Environmental Engineering Reference
In-Depth Information
2.3 Eruptive style
Are super-eruptions qualitatively different from lesser eruptions? To begin to
address this question, we look
first at the historical record. The largest eruption
of the past century is that of Mount Pinatubo in 1991 (M 6.1). The climactic
eruption of 15 June 1991 followed a crescendo in precursory activity over the
preceding months. The most intense phase of the Plinian eruption lasted around
3.5 hours (Scott
et al
.,
1996
) and was accompanied by the collapse of parts of
the ash and gas plume, which generated substantial pyroclastic density currents
and a co-ignimbrite ash cloud. A 2.5-km-wide crater was formed as a result of
the eruption.
To get close to the
M
7 class in the modern period for which we have some
eyewitness accounts, we turn to the eruption of Tambora (Sumbawa island,
Indonesia) in 1815 (
M
6.9). From what we can infer from the deposits and the
contemporary reports, the eruption was of a similar style to that of Pinatubo in
1991, with increasing eruption intensity heralding a collapse of a Plinian eruption
column that generated voluminous pyroclastic density currents and associated
co-ignimbrite plumes (Sigurdsson and Carey,
1989
; Oppenheimer,
2003
). Unfor-
tunately, there are no clear indications of the duration of this paroxysmal phase,
so it
cult to say to what extent the greater size of Tambora relative
to Pinatubo is due to intensity or duration; but it is probably a combination of
both. A comparable
M
7 eruption, of Rinjani volcano on the Indonesian island
of Lombok, has been identi
is dif
ed and dated to the mid-thirteenth century (Lavigne
et al
.,
2013
).
While intensities and durations of lesser events are somewhat constrained,
these parameters can only be inferred for eruptions of
M
8 and above. In the
case of the 0.76-Ma-old
M
8.3 Bishop Tuff eruption (associated with Long Valley
caldera, California) it has been estimated that the duration of the climactic eruption
may have been less than a week (Wilson and Hildreth,
1997
). Other super-
eruptions may have involved intermittent bursts of activity that spanned a few
years (Wilson,
2008
), or possibly a few centuries (e.g. Ellis
et al
.,
2012
; Svensson
et al
.,
2013
).
2.3.1 Wet versus dry eruptions
The most recent documented super-eruption is the
M
8.1 Oruanui eruption of
Taupo volcano in New Zealand, which took place
c
. 25.4 ka ago. Triggered and
modulated by rift tectonics (Allan
et al
.,
2012
), episodes of the eruption involved
interaction between magma and lake water (Van Eaton and Wilson,
2013
). As a
consequence, some of
the pyroclastic deposits are particularly
fine-grained,
