Environmental Engineering Reference
In-Depth Information
of pyroclastic (fall and current) deposits over a suf
ciently representative geo-
graphic range. Then, estimates of relevant densities of the diverse materials are
required to convert to mass (as required in
Equation (2.1)
). For distal tephra fall
deposits, it may be feasible to make direct measurements of mass per unit area. In
all cases, an extrapolation or model will be required to account for unmapped
material, whether it be the very thin (and potentially unpreserved) ultra-distal ash
fall deposits, or proximal ignimbrite with unseen bases resting on unknown pre-
eruption topography (e.g. Bonadonna and Costa,
2013
; Burden
et al
.,
2013
). Many
of the published estimates of super-eruption magnitudes are based on limited
deposit data and/or
'
back of the envelope
'
calculations of the volumes of intra-
caldera deposits, out
ow sheets and tephra fallout (see, e.g., Rose and Chesner,
for a digest of estimates of the magnitude of the Youngest Toba Tuff eruption).
2.1.1 Collapse calderas
Super-eruptions are typically caldera-forming events, given the volumes of
magma that are expelled from the crust. Thus, the most distinctive feature of a
super-volcano is often its sizeable caldera (e.g. Geyer and Marti,
2008
). These are
generally at least 20 km across; some are closer to 100 km in diameter, though
such large structures are often complex resulting from super-imposition of craters
formed during more than one episode of collapse (Lipman,
1997
). The timing
of collapse can be critical in the dynamics of very large eruptions since the
ring fractures developed above the evacuating chamber can promote even higher
magma discharge rates associated with prodigious discharge of pyroclastic currents
at the surface. The depression formed can also end up accommodating a substantial
fraction
-
as much as a third to a half
-
of the ejecta (Mason
et al
.,
2004
). Many
large calderas today feature a
that typically domes the intra-
caldera deposits upwards. The precise origins of such structural uplift are uncertain
but probably re
'
resurgent centre
'
ect a combination of magma reservoir recharge and volatile
exsolution (Kennedy
et al
.,
2012
; Wilcock
et al
.,
2013
).
2.2 Magma bodies associated with super-volcanoes
Considering the great mass of magma disgorged by a super-eruption, and its exclu-
sively silicic (dacitic through rhyolitic) petrological af
nities, it is clear that the
magma involved was assembled prior to eruption. A range of geochemical, mineral-
ogical and geochronological studies indicate that the associated timescales of assem-
bly of very large magma chambers may take anything from hundreds of thousands of
years to just a few thousand years (e.g. Wilson and Charlier,
2009
;Druitt
et al
.,
