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

3A1_3C-O4

Room A5

Date/Time: July 23 9:30-9:45

Caldera development during explosive eruptions
John Stix
McGill University, Canada
E-mail: stix@eps.mcgill.ca

An interesting question is the likelihood that an explosive eruption results in caldera formation. Large eruptions
emitting 102 -103 km3 of magma invariably form substantial surface subsidence structures, but smaller eruptions
emitting 5-10 km3 may not. These smaller events show variable behaviour, which is generally unpredictable,
ranging from well-developed caldera structures (e.g., Pinatubo) to no calderas at all (e.g., Huaynaputina). For such
events, is it possible to establish, during or even prior to the eruption, the probability that a caldera will form? Suchan assessment is important, as the style of the eruption depends in part upon whether a caldera forms. One clear
control is depth to the top of the magma reservoir; as depth increases, surface subsidence is reduced due to thecrustal column between the surface and the top of the reservoir. A second important consideration is the amountof eruptable magma in the reservoir. An eruption that efficiently drains such eruptable magma from the reservoir
may suddenly cease, putting a halt to caldera development. The caldera volume is commonly smaller than thevolume of erupted magma, and this is a reflection of the mismatch between efficient magma extraction and surface
caldera response. There is general recognition that many volcanic systems are underlain by a series of magma
reservoirs stacked vertically in the crust and connected at times. There may thus be a balance between the relativeroles of a shallow reservoir at 4-10 km depth and a second reservoir at deeper crustal levels which together may
influence surface conditions including caldera subsidence. If the shallow chamber is small or largely crystallized,then the deeper reservoir may play a greater role in controlling the nature and size of the eruption. If the shallow
chamber is large or largely liquid, then its influence will dominate. A fundamentally important aspect of this stacked
magmatic system is that the shallow and deeper reservoirs are connected, allowing magma to be transferred fromthe deep to the shallow reservoir. The connection may be established, before, during, and/or after the eruption.
Large explosive eruptions commonly appear to have this connection established prior to the eruption, allowing deep
magma to (a) interact and mobilize shallow magma and (b) trigger the eruption. Magma erupted from the shallow
reservoir unloads the system, providing a feedback for inflow of magma from below, perhaps at accelerating rates
at later stages of the eruption and immediately afterward. Forecasting caldera formation during an eruption may bepossible. Since surface caldera development is commonly a late-stage eruptive phenomenon which is preceded bysubsurface faulting and subsidence, geophysical approaches (e.g., seismology, geodesy, gravity) may be used toidentify subsurface processes such as faulting and development of voidspace which migrate upward in time.

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