During an eruption at the Bocca Nuova, one of the summit craters of
Mt. Etna, in October–November 1999 a part of the crater floor
near its WNW rim was uplifted to form a dome-shaped feature that consisted
of older lava and pyroclastics filling the crater. This endogenous dome
grew rapidly over the crater rim, thus being perched precariously over
the steep outer slope of the Bocca Nuova, and near-continuous collapse
of its steep flanks generated swiftly moving pyroclastic avalanches
over a period of several hours. These avalanches advanced at speeds
of 10–20 m/s and extended up to 0.7 km from their source on top
of lavas emplaced immediately before. Their deposits were subsequently
covered by lava flows that issued from vents below the front of the
dome and from the Bocca Nuova itself. Growth of the dome was caused
by the vertical intrusion of magma in the marginal western part of the
crater, which deformed and uplifted previously emplaced, still hot and
plastically deformable eruptive products filling the crater. The resulting
avalanches had all the characteristics of pyroclastic flows spawned
by collapse of unstable flanks of lava domes, but in this case the magma
involved was of mafic (hawaiitic) composition and would, under normal
circumstances, have produced fluid lava flows. The formation of the
dome and the generation of the pyroclastic avalanches owe their occurrence
to the rheological properties of the eruptive products filling the crater,
which were transformed into the dome, and to the morphological configuration
of the Bocca Nuova and its surroundings. The density contrast between
successive erupted products may also have played a role. Although events
of this type are to be considered exceptional at Etna, their recurrence
might represent a serious hazard to visitors to the summit area.
Mt. Etna; Bocca Nuova; endogenous lava dome; pyroclastic avalanches;
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