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Flank
Eruptions
Mount Etna erupts both from the summit craters and from vents away from the summit craters, which are generally considered flank vents. It has been correctly observed that some activity on the flanks appears to be closely related to the summit craters, and thus to the central conduit system. In these cases, flank activity is almost always preceded by increased summit activity by days to months, and summit activity ceases once flank eruption has begun, or changes from Strombolian-effusive to strong degassing, often carrying some ash. Flank vents may form, in this type of eruption, very close to the summit and practically "take over" their activity, as has occurred in February 1999. This type of activity, which may continue for months or even years, is the classical "subterminal" eruption as defined by the volcanologist Rittmann, and is nothing else than some kind of summit activity, even though occurring from vents outside the summit craters. The output is generally fairly low (from less than one cubic meter per second to a few cubic meters per second) and is considered to represent the mean rate at which magma is ascending to the surface from depth. It should be noted, though, that this magma ascent rate is not all too regular. "LATERAL"
ERUPTIONS What
causes such eruptions? There are two main reasons for these eruptions
to occur. One is the response of the volcano and its fracture systems
to variations in the volume of magma rising within the central conduit
system. The uprise of a voluminous "batch" of magma, especially
when gas-rich, may notably increase the hydrostatic pressure on the
conduit walls and cause the mountain to fracture radially, in a process
where magma injection into the flanks is active and fracturing
is passive. The other process which may lead to "lateral"
erputions is a change in the structural stability of the volcano, caused
by tectonic movements. A decrease in the structural stability of the
volcano may generate fractures at depth into which magma can intruce
more or less passively, and there it may remain for weeks before it
erupts, as supposedly occurred in 1991, before the 1991-1993
flank eruption. The
evolution of a "lateral" eruption from the initiation of dike
intrusion to the arrival of magma in a newly opening eruptive fissure
has been documented extraordinarily well in 1983 on Etna's south flank
(Murray and Pullen 1984). The path of the subterranean dike, as it propagated
towards the eruption site, could be followed by monitoring the ground
deformation (a levelling network was set up in the area by John Murray
and his colleagues months before, because an eruption was considered
likely to occur there in the future). It was striking to see that, although
there was no activity at the SE Crater before and during the 1983 eruption,
the dike originatted from a point very close to that crater, at a depth
of about 1 km. For the first kilometer, the path of the dike ran parallel
to the fractures of 1978 and 1979 eruptions, then, east of Torre del
Filosofo, it split into two branches, one continuing straight southeastwards
while the other turned southwestwards, and then, still another kilometer
downslope, southwards. The southeastern branch turned south at Belvedere
and ended somewhere to the south without reaching the surface. Although until 2001 the path of a feeder dike for a flank eruption was never again traced in as much detail as in 1983, geophysical monitoring has allowed the documentation of further intrusions in other cases thereafter. All flank eruptions during that period through 2001 thus seem to originate from the SE Crater, only one intrusion which did not result in a flank eruption, in September 1986, originated at the NE Crater (Murray 1990). Some were, in fact, characterized by the slow draining of the central conduit system (1983, March-July 1985, 1986-1987, 1991-1993) while others were caused by the fracturing of the volcano under the hydrostatic pressure of a newly arrived, voluminous and gas-rich batch of fresh magma (e.g., 1989). In 2002, eruptive activity on the Northeast Rift was again closely related to the NE Crater, while activity on the S flank during the same eruption was completely independent of the central conduit system. A typical eccentric eruption at Etna occurred in early 1974 on the western flank of the volcano. These amateur photos, taken by Renato Bernardini (University of Catania), show a small cinder cone growing by intense Strombolian activity amidst a forest, February 1974. "ECCENTRIC"
ERUPTIONS In
spite of the enormous amount of new data provided by the 2001 and 2002-2003
eruptions, the problem of eccentric eruptions remains full of mysteries.
Do the conduits feeding such eruptions originate in a rather "individual"
manner from the deep storage area, or do they depart, still at considerable
depth, from the roots of the central conduit system? And which of the
eruptions of the past centuries were eccentric, which were lateral?
Is there a reliable method allowing to distinguish whether past eruptions
were eccentric or lateral? Possibly the explosivity of those eruptions,
which can be read from the size of the related cones, is an important
indicator, and the chemical composition of their products may be another.
Research in this field is not only of academic interest, but has important
implications for hazard assessments at Etna. If eccentric eruptions
are always as explosive as those of 2001 and 2002-2003, they represent
a significant hazard to human infrastructures and traffic. It is therefore
necessary to understand how frequent such eruptions are in the long
and short term. During the 110 years before the 2001 eruption there
had been only one single eccentric eruption, which furthermore was studied
by an earlier generation of scientists, and before most of the current
concepts of Etna's dynamics were developed. At the beginning of the
new millennium eccentric eruptions thus received very little - if any
- attention and were not even mentioned in most publications of the
20 years preceding the 2001 eruption.
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Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology" |
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set up on 1 June 1999, last modified on 8 February 2004
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