Page 174 - Fundamentals of Physical Volcanology

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ERUPTION STYLES, SCALES, AND FREQUENCIES 151

In reality Vulcanian explosions are not always the magma is much greater and the clasts which are
caused solely by magmatic gases; interaction with erupted are considerably smaller. This allows trans-
groundwater also plays a role (section 7.3). The port of much of the erupted mass upwards in a con-
strong link between composition and volcanic vecting eruption plume and dispersal over a wide
activity suggests, though, that the viscosity of the area (section 8.2). So a key issue is the factors which
magma and its influence on bubble rise speed control the degree of fragmentation and hence the
play an important role even when groundwater clast-size distribution of the erupting material.
is involved in an eruption. The fragmentation process is still not well under-
stood, but considerable advances in understanding
were made during the 1990s. It is thought that frag-
10.4.3 Chemical composition and sustained
mentation can occur through two primary mech-
explosive eruptions
anisms: rapid acceleration or rapid decompression
We have seen that sustained explosive eruptions of magma. Rapid decompression is a likely trigger
occur when the rise speed of magma is sufficiently for fragmentation and explosive activity in situ-
great to prevent significant segregation of mag- ations where there is a rapid reduction of confining
matic gas bubbles from the magma in which they stress on the magma – such as when a lava dome
originated (section 5.5). A range of sustained collapses (section 8.4.2.2; Fig. 8.10) or during slope
explosive eruptions can occur which vary con- failure such as that which triggered the initial lateral
siderably in character. Hawaiian eruptions involve blast during the May 1980 eruption at Mount St
the eruption of relatively coarse clasts at relatively Helens. Rapid acceleration is considered to be the
low exit velocities, produce low eruption plumes more likely cause of fragmentation in Hawaiian and
and dominantly generate lava flows. These erup- many Plinian eruptions. In this case, vesiculation
tions are associated with basaltic magmas. Other sus- due to exsolution of gas from rising magma is
tained explosive eruptions ranging from subPlinian, delayed until a high degree of supersaturation is
through Plinian to ultra-Plinian are more normally reached. Rapid exsolution then causes the devel-
associated with intermediate to evolved magma. opment and rapid acceleration of a magmatic foam
They generate greater plume heights, produce finer (section 5.6). Rapid acceleration results in high
clast-size distributions, and are dominated by fall strain rates which induce stresses across the bubble
deposits which are much more widely dispersed walls sufficient to cause brittle failure and fragmen-
than those produced in Hawaiian eruptions. It has tation. The viscosity of the magma involved is con-

been commonly assumed that the difference in sidered to be crucial to the nature of this process.
viscosity of the erupting magma is what controls The viscosity of magmas can change dramatically
whether an eruption is Hawaiian or Plinian. However, as water exsolves from them. This effect is small
both the viscosity and the magma gas content are in basaltic magmas and so basaltic fragmentation
lower in basaltic magmas and both are likely to play occurs progressively by the thinning and tearing of
some role in determining the details of the eruption bubble walls and produces relatively large and fluid
dynamics. lava clots. Exsolution of water from more evolved
magmas, though, causes a dramatic increase in the
magma viscosity (section 10.2 and Fig. 10.4). The
ROLE OF VISCOSITY
high viscosity of these magmas means that rapid
A key difference between Hawaiian eruptions and acceleration leads to strain rates which are high
the spectrum of Plinian eruptions is the degree of enough to cause the magma to fail in a brittle fash-
fragmentation of the erupting clasts (section 6.6). ion. This causes more complete fragmentation and
Hawaiian eruptions produce coarse clasts which the generation of smaller clasts. The highest strain
can be carried typically only a few hundred meters rates are likely to cause the highest degree of
above the vent in incandescent lava fountains fragmentation. The difference between elastic and
(Fig. 1.1) and which then fall back around the vent brittle fragmentation can be likened to the behavior
forming cones and lava flows (see section 9.2). of “silly putty”. When “silly putty” is stretched slowly
In Plinian eruptions the degree of fragmentation of (i.e., at low strain rates) it continues to stretch until