Conduit flow experiments help constraining the regime of explosive eruptions

It is currently impractical to measure what happens in a volcano during an explosive
eruption, and up to now much of our knowledge depends on theoretical models. Here we
show, by means of large‐scale experiments, that the regime of explosive events can be
constrained on the basis of the characteristics of magma at the point of fragmentation
and conduit geometry. Our model, whose results are consistent with the literature, is a
simple tool for defining the conditions at conduit exit that control the most hazardous
volcanic regimes. Besides the well‐known convective plume regime, which generates
pyroclastic fallout, and the vertically collapsing column regime, which leads to pyroclastic
flows, we introduce an additional regime of radially expanding columns, which form when
the eruptive gas‐particle mixture exits from the vent at overpressure with respect to
atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which
favors the formation of density currents resembling natural base surges. We conclude that
a quantitative knowledge of magma fragmentation, i.e., particle size, fragmentation
energy, and fragmentation speed, is critical for determining the eruption regime.

Research was partially funded by DPC-INGV
agreement 07‐09 and MUR PRIN 06.

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