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8 Pyroclastic falls and

pyroclastic density currents

8.1 Introduction
8.2.1 Fallout from the rising eruption column
Chapter 6 discussed the upward transport of clasts Chapter 6 explained that an eruption column can
in eruption columns, their eventual release from be divided into three regions: the gas-thrust region,
such columns, and the factors that may cause the convective region and the umbrella region. The
columns to become unstable, leading to pyroclastic width of an eruption column increases with height
fountaining that feeds pyroclastic density currents. in the gas-thrust and convective regions, becoming
This chapter looks in more detail at all of these even wider in the umbrella region, where it com-
processes and at the mechanisms of emplacement monly spreads preferentially downwind (Fig. 6.5).
of the resulting deposits. Figure 8.1 shows the same eruption column seen
from directly upwind of the vent. In the gas-thrust
and convective regions the expansion in the erup-
8.2 Fallout of clasts from eruption columns tion column is caused by the entrainment and heat-
ing of air from the surrounding atmosphere. The
Chapter 6 showed how the terminal velocity of a
clast within the eruption column, which is a func-
tion of its radius, density, and shape, largely deter-

mines the height above the vent to which it can be
carried. The general pattern is for the largest clasts Umbrella Region
to fall out from lower levels with smaller clasts
being carried to greater heights. The exact height
reached by a clast of given size and type, however,
depends on the eruption conditions, as discussed Convective Region
further below, and the location at which it lands
depends on the state of the atmosphere, especially
the wind. The distribution of fallen pyroclasts Gas-Thrust or
on the ground can be used to infer at least some Inertial Region
of the properties of the eruption that produced
them. This is particularly important for volcanoes Vent
that erupt very explosively, but do so only very
Fig. 8.1 A Plinian eruption cloud viewed from upwind of
rarely (perhaps at intervals of tens to hundreds of
the vent. The progressively greater lateral spreading of
thousands of years), because it means that the prod-
the cloud continues for most of its rise, but eventually
ucts of prehistoric eruptions can be analyzed in
incorporation of air that brings with it the momentum of the
order to gain some idea of what to expect in the wind causes the top of the cloud to be carried away in the
future. downwind direction and spreading becomes negligible.