This section consists of three parts. The first provides information about the way magma moves into the volcano and where it comes from; the second gives a general descriptions of the various styles of activity and eruption types observed at Etna. The third, chronological part leads to a series of pages dealing with some of the more important eruptions during the historical period until early 1971, ending just before the major eruption initiated in early April 1971.

 

Among the events discussed will be the 1669 eruption that threatened Catania and destroyed 16 villages, and most eruptions during the 20th century, ending with the long period of summit activity that lasted from 1955 until 1971. Eruptions since 1971 are treated in a separate section, since they represent a unique period in the history of Etna, both for the variety and frequency of eruptive activity and for the enormous progress made in the studies of Etna and its dynamics.

Part 1. Before Etna erupts: the storage and transport of magma

Etna erupts almost continously, but where does the magma feeding its activity come from, and how does it move into the volcano and up to the surface? Does Etna, like many other volcanoes, have a "magma chamber" where the mixture of gas and molten rock accumulates and evolves before rising to the surface?

At many volcanoes, modern seismological methods have helped revealing the location of what is popularly named "magma chambers" at relatively shallow depth (a few kilometers) - since these areas where magma accumulates before eruption are probably much more complex than anything having the simple shape of a "chamber", it is more appropriate to talk of "magma storage areas" or "magma reservoirs". At volcanoes like Kilauea on Hawaii and Vesuvio in Italy, magma reservoirs have been located at depths of 2-10 km. In the case of Etna, no permanent shallow reservoir appears to exist, at least none of significant dimensions.

Part 2. Etna's eruptions: an introduction to eruptive styles and eruption types

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 above). 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, 1960, 1964, 1995-1999.

   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, and 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).

   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: 1892, 1950-1951, 1978-1979, 1983, 1989, 1991-1993, and many more.

   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. Many 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 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.

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.

The July-August 2001 eruption was a unique 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.

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.