbrella region

Convective region

Fig. 6.5 The shape of an eruption
column seen from a direction at right
angles to the direction that the wind is
blowing. The column expands as it
Inertial region
rises and is eventually blown sideways
by the wind when it reaches its neutral
Vent buoyancy height.

wall – there is now a flexible boundary where the gas–magma mixture has as it exits from the vent.
volcanic material is mixing with the surrounding This lowest part of the plume is usually called either
air. the gas-thrust region or the inertial region
A second major effect of entrainment is that the (Fig. 6.5) and generally dominates the rise of the
heat energy contained in the erupted gas–magma plume for the first few kilometers above the vent.
mixture is shared with the entrained air. Typically As the plume entrains air and the upward velocity
the erupting jet will have a temperature of ∼900– of the plume decreases due to momentum sharing,
1150°C. The temperature of the entrained air the effects of the thermal buoyancy of the plume
depends on the geographical location of the vent (usually) take over. The zone in which the plume
but typically will be ∼0°C. The volume of air which rises as a result of buoyancy is referred to as the
is eventually entrained by the eruption column can convective region (Fig. 6.5).

vary widely depending on the mass flux and gas As the plume continues to rise in the convective
content of the erupting magma but, in general, will region it entrains more air and the temperature of
5
be between 100 and 10 times the volume of gas the plume decreases. The temperature decreases
released in the eruption. Thus when the heat from due to two effects: the sharing of the heat with
the eruption jet and clasts is shared with the progressively more air and the expansion of the
entrained air the temperature of the column as a plume as the atmospheric pressure decreases. As the
whole ends up being only slightly greater than that plume rises and the atmospheric pressure acting on
of the surrounding air. This temperature contrast it decreases, the plume expands. Expansion of gases
is, however, great enough to make the material in causes a decrease in the internal energy, and there-
the eruption column less dense than the surround- fore the temperature, of the gases, and also of the
ing air, and this thermal buoyancy causes continued pyroclasts which are still being carried in suspen-
plume rise. This effect is the same as that which sion in the plume. These pyroclasts are continually
allows a hot air balloon to rise. being lost from the plume as it rises, because the
In thinking about this rising column or plume of ability of the gas to support the clasts decreases as
material therefore we can define two main causes its speed and density decrease. Actually, the cool-
for its rise and define regions in which each is ing of the gas increases its density somewhat, but
dominant. Immediately above the vent the plume this is counteracted by the decreasing atmospheric
rises because of the initial momentum that the pressure. Thus as the plume rises, cools, and deposits