Page 170 - Fundamentals of Physical Volcanology

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ERUPTION STYLES, SCALES, AND FREQUENCIES 147

Figure 10.2 shows the general trend resulting Table 10.2 Calculations of the minimum water and carbon
from all of the above factors, in which the viscosity dioxide contents in erupting magma required to allow
of primitive magmas is smallest and viscosity pro- explosive volcanic eruption to occur at a range of
water depths.
gressively increases for more evolved or silica-rich
magmas. For comparison, the viscosity of water is
Water Total water Minimum Minimum
10 −3 Pa s; this means that the least viscous magmas depth (m) pressure (MPa) H O content CO content
2
2
have viscosities 100 to 1000 times as great as water
10
whereas rhyolites are more than 10 billion (10 ) 30 0.3 0.12 0.18
times as viscous as water! 100 1.0 0.36 0.62
300 3.0 0.99 1.87
1000 10.0 3.10 6.50
10.3 Chemical composition and 3000 30.0 9.27 22.42
effusive eruptions

10.3.1 Conditions of effusive eruption rising relatively slowly and thus is less likely in fast
rising, low-viscosity magmas. Effusive eruptions can
One fundamental distinction between eruption
also result when slowly rising magma that has been
styles is whether an eruption is effusive or explo-
progressively degassed by a series of transient explo-
sive. The explosive character of eruptions results
sions is finally erupted onto the surface.
from the fact that rising magma contains gas dis-
• Submarine eruptions are commonly effusive in
solved within it (Chapter 5). As a result of this the
character (section 9.2). Here effusion occurs not
majority of subaerial, and even some submarine,
because the gas content of the magma is low but
eruptions on Earth are explosive. There are four
because exsolution of gas from the magma is sup-
main circumstances in which effusion may occur
pressed due to the pressure of the water overlying
rather than explosive activity.
the vent. Table 10.2 gives the pressure at various
• If the gas content of the rising magma is very depths in the ocean and the total (exsolved plus
small then exsolution will occur but the resulting remaining dissolved) amounts of either water or
gas bubble volume fraction will be insufficient to carbon dioxide that a magma would need to con-
cause fragmentation (Chapter 6). Computer model- tain to guarantee an explosive eruption. Compari-
ing suggests that the gas content must be less than son of Tables 10.1 and 10.2 demonstrates that only

about 0.02 wt% in order for this circumstance to ∼500 m of water depth is needed to suppress explo-
occur. As shown above, even primitive magmas sive activity completely in most basalts, and that
have typical gas contents of at least 0.1 wt%, and so even the most gas-rich rhyolite could not erupt
effusive eruptions are very unlikely to be caused explosively under more than ∼2300 m of water.
solely by a small initial magma gas content. • Effusion may also occur if the viscosity of the
• If the magma loses enough of its gas at some stage erupting magma is sufficiently great to inhibit
during its ascent to the surface the gas content fragmentation. This can occur in eruptions of very
of the erupting magma may fall below the level at viscous magmas in the dacite to rhyolite compo-
which fragmentation can occur. Gas loss may occur sitional range (Figs 10.1 & 10.2). Effusion of such
during storage in a magma chamber or through magma produces steep-sided lava domes (some-
permeable conduit walls during ascent of the times higher than they are wide; Fig. 1.3) contain-
magma to the surface. There may be some indirect ing gas bubbles in which the pressure is significantly
chemical controls on this gas loss. For instance, the greater than atmospheric pressure. Chilling of the
gas content of a magma controls the depth at which outer surface of the dome increases the magma vis-
supersaturation and exsolution can occur (section cosity even further and prevents bubbles bursting
5.3) and thus whether a gas phase is present in the at the surface. However, if part of such a dome col-
magma chamber at all. Significant gas loss through lapses due to mechanical weakness, the release of
the conduit walls can occur only if the magma is pressure may trigger fragmentation giving rise to