of Mount Etna
Among the most fascinating aspects of Etna, from a volcanological standpoint, are the great variety of eruptive styles it displays, and the fact that there are various types of eruptions, based on their location and phenomenology. During the past decades, various classifications have been proposed, distinguishing eruptions according to location and eruptive styles. However, a classification based on the eruption location and inferred dynamics of magma transport is more difficult than one based simply on eruption styles, because it implies more knowledge of Etna's plumbing system than we actually have (see "The search for the hidden magma chamber"). I will nonetheless point out a few facts that give certain clues to the various types of eruption, resulting from differential magma movements within the volcano.
Before this, let us see how Etna's diverse eruptions have been classified by previous researchers. The best known classification scheme for Etnean eruptions is that of Rittmann (1964, 1973) and many authors following him, which distinguishes four eruption types:
In an attempt to simplify this classification scheme, Romano and Sturiale (1981, 1982) proposed that flank eruptions be distinguished in radial and regional or eccentric, the latter being mainly controlled by tectonic structures (regional fault systems). These authors noted that intermediate types of eruptions could occur and mentioned the 1669 eruption as an example.
Generally it has become common to talk of summit eruptions, if the activity occurs at the summit craters or at vents that lie very close to them (although there is no clear definition of the minimum distance from the summit to speak of flank rather than summit eruptions), and flank eruptions, if the activity occurs away from the summit area. The subdivision of flank eruptions into lateral and eccentric seems useful in the light of the most recent flank eruptions.
July-August 2001 eruption was the first documented
example of simultaneous lateral and eccentric activity from different
systems of eruptive fissures. While activity at fissures extending from
the SE Crater toward south (at 3050, 2950 and 2700 m elevation) and northeast
(at 2600 m elevation in the Valle del Leone) was directly fed from the
conduit of this crater (a classical case of lateral activity), magma that
erupted from the lowermost two fissures on the southern flank (at 2570
and 2100 m elevation) rose through a separate pathway and thus represented
an eccentric eruption.
kind of classification of Etnean eruptions is based on the type (or style)
of activity observed during an eruption, nonwithstanding its location
(summit or flank). Etna displays a wide range of eruptive styles, from
very slow, non-explosive emission of lava over mild Strombolian explosions
(often accompanied by slow lava effusion) to much more violent activity
which either consists of discrete explosions (which would be a more intense
variety of Strombolian activity) or continuous uprush of magma forming
lava fountains. These may or may not be accompanied by the formation of
tall columns of gas and tephra and fast-moving lava flows; there are,
however, examples of lava flows formed by such eruptions that were thick
and short and were formed by the rapid accumulation of still-fluid bombs
and scoriae. Such flows are called "clastogenic"; the most recent
examples were generated at the Voragine in 1998 and 1999.
Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"
Page set up on 3 February 1999, last modified on 8 February 2004