Page 146 - Fundamentals of Physical Volcanology

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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 123

Fig. 8.13 The deposit from a pyroclastic density current erupted from Mount St Helens volcano in August, 1980. The current
bifurcated just before coming to rest, forming two lobes, each about 10 m wide. The coarse, rubbly levées are about 2 m high
and the mainly fine-grained central channel deposit is ∼1 m deep. (Photograph by Lionel Wilson.)

be related to the height from which it is released. and magma volatile contents (Table 8.1). If we

The wind blows all clasts downwind, but if their add up the deposit volume from the isopachs,
positions on the ground are measured at right- convert this to a corresponding total mass, and
angles to the wind direction we obtain a record then divide the mass by the mass flux, the dura-
of their release distances and hence the release tion of the eruption can be calculated. Finally,
heights. the asymmetry of the deposit, together with the
• The release height of a clast of a given size and cloud height, can give us an estimate of the wind
density is an indicator of the rise speed of the col- speed during the eruption.
umn at that height. Low down in the column, • When a mixture of gas and pyroclasts forming
the rise speed depends mainly on the speed of an eruption column in a steady explosive erup-
the gas coming out of the vent and hence on the tion cannot obtain enough buoyancy from
magma volatile content. Higher in the column, mixing with the surrounding air as it emerges
the rise speed depends mainly on the heat flux from a vent, the lower part of the column
driving the column and hence on the mass flux of collapses to form a fountain over the vent. The
magma being erupted from the vent (Fig. 8.6). upper part of the column continues to convect
• Observations of pyroclast dispersal patterns and drifts away in the direction of the wind. The
(Fig. 8.5) can be compared with theoretical fountain now being fed by the eruption leads to
predictions (Fig. 8.7) to deduce erupted mass the formation of one or more pyroclastic density
fluxes, which also define the column heights, currents.