IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_3C-O1

Room A5

Date/Time: July 23 8:45-9:00

A thermomechanical perspective on caldera formation and classification
Shanaka L de Silva, Patricia M GreggOregon State University, U.S.A
E-mail: desilvas@geo.oregonstate.edu

Decades of research have led to a mature understanding of caldera formation and evolution. In particulara temporal model for caldera formation (Smith and Bailey, 1968) is now well understood and morphologicalclassifications based on geological studies (e.g. Lipman, 1997; Cole et al, 2005), recently informed by analogue
models (Acocella, 2007), capture the final surface manifestations and styles of collapse and inferred relationships tochamber depth, size, and shapes (Lipman, 2000; Roche et al., 2000; Kennedy, 2000). However, our understandingof the processes that lead to these morphologies and the temporal evolution remain murky. In particular, what
initiation and triggers catastrophic caldera collapse is unclear. The general two stage model of collapse (Stages 2and 3 of Smith and Bailey, 1968) is the paradigm that prevails (Druitt and Sparks, 1984; Kennedy and Stix, 2003).
In this model an initial eruption is triggered by overpressure in the magma chamber. Evacuation of magma andbleeding off the overpressure leads to an underpressured condition in the magma reservoir that triggers caldera
collapse and pyroclastic flow generation. However, it has been increasingly recognized that many of the largest
calderas do not conform to this two-stage model and involve catastrophic caldera collapse at the inception oferuption (Christiansen, 2005; de Silva et al., 2006). Moreover, many of these large calderas exhibit a significant
structural resurgence that smaller systems do not. Large caldera collapses result in collapse of the entire upper
crust above the magma system, smaller calderas form within the edifice. The details of the physical volcanology ofthe eruptions and the resulting ignimbrites are quite different.

These differences between large and small calderas can be understood from a thermomechancal perspective.
Generation of viable large magma bodies requires elevated thermal fluxes and thermally mature crust (de Silva andGosnold, 2007; Annen, 2009) that plays a fundamental role in the rheology of magma/host rock interface (Jellinekand DePaolo, 2003) and ultimately controls magma reservoir growth, pressure evolution, surface uplift, faulting,and caldera size (Gregg et al, 2012, 2013). In particular thermomechanical considerations suggest that in large
systems the catastrophic caldera eruptions are "externally" triggered by mechanical failure of the roof. Smaller
systems are "internally" triggered by overpressure within the magma reservoir. These different modes of triggeringlargely control the final form of the calderas and the character of the eruption and associated pyroclastic deposits.
We suggest that catastrophic calderas can be fundamentally divided into two size divisions with an approximateboundary at 10km diameter, 100 km3 and VEI=7. Ultimately this is a thermal or thermomechanical divide.

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