Environmental Engineering Reference
In-Depth Information
2012
). In the case of long-lived crystal-richmagmas (
) that lack the mobility
to erupt, rejuvenation may occur as a result of intrusion by hotter ma
'
mushes
'
c or intermedi-
ate magmas (e.g. Bachmann
et al
.,
2002
; Wotzlaw
et al
.,
2013
), potentially on short
timescales (Burgisser and Bergantz,
2011
). A related question concerns the particular
circumstances that
'
keep the lid on
'
a reservoir of buoyant magma, which may
ultimately feed a super-eruption. While super-volcanoes seem consistently to be
sited on thick, relatively low-density crust, which promotes accumulation of magmas
at depth, super-volcanoes do not only produce super-eruptions
the massive events
are interspersed with smaller intrusions, explosions and extrusions (see, e.g., Charlier
et al
.,
2005
on the
-
Taupo volcano).
Radioactive-decay series have been widely used to constrain the evolutionary
timescales of magma bodies. An important consideration in such studies is to
discriminate between the timescales of the chemical processes that generated
the silicic magmas responsible for super-eruptions, and the potentially more rapid
timescales of assembly and rejuvenation of an eruptible reservoir (Allan
et al
.,
2013
).
For instance, rubidium
'
hyperactive
'
strontium isotope measurements on minerals and glasses
from the Bishop Tuff indicate ages at least a million years older than the date of the
eruption itself (e.g. Halliday
et al
.,
1989
; Christensen and Halliday,
1996
) suggesting
a very long-lived magma system that was intermittently tapped during eruptions.
Uranium
-
thorium ages on zircon and allanite crystals isolated
from tephra deposits have also revealed timescales of a few hundreds of thousands of
years of crystallization prior to eruption. Vazquez and Reid (2004) found evidence
for part of the magma body associated with the
M
8.8 Youngest Toba Tuff being in
place up to 150 ka before eruption. A geochemical and geochronological study of
zircon crystals from the largest known super-eruption deposit, the 28 Ma,
M
9Fish
Canyon Tuff (associated with La Garita caldera, Colorado), suggests a 440 ka
magmatic history, a significant part of which was taken up by reheating of the pluton
that had reached up to 80% crystallinity (Wotzlaw
et al
.,
2013
).
-
lead and uranium
-
2.2.1 Eruption triggers
Emerging evidence concerning the thermo-mechanics of magma chambers may
give some meaning to the discrimination between super-eruptions and
'
eruptions. Caricchi
et al
.(
2014
) have modelled the thermal properties of host
rocks and their response to repeated magma intrusions that incrementally in
'
ordinary
ate
the chamber. They argue that
M
6 eruptions tend to be triggered as a result of the
overpressure that develops in smaller chambers in the upper crust as they are
repeatedly supplied with magma. This behaviour re
ects the elastic behaviour of
the relatively cold wall rocks and their propensity to fail under tensile stresses.
Such eruptions can be relatively frequent but only partially empty the chamber
