The mechanics of large volcanic eruptions

The mechanical conditions for a volcanic eruption to occur are conceptually simple: a magma-driven fracture (normally a dyke) must be able to propagate from the source to the surface. The mechanics of small to moderate (eruptive volumes less than 10 km3) is reasonably well understood, whereas that of large eruptions (eruptive volumes of 10–1000 km3) is poorly understood. Here I propose that, while both large and small eruptions are primarily driven by elastic energy and may come from the same magma chambers and reservoirs, the mechanisms by which the elastic energy is transformed or relaxed in these eruptions are different. More specifically, during small to moderate eruptions, the excess pressure in the source (the primary pressure driving the eruption) falls exponentially until it approaches zero, whereby the feeder-dyke closes at its contact with the source and the eruption comes to an end. Under normal conditions, the ratio of the eruptive and intrusive material of the eruption to the volume of a totally molten shallow basaltic crustal magma chamber (at the common depth of 1–5 km) is about 1400, and that of a partially molten deep-seated basaltic magma reservoir (in the lower crust or upper mantle) is about 5000. Many magma chambers are partially molten, in which case the ratio could be close to that of reservoirs. Most magma chambers are estimated to be less than about 500 km3, for which the maximum eruptive volume would normally be about 0.4 km3. An eruptive volume of 1 km3 would require a totally molten chamber of about 1400 km3. While chambers of this size certainly exist, witness the volumes of the largest eruptions, large eruptions of 10–1000 km3 clearly require a different mechanism, namely one whereby the excess pressure maintenance during the eruption. I suggest that the primary excess-pressure maintenance mechanism is through caldera subsidence for shallow magma chambers and graben subsidence for deep-seated magma reservoirs. In this mechanism, it is the subsidence, of tectonic origin, and associated volume reduction (shrinkage) of the magma source that drives out an exceptionally large fraction of the magma in the source, thereby generating the large eruption. Most explosive eruptions that exceed volumes of about 25 km3, and many smaller, are associated with caldera collapses. The data presented suggest that many large effusive basaltic eruptions, in Iceland, in the United States, and elsewhere, are associated with large graben subsidences In terms of the present mechanism, successful forecasting large of eruptions requires understanding and monitoring of the volcanotectonic conditions that trigger large caldera and graben subsidences.