Page 186 - Fundamentals of Physical Volcanology

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ERUPTION STYLES, SCALES, AND FREQUENCIES 163

version of what happens during the formation of a magma added to the chamber prior to eruption
pahoehoe compound lava flow field. Large lava will be approximately the same as the volume
flows are emplaced, then inflated, and finally used which is erupted (as long as the behavior is elas-
as lava tubes to feed more large flows. In such a case tic) then such models also explain why larger
the limiting factor on the total erupted volume chambers generate larger eruptions.
would be how much magma could be generated in • The largest volcanic eruptions are of only two
the plume head and how continuously it could be types because these eruptions represent special
supplied to the dike system. Any significant break cases in which conditions are such that exces-
in supply could cause cooling and solidification of sively large amounts of magmatic material can be
the dike system and interrupt activity until stresses erupted.
built up within the lithosphere to the point that a • Very large volume ignimbrite-forming eruptions
new dike propagated. are “inelastic” events in which eruption causes
caldera formation to occur. Such eruptions can
produce volumes which greatly exceed the vol-
10.9 Summary umes generated in “elastic” eruptions. They can
occur if there is a gas phase present in the magma
Observational data have demonstrated three fun- chamber when the initial overpressure has been
damental features of the behavior of volcanic relieved. The expansion of the gas phase drives
systems: the magma–gas mixture out of the chamber. This
can continue as long as gas continues to exsolve
1 that there is a link between the magnitude and from the magma. The chamber pressure will con-
frequency of activity such that small eruptions tinue to decline as more magma is removed and
occur frequently and larger eruptions occur less if it declines sufficiently the roof of the chamber
frequently; may fail causing caldera collapse. This roof col-
2 that the volume of magma erupted from a given lapse can then cause huge quantities of magma
volcano is commonly linked to the size of the to be driven from the eruption at very high rates
magma chamber feeding it such that large cham- causing very large volume ignimbrite-forming
bers feed large eruptions; eruptions.
3 that the largest eruptions in the geological record • Flood basalt eruptions are associated with the ini-
are of two distinct types: ignimbrite-forming erup- tial impingement of a mantle plume on the litho-
sphere and represent events in which magma is
tions and flood basalt eruptions.
erupted directly from the base of the lithosphere
These points can be explained as follows:
with no significant shallow storage occurring.
• Simple models of magma chamber failure explain Opinion differs about the mechanism of these
the first two points. For a chamber to erupt it first eruptions. One view is that the eruptions occur
inflates as magma is added to it. Once the pres- at extremely high eruption rates but last no more
sure exceeds a critical point determined by the than a matter of days. In the other view eruptions
strength of the chamber walls, the walls fail and may occur at much slower rates over a period of
an eruption or intrusion will occur. The larger years to decades. Both possible styles of eruption
the chamber the more magma must be added to are analogous to behavior observed in small-scale
it before failure occurs. Assuming that the rate at basaltic eruptions. The former would be equival-
which magma is supplied to a chamber does not ent to the elastic eruptions described in section
vary very greatly between different volcanic sys- 10.7 and the volume erupted would be limited by
tems, this means that the repose time between the size of the magma storage area at the base
events will be greater for larger magma cham- of the lithosphere. The latter would be limited
bers, i.e., the frequency of eruptions from large by how long magma could be continuously sup-
magma chambers is smaller than from smaller plied through the feeder dike system from the
magma chambers. Furthermore, as the volume of mantle source zone. A break in supply would