Large silicic magma bodies and very large magnitude explosive eruptions.
Abstract
An enduring and fascinating problem is how to generate very large silicic magma bodies capable of feeding eruptions of hundreds or thousands of km3 of magma in the form of ignimbrite and tephra fall with associated caldera formation. We outline a conceptual model based on geological, geophysical and petrological evidence, and framed by physical models of fluxing magmas through the crust. Primitive basaltic magmas flux into the base of the crust where they stall, cool and crystallize, create mushy hot zones containing residual differentiated mafic to intermediate melts. These melts separate, ascend and accumulate at higher levels (broadly the middle crust) where the same process is repeated to generate mushy, hot-zone silicic melts through a combination of fractional crystallization, reactive flow and crustal assimilation. These silicic, hybrid melts separate into melt layers that accumulate at the top of the mush. The combination of melt and mush rheology means that middle crustal hot zones need long time periods (105 to >106 years) to form separated layers which have volumes commensurate with large magnitude (M>8) ignimbrites. As the silicic melt layer grows buoyancy forces lead to instability and incipient formation of Rayleigh Taylor (RT) instabilities deforming overlying hot crust. Petrological evidence for very rapid accumulation of shallow silicic magma bodies favours transport of silicic magma from the middle crust through dykes formed above incipient RT instabilities rather than diapirs. Rapid magma transport enables fluxes that are orders of magnitude higher than time-averaged silicic melt generation in middle crustal mush. Consequently, advection of heat combined with downward subsidence of crust enables a large shallow magma chamber to form at typical depths of 5 to 10 km and typically erupt within decades or centuries. Remobilization of mush or magma by reheating during replenishment at shallow levels can occur, but its role is argued to be secondary. Observations of magma mixing, re-heating and disequilibria may be the consequence of disruption of the trans-crustal magma reservoirs due to the dynamics effects of magma ascent induced by buoyancy rather than being a primary cause of silicic magma chamber formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV018...04S
- Keywords:
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- 1922 Forecasting;
- INFORMATICS;
- 5480 Volcanism;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY