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152 CHAPTER 10

low viscosities. It is possible that the lower gas con-
10 8 tent of basaltic magmas also affects the fragmenta-
tion process. The amount of gas within the magma
affects the energy released during ascent and thus
Rhyolite
10 6 is likely to affect the amount of acceleration occur-
ring prior to fragmentation (section 6.4.1) and
Viscosity (Pa s) 10 4 Andesite magma, with higher gas contents causing higher
hence to affect the strain rates experienced by the
Dacite
strain rates and greater fragmentation.
The gas content of the magma also influences
the exit velocity of material in sustained erup-
100
tions by affecting the depth at which fragmentation
Basalt
occurs and the total energy release (section 6.4.1),
with lower gas contents leading to lower exit vel-
1
ocities. Typical exit velocities in Hawaiian erup-
Komatiite
tions, where gas contents are low, are ∼100ms −1
0.01 whereas in Plinian eruptions they are more typ-
−1
0 0.5 1 1.5 2 2.5 3 ically ∼300ms . This difference affects the ability
of the erupting jet to entrain air and develop a
H 2 O (wt%)
stable eruption plume. The low gas content of
Fig. 10.4 Variation of the viscosities of various magmas basaltic magmas makes the development of a stable
with their dissolved water content at a constant eruption plume unlikely, and the lava fountain
temperature. Decreasing the water content increases the
characterizing a Hawaiian eruption is, in effect, the
viscosity, especially when the magma is silica-rich. (Based
basaltic equivalent of a collapsed eruption column
on fig. 5 in Spera, F.J. (2000) Physical properties of magmas.
(section 6.7.1).
Encyclopedia of Volcanoes. Academic Press, pp. 171–190,
copyright Elsevier (2002).)
10.5 Summary of compositional controls on
it eventually tears apart. If it is stretched rapidly eruption character
(i.e., at high strain rates) the putty breaks after only

a small amount of stretching producing a sharp The previous sections have shown that chemical
edge, i.e., it has “broken” in a brittle fashion. For
composition can play a crucial role in determining
basaltic magmas strain rates and magma viscosity
the detailed character of volcanic eruptions. The
are too small to allow brittle failure to occur and the
link between chemical composition and eruption
magma stretches and tears producing large, fluid
style is to a large degree due to the links between
lava clots. In more evolved magmas the increase in
chemical composition and two key physical prop-
viscosity caused by the exsolution of water means
erties of the erupting magma – the viscosity and
that strain rates may be high enough to cause brittle
magma gas content. Primitive magmas generally
fracturing of the magma resulting in the formation
have low viscosities and low magma gas contents
of small clasts. This difference is fundamental to
while progressively more evolved magmas have
controlling the style of the resulting eruption.
progressively higher viscosities and gas contents.
This section summarizes the links between compo-
sition and eruption style described in the previous
ROLE OF GAS CONTENT
sections.
The magma gas content may also play a role in con-
trolling the style of sustained explosive eruptions. • Chemical composition has relatively little
For instance, basaltic magmas differ from more influence in determining whether an eruption is
evolved ones in having low gas contents as well as effusive or explosive in character. This is more