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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 117

Fig. 8.10 A pyroclastic density current
erupted from the crater of Mount St
Helens volcano on August 7, 1980.
The dense, ground-hugging current
is almost completely hidden by the
low-density convecting cloud formed
above it as hot gases lift small particles
out of the body of the current.
(Photograph by Peter W. Lipman,
courtesy of U.S. Geological Survey,
Cascades Volcano Observatory.)

rents moved downslope at right-angles to contours The method was to equate the potential energy
in a ground-hugging fashion. Observations of pyro- needed to raise material to the height h of the ridge
clastic density currents that have been seen in his- (gh, where g is the acceleration due to gravity) to
torical eruptions confirm this tendency (Fig. 8.10), the kinetic energy of material approaching the
2
but also show a cloud of gas and fine material form- ridge at speed v (0.5 v ). For a ridge 1500 m high
−1
ing a convecting cloud above the basal part of the this would imply a speed of ∼170ms . Speeds

current. This strongly implies that there is a vertical approaching 100 m s −1 have been observed for
variation of properties within the density current, historic pyroclastic density currents, so this was
i.e., there is density stratification. Other features of assumed to be plausible. However, the realization
pyroclastic density current deposits support this that such currents have a vertical density stratifi-
idea. Thus in places where the bulk of a deposit lies cation casts doubt on the reliability of this kind of
in a valley, there may be a veneer of pyroclastic calculation. It may be that only the upper parts of a
material on top of ridges on either side of the valley vertically extensive current cross the ridge, imply-
or in adjacent valleys. And in some cases it has been ing less vertical rise of material and hence a smal-
observed that pyroclastic density currents reaching ler speed. Nevertheless, it is clear that speeds up
an ocean or lake shore split into a component that to ∼100ms −1 are a common feature of all types of
travels under the water, following the topography, pyroclastic density current, and this must be linked
and a component that travels for some distance to the ways in which they form.
across the water surface.
In some instances, ignimbrites are found in adja-
8.4.2 Origins of pyroclastic density currents
cent valleys separated by ridges up to 1500 m high.
At one time this was taken to indicate that the ridge Chapter 6 introduced one mechanism for form-
height could be used as a measure of the speed of ing these features: the collapse of Plinian and
the pyroclastic density current forming the deposit. sub-Plinian eruption columns to form pyroclastic