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Spectacular reproduction of a fresco in the Cathedral of Catania, showing the 1669 eruption of Etna whose lava flow caused significant damage in Catania and destroyed numerous villages. This drawing shows, to the north of the city, the still-barren lava flow erupted a few centuries earlier (possibly in 1381) from a fissure near the village of Mascalucia, which entered the sea at Ognina, now a densely populated part of Catania. Also note the medieval architecture of the cathedral (large tower in the lower center) which was destroyed in the 1693 earthquake, along with large part of the city. The Castello Ursino (seen surrounded by the 1669 lava flow in lower left of the image) survived both the eruption and the earthquake.

 

Eruptions of Etna
(A) Magma storage and ascent, and types of eruptions
(B) Historical eruptions - for eruptions since 1971 click here

The coast at Ognina, the easternmost part of the town of Catania, has spectacular outcrops of lava which is of an eruption in the 12th century (often cited as "1381 eruption"). Much of this lava has ropy pahoehoe surfaces. At the time of the eruption,the area was fortunately uninhabited, but a similar eruption, were it to occur today, would have devastating consequences.

An old sketch showing Etna and Catania during the 1669 eruption. In this bird's eye perspective the view is towards NNW, showing the fuming summit of Etna in the background while an eruption column is rising from a crater on the S flank, now called Monti Rossi, shown in the middle ground. This crater is feeding a large lava flow which has reached, and partly surrounded, the city of Catania, in the foreground.

Eruption in the Valle del Bove in 1819. This sketch neatly shows an upper explosive vent, at the SE base of the main summit cone, and a lower fissure in the upper Valle del Bove, similar to the vent alignment during the 1991-1993 eruption in the same general area. This sketch was drawn by C. Maravigna, author of a contemporaneous description of the eruption.

Spectacular photo, taken by photographer Salvatore Tomarchio, showing the initial stage of the eruption that lasted from November 1950 until December 1951, one of the most voluminous eruptions of Etna in the past 300 years. Eruptive fractures opened in the northwestern part of the Valle del Bove, and lava covered more than one-third of the surface of the Valle, at times advancing uncomfortably close to the village of Fornazzo.

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. Click here for more detail about this 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.

Part 3. Etna's eruptions from Ancient times until early 1971
(click here to go to eruptions since 1971)

The record of Etna's eruptions goes back to about 1500 B.P., and its list of documented eruptions is the longest of any volcano in the world. This record is clearly dominated by flank eruptions which caused much more alarm among the people living near the volcano, so that the picture of Etna's historical eruptions is far from complete, especially during the period prior to the 17th century. You may see lists of eruptions before 1900, and from 1900 to present; these will contain more and more links to selected eruptions with brief descriptions, flow maps and images (and/or photos). The following is a brief analytical summary of the eruptive activity since 1600.

The past 400 years in the eruptive history of Etna are characterized by several changes in the eruptive behavior of Etna, which coincide with changes in the output rate (that is, the amount of lavas and pyroclastics produced in a given period). During the period from about 1600 until 1669, eight flank eruptions occurred in various sectors of the volcano, and some of them were of unusually long duration (1614-1624, 1634-1638, 1651-1653) and volume (1614-1624: about 1-2 km³; 1669: about 1 km³). The mean output rate during the period 1600-1669 was calculated by Hughes et al. (1990) at 1.19 m³ s^-1, considerably higher than during any period after 1669. The culminating and concluding event of that period was the devastating eruption of March-July 1669 which drained the central conduit system to the point that the summit cone collapsed, and the shallow plumbing system underwent a profound change. For nearly a century following the 1669 eruption, Etna's output was presumably very low (Hughes et al., 1990), and there were only three relatively minor flank eruptions in 1689, 1702 and 1755, all of which occurred within the Valle del Bove on the eastern flank of the volcano and whose volumes are unknown. Furthermore, there is no information about the amount of eruptive products produced during that period by the near continuous activity at the Central Crater which led to the construction of a new summit cone after the 1669 collapse.

The 1755 flank eruption was followed by several years of near-continuous summit activity including several overflows of lava. Then, in a period of three years, no less than four flank eruptions occurred in various sectors of Etna: February-March 1763 (W flank), June-September 1763 (S flank), 1764-1765 (N or NE flank), and 1766 (S flank). This peculiar "cluster" of flank eruptions heralded a period when the eruptive behavior of Etna was remarkably regular and the output rate was distinctly higher than during the period 1669-1755. From 1767 until 1865 there were nine clearly isolated flank eruptions, separated by intervals of about 10 years: 1780, 1792-1793, 1802, 1809. 1811-1812, 1819, 1832, 1843, 1852-1853, and 1865. The last eruption of that period, in 1865, emitted some 100 x 10⁶ m³ of lava and pyroclastics and thus was among the largest since 1669 (the volume of the June-September 1763 eruption is estimated at 100 x 10⁶ m³ as well, but much of the lava of that eruption is buried under more recent lavas, so that this estimate is very approximate). A few weeks after the end of that eruption, a locally devastating earthquake (Magnitude 4.7) killed more than 70 people near the village of Macchia on the E flank of Etna (Boschi et al., 1995). This earthquake was related to displacements on a fault belonging to the "Timpe" fault system which lies at the intersection of the two major fault systems of eastern Sicily, the Malta Escarpment and the Messina-Giardini (or Messina-Etna) fault systems. Earthquakes originating from "Timpe" fault system have on various occasions followed eruptions on Etna (Gresta et al., 1987); Hirn et al. (1996) also recognized a possible correlation of periods with high output and large regional earthquakes. However, a clear causal relationship has so far not been confirmed due to insufficient data.

(To be continued)