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6 Steady explosive eruptions

6.1 Introduction about the mass flux through the dike system. The
mass flux is simply the mass of material passing any
The previous chapter discussed how, as magma given point in the system in a given amount of time.
rises towards the surface, the volatiles dissolved The mass of material entering the dike system must
within it begin to exsolve forming gas bubbles. be the same as the mass erupted at the vent (unless
Continued rise of the magma leads to further ex- magma is intruded or stored somewhere on the
solution of gas and growth of gas bubbles through way). This means that, if the mass flux could be
diffusion, decompression and bubble coalescence. measured at a whole series of points in the system,
This chapter considers what happens in eruptions it would be found that its value was the same at
in which the rise speed of the magma is sufficiently every point. Figure 6.1 shows a schematic view of a
great to ensure that bubble coalescence is minor
during rise of the magma (see sections 5.4 and 5.5),
so that a more or less steady spray of gas and en-
trained magma clots emerges from a surface vent
ρ B , u
at a speed, an exit velocity, that can vary from sev-
eral tens to several hundreds of meters per second.
How gas in the magma behaves during the early

stages of magma ascent is considered first, followed l
by consideration of how the gas affects the way
the gas–magma mixture rises after fragmentation.
Finally we look at what happens to the gas–magma
mixture when it is erupted. The physical processes
discussed in this chapter are fundamental to a wide
range of styles of volcanic eruptions – from Ha-
waiian lava fountains (Fig. 1.1), through subPlinian
and Plinian eruptions (Fig. 1.2), to ignimbrite-
t
forming eruptions (Fig. 1.11).

6.2 Influence of gas bubbles prior to magma
fragmentation

As soon as gas bubbles begin to nucleate in a rising
Fig. 6.1 Schematic view of a dike of length l along strike
magma they start to influence the magma rise and width t in which magma of density ρ rises at an average
B
speed. A simple way to illustrate this is by thinking speed u.