Eruptions of Mount Etna
Types and styles of eruptions at 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:

1. terminal eruptions that occur from one or more of the summit craters. Examples: 1787, 1955-1971, 1995-2001.
2. subterminal eruptions occur from vents very close to the summit craters, and "take over" their activity. Until recently, the NE and SE Craters have been considered subterminal vents, but they are now classified rather as true summit craters. Eruptions classically considered "subterminal" occurred in 1975-1977 near the NE Crater, in April to early May 1971 (the first phase of the major 1971 eruption; the second phase, in early May to June 1971 involved activity from fractures on the ENE flank), and between February and November 1999 at the base of the SE Crater. Other, mostly small-volume, eruptions that are generally considered subterminal rather than flank eruptions occurred in 1956, 1964 and 1968 to the east and southeast of the summit.
3. lateral eruptions are fed by dikes radiating away from the central conduit system and occur at some distance from the summit craters. There may be types of eruptions intermediate between subterminal and lateral. Examples: 1928, 1950-1951, 1978-1979, 1983, 1989, 1991-1993, and part of the 2001 and 2002 eruptions.
4. eccentric eruptions are not genetically linked to the (upper part of) the central conduit system, but are fed by conduits originating at depth, rather than by radial dikes. Some of these eruptions do not occur from fissures with many vents (as is typical in many lateral eruptions), but from isolated vents, and tend to build relatively large pyroclastic cones. Many of the large cones on the middle to lower flanks of Etna are thought to have formed in this manner. Eruptions that have been classified as "eccentric": 1669, February-March 1763 (western flank), June-September 1763 (southern flank: Montagnola), 1974, 2001 (lowermost vents at 2570 and 2100 m), 2002-2003 (southern flank at 2800-2700 m).

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.

The 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.
Similarly, the 2002-2003 eruption consisted of simultaneous lateral and eccentric activity with magma draining from the conduit of the NE Crater into the NE Rift and a different magma issuing from vents on the upper southern flank. This complication and the fact that during the past few decades the summit craters have shown an ever wider spectrum of types of activity makes any classification rather difficult.

Another 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"