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THE ROLE OF VOLATILES 73

0.03 0.1 0.3 1 3 10 30
10 m 10 m

1 m 1 m

Fig. 5.8 The influence of magma rise
speed on bubble growth for initial 0.1 m 0.1 m
magma water contents of 1 and 2 wt%
in a basaltic magma. At magma rise Final bubble diameter
−1
speeds greater than ∼1ms the
bubbles grow by diffusion and 10 mm 10 mm
decompression in the normal way, but
at smaller rise speeds there is time for
significant amounts of bubble 2%
1 mm 1 mm
coalescence and very much larger
1%
bubbles can be present in the magma
reaching the surface. (Modified from
fig. 8 in Wilson and Head (1981)
0.1 mm 0.1 mm
Ascent and eruption of basaltic magma
0.03 0.1 0.3 1 3 10 30
on the Earth and Moon. J. Geophys.
–1
Res. 86, 2971–3001.) Magma rise speed (m s )
more gas-rich magmas there is a greater number An early observation about all types of pyroclasts
density of gas bubbles in the magma at any given was that almost all of them contained a network of
depth, and thus bubble coalescence becomes more vesicles (see Fig. 5.9), these being the holes left
likely for a given rise speed. Thus the rise speed behind by bubbles of volcanic gas that were trapped
must be proportionately larger to prevent coales- in the pyroclast as it was erupted. Commonly, but
cence. The critical rise speed is smaller when not universally, the bubbles are interconnected,
the magma viscosity is greater. This is because the so that the volcanic gas has been lost and replaced

greater magma viscosity increases the drag on the by air, and equally commonly the vesicularities of
bubbles and reduces their speed, so even if the rise pyroclasts, in other words the volume fractions
speed of the magma is small the bubbles move so of the clasts that consist of the bubble spaces, are
slowly that little coalesce can occur. between 70 and 80%. This led to the idea that frag-
mentation occurs when the gas bubbles in a magma
have grown so much that they become very closely
5.6 Magma fragmentation and the influence packed, so that the liquid walls between the larger
of volatiles on eruption styles bubbles collapse, allowing the large bubbles to join
together. Figure 5.9 shows the kind of bubble
A critical consequence of the growth of gas bubbles growth history that could lead to this. Clearly not all
in magmas is their ability to cause the magma to of the bubbles will be connected together, and so
fragment, that is, to change from being a continu- the pyroclasts that are formed would be expected
ous liquid containing gas bubbles of various sizes to to contain a lot of trapped bubbles, just as is
being a continuous body of gas in which pyroclasts observed.
– droplets or clots of the liquid – are carried along However, there tends to be a difference between
by the gas. Fragmentation may occur for any one of the size distributions of trapped bubbles in pyro-
a number of reasons, and these tend to be linked to clasts derived from magmas of different composi-
magma composition, but not exclusively so. tions. Pyroclasts of more evolved, highly viscous