Page 94 - Fundamentals of Physical Volcanology

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THE ROLE OF VOLATILES 71

way nucleation proceeds depends on the avail-
ability of sites for nucleation and the degree of
supersaturation of the magma at any given time.
Nevertheless, in general the magma will contain
some relatively large bubbles that formed some
time ago at deeper levels beneath the surface and
which have been growing through diffusion and
decompression, and also some very small bubbles
which have just formed. A spectrum of bubble sizes
will exist between these two extremes (Fig. 5.6).
All of these bubbles are rising relative to the
magma because of their buoyancy but the larger
bubbles are rising faster than the smaller ones
(eqn 5.12). This means that the larger bubbles can
overtake the slower, smaller bubbles. When this
happens there are two possibilities. The small bub-
ble may be swept around the larger one, effectively
trapped in the liquid magma moving sideways and
down to let the larger bubble pass, and thus may be
left behind as the large bubble continues to move
upward. But if the small bubble is close enough to
the large one it may be swept into the wake of
the large bubble. This wake consists of some liquid
magma that is effectively trapped behind the large
bubble and is moving up with it through the rest of
the liquid. The small bubble is no longer left behind
but instead rises slowly through the wake liquid
and eventually collides with the bottom of the large
Fig. 5.6 The typical distribution of bubble sizes within
magma rising toward the surface. The largest bubbles are bubble and coalesces with it, forming a single, lar-
those that formed earliest at the greatest depth beneath the ger bubble. This new, larger bubble then rises even

surface and have grown by diffusion and decompression; faster, overtakes more bubbles and coalesces with
the smallest bubbles are those that have most recently some of them, growing larger and moving even
nucleated. The length of the arrows extending from each
faster and so on. This runaway process can lead to a
bubble reflects the relative rise speeds of the bubbles
situation where a single large bubble (called a slug)
through the magma, largest bubbles having the greatest
can fill the whole width of the dike or conduit,
rise speeds.
absorbing all of the smaller bubbles ahead of it as it
continues to rise.
rises by nearly twice its own diameter in one sec- In basaltic magmas, a critical factor in determin-
ond in the basalt but by only about one-twentieth ing whether bubble coalescence can occur is the
of one percent of its own diameter in the rhyolite. rise speed of the magma. A simple example will
Figure 5.6 shows schematically what might be illustrate why this is the case. Consider magma
seen if a “snapshot” could be taken of the bubbles rising over a distance of 500 m. If the rise speed of
within magma which is rising towards the surface. the magma is 1 m s −1 then it takes 500 seconds for
At any given time the magma will contain a popu- the magma to rise 500 m. If the magma rise speed is
−1
lation of bubbles of various sizes. Once bubbles only 0.1 m s then it takes 5000 seconds to move
start to form at the exsolution level, new bub- the same distance. If a bubble in the magma is rising
−1
bles continue to nucleate until the magma is finally relative to the magma at a speed of 0.01 m s then
erupted. However, as was seen earlier, the exact in 500 seconds it will rise a distance of 5 m through