Page 91 - Fundamentals of Physical Volcanology

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68 CHAPTER 5

the molecular level, means pushing the volatile bubbles may nucleate at more or less the same time
molecules back into the liquid. The spontaneous throughout the magma, and this will mean that the
aggregation of molecules of a volatile species distances between the bubbles will be less than
into bubbles in this way is called homogeneous if nucleation has taken place in equilibrium with
nucleation. the decreasing pressure. Of course, there still has
The nucleation process is greatly aided if there to be some movement of the volatile molecules
is some, preferably irregular, surface onto which to reach a nucleation site even if the magma is very
the volatile molecules can gather to minimize the supersaturated, and that movement, by diffusion
effects of surface tension, in which case heteroge- through the liquid magma, takes a finite time. So
neous nucleation takes place. Thus, nucleation is the greater the rise speed of the magma toward the
helped by the presence of solid crystals, and such surface, the more out of equilibrium, and hence
crystals are commonly present in many magmas, the more supersaturated, the magma is likely to
especially if the magma has been stored in a magma become. At all stages in the rise of a magma that is
chamber before eruption for long enough to have exsolving volatiles there will be a competition
cooled below its solidus temperature, so that at between adding more molecules to existing bubbles
least one mineral has started to crystallize. This use and nucleating new bubbles. The distance between
of crystals in magmas as nucleation sites for gas bubbles is a major factor in determining how effi-
bubbles has analogies with the way water vapor ciently volatile molecules can reach the nearest
condenses onto dust motes in the atmosphere to existing bubble or new bubble nucleation site.
form raindrops. Of course, any magma chamber Thus the presence or absence of crystals (and also
or dike must have walls, and at first sight these are whether there are a large number of small crystals
obvious potential sites for bubble nucleation (the or a small number of large crystals) will have a
next time you hold a glass of champagne look major influence on how bubbles form and then
closely at the streams of bubbles nucleating at grow.
irregularities in the wall of the glass). However,
the magma immediately adjacent to the wall will be
relatively cool and viscous, and these factors re- 5.5 Bubble growth
duce the ability of volatiles to migrate through the
magma to the otherwise attractive nucleation sites. Once gas bubbles have formed within a rising
If there are no aids to nucleation in a magma magma, the bubbles grow progressively through

then there may be a nontrivial delay in the onset of some combination of three processes: diffusion of
bubble formation, and the magma may become more gas into existing bubbles; decompression
very significantly supersaturated, by as much as and expansion of the gas already in bubbles; and
∼100 MPa, before bubbles begin to form. The coalescence of bubbles.
balance between supersaturation pressure ∆P and
bubble radius r is
5.5.1 Growth by diffusion
∆P = 2σ/r (5.4) Growth by diffusion involves the migration into
a bubble of molecules of the volatile compounds
where σ is the surface tension, typically 0.05– still dissolved in the surrounding magma. The main
−1
0.1 N m . With ∆P = 100 MPa, the initial bubble volatile entering the bubble will be the one that
−9
sizes will be only a few nanometers (1 nm = 10 m). has first become supersaturated and so caused the
However, if heterogeneous nucleation on magma nucleation of the bubble, but some molecules of
crystals is taking place at a small supersaturation, any other volatiles present will also enter the bub-
say 1 MPa, then the nucleating bubbles will be ble. The diffusion process of any one volatile is
more like 1 µm in size. influenced by many factors, especially the magma
If the magma has become very supersaturated composition, the magma temperature, and the mix-
before bubbles start to form, a large number of ture of other volatile species present.