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velocity decreased, the clast reached a height 6.7 Unstable eruption columns
within the plume where the gas stream velocity
has declined and become equal to the terminal In a stable eruption plume, air entrained into the
velocity of the clast. The clast is then suspended at column is heated enough to be thermally buoyant
this height above the ground and cannot be carried despite the load of entrained pyroclasts that it is car-
any further upward (points A and B in Fig. 6.8). The rying (see section 6.5). This section looks at what
two smaller clasts in Table 6.3 would continue to be happens to an eruption plume if it cannot achieve
carried higher in the plume because their terminal thermal buoyancy.
velocities are lower. As the plume rise speed con-
tinues to decrease, a point will be reached first
6.7.1 Plume density and column stability
where the terminal velocity of the intermediate size
clast equals the rise speed of the plume and the We start by considering how the density of the
clast becomes suspended at this height (points C eruption plume varies with height. Table 6.4 shows
and D in Fig. 6.8). Finally a height is reached where typical bulk densities for gas–magma mixtures as
even the smallest clast can no longer rise (points they leave an eruptive vent. From these values it is
E and F). As the two different cases in Fig. 6.8 illus- clear that, even in the most gas-rich eruptions, the
trate, the height gained by a clast of a given size gas–magma mixture is denser than air when it is
depends on the eruption conditions: the erup- erupted from the vent. It has been shown that rise
tion with a larger mass flux (eruption 1) produces of the plume in the lowest few kilometers occurs
a higher plume and carries clasts of a given size because of the initial momentum of the erupted
higher above the vent than the smaller mass flux material. During this initial rise, entrainment and
plume (eruption 2). heating of the air occurs and usually leads to a situ-
So what is the fate of these suspended clasts? ation where the density of the plume material
The eruption plume is a highly turbulent place, so a becomes slightly smaller than that of the surround-
clast that has reached its maximum height will not ing air and hence the plume is able to rise due to
be passively suspended at this height but instead thermal buoyancy. A situation may arise, however,
will be constantly moved around by eddies within in which entrainment causes the rise speed to decline
the plume. Eventually, the clast is likely to find itself to a very small value because of the momentum-
at the edge of the eruption plume. If it is nudged sharing (eqn 6.6) before the plume has been able to
to the edge of the plume by the turbulence, it will entrain and heat enough air to become thermally

leave the plume because it no longer has the sup- buoyant. In other words, the plume reaches a
port of the rising gas. The clast will then fall toward point where its rise speed is negligible but the bulk
the ground. So for any particular plume speed, and density of the plume material (the overall density of
therefore any particular height above the ground, the mixture of gas and pyroclasts) is still greater
there is a maximum size of clasts of any given than that of the surrounding air. In this situation
density which can be supported at that height. the plume can rise no further and the material in it
The size of clast that can be supported decreases
with height (because plume rise speed decreases –
Table 6.4 The bulk density, ρ , of a gas–magma mixture as
Fig. 6.8) so that large clasts will fall out from the B
it is erupted for four different exsolved water contents, n.
plume at smaller heights above the vent than will
For comparison, the density of air at the Earth’s surface
small clasts. In practice, many particles are likely to is ∼1.2 kg m .
−3
fall out from the plume before they reach their max-
− −3
imum achievable heights because the turbulence of n (wt%) ρ ρ (kg m )
B
the plume causes them to reach the edge of the
1 18
plume and be pushed out into the surrounding, still
3 6
atmosphere prematurely. The deposition of tephra
5 3.6
from eruption plumes is discussed in more detail in
7 2.6
Chapter 8.