How to Make a Super Eruption: The Roles of Magma Supply and Crustal Structure
Abstract
There are two requirements for producing very large (ca. 1000 cubic km) catastrophic eruptions: (1) there must be a large volume of magma stored in the crust within several kilometers of the surface, and (2) it must be possible to erupt a large fraction of the magma in the chamber. A model can be constructed to help understand the circumstances under which these two requirements are met. Critical information comes from isotope geochemical studies of magmatic systems, models for magma generation in the mantle, energy balance considerations for magma being transported through or stored in the crust, and the properties of wallrocks surrounding a magma chamber. The single most important parameter is the amount and duration of magma supplied to mid-crustal levels in continental crust. Magma supply is determined by magma generation rates in the mantle, and can be amplified by melting in the lower crust if the ambient lower crustal temperature is high enough. Mantle magma supply to individual volcanic systems not associated with large mantle plumes is generally limited to about 0.03 cubic km per year, with values in the range 0.0001 to 0.01 ckm/yr being more common. Isotope studies show that in regions of thick, hot continental crust, the mantle magma supply can be enhanced by factors of 2 to 10 by melting in the lower crust. A high magma supply assures that magma can accumulate in the crust at mid- to shallow crustal levels; the continuing addition of new magma being sufficient to overcome losses due to crystallization and small eruptions. With a supply of 0.01 ckm/yr it requires a minimum of 100,000 years to accumulate 1000 ckm of magma, hence it is a requirement of very large eruptions that magma be stored in the crust for a long time. Extended storage is a special circumstance; most magma supplied to the mid-crust is either rapidly erupted or crystallizes, as in andesitic and basaltic volcanoes where U-Th isotopic disequilibrium that requires minimal storage time. Magma can accumulate in the middle and upper crust, even though it is buoyant and eruptable, if the wallrocks behave viscoelastically and have a sufficiently low yield stress (Jellinek and DePaolo, Bull. Volc., 2003), which means that the wallrocks have to be pre-heated, presumably by precursory magmatic activity. Once a magma chamber becomes moderate in size, if it is filled by relatively high-silica magma, eruption is suppressed due to limitations on the overpressure that can be achieved to drive dike propagation away from the chamber. When this condition is reached, magma can accumulate as long as the supply is maintained, and it becomes possible to accumulate up to ca. 10,000 ckm of magma in a single chamber. To erupt a large fraction (order half) of magma stored in the chamber requires that the roof detach during eruption; hence it is impossible to have a large eruption without forming a caldera. The exact requirements for roof detachment have not been worked out, but general conditions can be described based on observations of caldera dimensions and estimates of magma chamber depths.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V24C..06D
- Keywords:
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- 1037 Magma genesis and partial melting (3619);
- 1040 Radiogenic isotope geochemistry;
- 8428 Explosive volcanism;
- 8439 Physics and chemistry of magma bodies;
- 8440 Calderas