Page 97 - Fundamentals of Physical Volcanology

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74 CHAPTER 5

(d)

(c)

Fig. 5.10 Pumice clasts in the deposit from a pyroclastic
density current erupted at Mount St Helens volcano,
Washington State, USA, in August, 1980. Scale is in inches.
(b) (Photograph by James W. Head III.)

the greater likelihood of supersaturation and hence
the nucleation of large numbers of small bubbles,
and the reduced ability of volatile molecules to
diffuse through viscous magmas as compared
with basaltic ones. This suggests that in the more
evolved magmas the cause of fragmentation is not
just the close packing of large bubbles. Instead, it

(a) may often be the fact that it is difficult to force
viscous liquid to flow through the narrow films of
liquid separating the bubbles. On the time scale of
the changing stresses to which the magma is sub-
jected as it accelerates upward through a dike toward
Fig. 5.9 Four stages in the growth of gas bubbles in a
magma leading to magma fragmentation and an explosive the surface (the subject of the next chapter) its
eruption. Bubbles start to nucleate in (a); older bubbles have rheology is no longer Newtonian, and it develops
grown by diffusion and decompression and new bubbles an effective strength. When the stresses exceed that
have nucleated in (b); the bubble number density is so large strength, the magma fractures as though it were
in (c) that bubbles are extremely crowded and the walls
a brittle solid; a similar situation was described in
between adjacent bubbles can start to collapse; in (d) so
Chapter 3, where rheology was a function of envir-
many bubble walls have collapsed that the magma has
onmental stresses in connection with the behavior
changed from a liquid containing bubbles to a gas
containing liquid droplets. of rocks in the upper mantle.
There is a final very important factor connected
magmas, called pumice clasts (see Fig. 5.10), gen- with gas bubbles and magma fragmentation. When
erally contain a smaller range of vesicle sizes than the fragmentation process occurs throughout the
more basaltic pyroclasts, usually called scoria (Fig. magma at some specific depth in a dike, every batch
5.11). This is due to the reduction in coalescence, of magma rising through the system undergoes the