Page 183 - Fundamentals of Physical Volcanology

9780632054435_4_010.qxd 12/10/2007 12:32PM Page 160

160 CHAPTER 10

The greater amount of deflation during eruption 18 east rift zone extends to a total length of 125 km,
is reflected in the greater volume of lava produced 75 km of which is offshore and has built a submar-
in this eruption – twelve times as much lava as dur- ine ridge (Fig. 10.10). Eruptions on this ridge can
ing eruption 19! The similarity between the actual occur at elevations which are lower than the elevation
behavior shown in Fig. 10.9 and the behavior of the top of the summit magma chamber, and it is
predicted by simple elastic models such as that inferred that large submarine eruptions periodically
described above is remarkable. cause excessive drain-down of the magma chamber
The type of eruptive activity just described is and hence caldera formation.
“elastic”, i.e., the inflation and deflation of the The condition that allows inelastic activity and
magma chamber is cyclic and not associated with caldera formation in association with large explo-
any significant permanent deformation; eruption sive eruptions is very different. It depends on the
ceases when the overpressure generated prior to idea that a gas phase exists in the upper part of
eruption has been relieved. Not all volcanic sys- the magma chamber at the time when the initial
tems, however, behave in such a simple way. In overpressure has been relieved. If no such gas
many eruptions this simple pattern of behavior is phase is present in the chamber once the overpres-
significantly modified because an eruption causes sure is relieved then the eruption must cease because
inelastic, i.e., irreversible, deformation of the vol- there is no pressure available to push the magma
canic edifice. These are eruptions in which activity upwards through the dike/conduit system. If,
continues even after the initial overpressure has however, the magma is initially supersaturated
been relieved and in which continued eruption in volatiles, or becomes so as the pressure in the
may eventually reduce the pressure inside the chamber falls (during the elastic phase of the erup-
chamber to the point where the roof collapses. tion), then the formation and growth of the gas bub-
Such “inelastic” eruptions seem to occur when bles will drive the gas–magma mixture out of the
3
erupted volumes exceed at least ∼10 to 50 km . chamber and allow eruption to continue even as
These inelastic eruptions include some of the the chamber pressure continues to decline. In fact,
largest eruptions known – the ignimbrite-forming this becomes a self-perpetuating behavior because
eruptions in Table 10.5 are all examples of inelastic the further reduction in the chamber pressure trig-
activity and are all associated with caldera collapse. gers further exsolution and so on. If this behavior
Inelastic events can also occur, though, in small continues for long enough then eventually the
basaltic systems – the current caldera at Kilauea reduction in pressure causes failure of the chamber

volcano, for example, formed during an eruption in roof and caldera formation. The amount of magma
1790. As inelastic eruptions are ones in which the which can be erupted will depend on the detailed
largest volume of material can be produced by any eruptive behavior. If exsolution of gas from the
given volcanic center, it is important to understand magma ceases before caldera collapse occurs then
the circumstances in which an eruption may con- the limiting factor will be the amount of gas initially
tinue beyond its elastic limit. dissolved in the magma and the chamber depth.
There are two main conditions in which an If caldera collapse occurs, the force of the unsup-
eruption may be able to continue after its initial ported roof rocks pushing down on the remaining
overpressure has been relieved. The first occurs magma in the chamber could force a high propor-
on shield volcanoes. If the vent at which eruption tion of that magma out.
occurs is at a lower elevation than the top of the
magma chamber then magma can continue to drain
from the magma chamber through the feeder dike 10.8 Eruptions of exceptional magnitude
system even after the overpressure has been relieved.
This is the mechanism envisaged at Kilauea volcano
10.8.1 Introduction
to explain caldera formation in 1790. Kilauea pos-
sesses two long rift zones – areas of dike intrusion The largest eruptions seen in the geological record
which extend laterally from the summit. Kilauea’s are ignimbrite-forming eruptions and flood basalt