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90 CHAPTER 6

600
50 (a)

500
40
Velocity
400
Plume height (km) 30 column Collapse 300 Exit velocity (m s –1 )

20
Plinian 200
10 100

Fountain
0 0
10 100 1000
Vent radius (m)

600
50 (b)

500
Velocity
40 column 400
Plume height (km) 30 Plinian Collapse 300 Exit velocity (m s –1 ) Fig. 6.10 Diagrams showing the variations of the

velocity of gas and pyroclasts exiting a vent, and
also of the height of the resulting Plinian eruption
column or pyroclastic fountain, with (a) changing
20
vent radius and (b) changing magma gas content.
200
In each of the cases shown conditions evolve from
a stable Plinian column though a collapse event to
10 100 a stable but much lower fountain. (Adapted from
fig. 15 in Wilson, L., Sparks, R.S.J. & Walker, G.P.L.
(1980) Explosive volcanic eruptions – IV. The
Fountain
control of magma chamber and conduit geometry
0 0
on eruption column behaviour. Geophys. J. Roy.
6 5 4 3 2 1 0
Astron. Soc., 63, 117–148, copyright Wiley-
Gas content (wt%) Blackwell Publishing Ltd.)
progressively during an eruption, the bulk density and a smaller exit velocity leads to less entrainment
of the erupted gas–magma mixture increases and (section 6.4.1). So, if the gas content decreases
more air must be entrained to ensure that the sufficiently during an eruption, this may also cause
plume will be able to rise by thermal buoyancy. the eruption plume to collapse. An example of this
Such an erupting jet, though, will entrain less not effect is shown in Fig. 6.10b. Here the mass flux
more air. This is because, with less gas exsolution, of the eruption is constant but the gas content of
the exit velocity of the erupting jet will be smaller, the magma decreases progressively. Initially the