A suggestive image of the first (subterminal) phase of the 1971 eruption, photographed on the evening of 6 April 1971 in the flat area at the base of the central cone (seen in the background), with the eruptive vents that have opened near the Etnean Observatory and the arrival station of the Etna cable-car. The right portion of the photograph is now occupied by the cone of the Southeast Crater, which was born during the second phase of the 1971 eruption. Photo taken by Giuseppe Scarpinati
40 years ago, on 5 April 1971, a new eruption began on Etna, which in its first phase affected the southeastern portion of the summit area (a so-called "subterminal" eruption), and then developed into a classical flank eruption with the opening of new eruptive fissures on the ENE flank. This was the first flank eruption of Etna since 20 years. To better understand the significance of this eruption, it is necessary to place it into the context of the eruptive activity of Etna during the 20th century, and compare the events before the 1971 with those following it.
Prelude: 20 years without flank eruptions, but with intense summit activity
During the first half of the 20th century, Etna produced eight flank eruptions, the latest of which, in December 1949, started in the summit area before shifting to the upper northwestern flank, and ending a few days later. One year later, on 25 November 1950, a much more significant eruption started in the Valle del Leone, on the upper ENE flank; this eruption was to last 13 months and caused widespread damage to human property around the village of Fornazzo. For its long duration and great volume of lava emitted (estimated around 150 million m3), this was one of the most significant eruptions of Etna in recent centuries.
After the end of the 1950-1951 eruption, the volcano was in repose for more than 3 years. A new eruptive period started in April 1955 at the summit craters. This activity continued with few interruptions for 16 years, mostly from the Northeast Crater, but with a number of culminating paroxysmal events from the Central Crater (mainly in 1956, 1960 and 1964). This long period includes also three minor subterminal lava flows, in 1956, 1964, and 1968, phenomena closely related to the summit activity (unlike typical flank eruptions, these outflows were not accompanied by the cessation of summit activity). Between January 1966 and the first days of April 1971, the Northeast Crater was the site of classical "persistent activity", consisting of frequent small Strombolian explosions accompanied by continuous lava emission at low rates (a few cubic meters per second).
This long period of summit activity resulted in dramatic changes in the morphology of Etna's summit area: the former Central Crater was filled to overflowing during the paroxysms of 1956, 1960 and 1964, although a huge pit - the Voragine ("big mouth"), born in 1945 - remained open in its northeastern quadrant. In the spring of 1968, a new pit crater formed to the west of the Voragine; this became known as Bocca Nuova ("new mouth"). The Northeast Crater built a cone around it that constantly grew taller and larger, and was surrounded by a voluminous apron of lava flows.
Activity at the Northeast Crater faded away during the first few days of April, and for a very short time, the volcano was silent. There were no signs of any kind that might have indicated an imminent major eruption; it has to be noted, though, that at the time no real seismic monitoring network existed at Etna - during the months prior to the eruption, one single seismic station was operating at Acireale, while a second, in Catania, apparently started working again shortly before the onset of the eruption (Azzaro & Barbano, 1996).
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First phase: subterminal eruption
The first, subterminal phase of the 1971 eruption started on 5 April and continued until 7 May. In this phase, eruptive activity occurred from four radial fissures located at the southeastern base of the central summit cone of Etna. Fissure #1 lay at 3000 m elevation, fissure #2 at 2960 m, fissure #3 at 3020 m, and fissure #4 at 2880 m.
Lava emitted from these vents flowed south, across the Piano del Lago (a gently sloping, generally flat area between 2500 and 2900 m elevation on the south flank of Etna), and ESE into the Valle del Bove. During this phase, the building of the "Etnean Observatory" (constructed at the end of the 19th century but never put into continuous use as a volcano observatory) was buried by lava flows along with the arrival station and several poles of the Etna cable-car. Small scoria cones grew around the eruptive vents, reaching heights of a few tens of meters. The most advanced lava flow, to the south, stagnated at 2175 m elevation, not far from the Rifugio Sapienza, a prominent mountain hut near the departure station of the Etna cable-car.
Second phase: flank eruption
On 7 and 8 May, a system of seven eruptive fissures starts to propagate from the site of the first phase of the eruption toward east-northeast, along the western rim of the Valle del Bove. The six major fissures are at 2680 m, 2580 m, 2450 m, 2300 m, 1840 m and 1800 m elevation. The lowermost of the fissures in Contrada Serracozzo (near the Rifugio Citelli, on the northern outer flank of the Valle del Bove), emit the most destructive lava flows, which during one month repeatedly threaten the villages of Fornazzo and Sant'Alfio, burying 2.5 km2 of cultivated land. On 14 May, a pit crater opens at the southeast base of the central cone, from which dense clouds of ash are emitted; this ash consists of pulverized old rock, no fresh magmatic material. Seven years later, this new vent will be formally named "Southeast Crater".
The eruption ceases after 68 days of continuous activity, on 12 June 1971. The lava has covered a total area of 7.5 km2; the maxium length of lava flows of the first phase is 4 km (down to 2175 m); and of the second phase, 7 km (minimum elevation reached is 600 m). The total volume of the eruptive products of this eruption is estimated at 40-75 million m3 of lava; to this have to be added 3 million m3 of pyroclastic material.
The 1971 eruption is the first in a long series of flank eruptions at Etna, which will continue until 1993 and consist of 13 discrete events. At no other time in the documented history of the volcano has there been a similar sequence of flank eruptions, and many of these events cause damage; a few seriously threaten population centers like Fornazzo, Randazzo, and Zafferana Etnea.
In fact, the 1971 eruption coincides with a significant change in the dynamics of Etna: many of the subsequent eruptions will occur in the eastern sector of the volcano, and nearly all of them will be structurally closely connected to the newly formed Southeast Crater, which since 1978 is the most active of the four summit craters of Etna. Furthermore, the magmas emitted since 1971 show a tendency toward more mafic (sometimes also called "basic") compositions, a sign of a more voluminous and rapid ascent of magmas from depth, which gradually will affect the entire feeding system of Etna.
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First phase of the 1971 eruption: Strombolian activity from various vents at the southeastern base of the central summit cone of Etna, whose outline can be discerned in the background. Photo taken on 8 April 1971 by Giuseppe Scarpinati
Second phase of the 1971 eruption: very fluid, degassed lava is emitted from the lowermost eruptive vents, at about 1800 m elevation in the Contrada Serracozzo area, on the northern outer flank of the Valle del Bove. Photo taken in late May 1971 by Giuseppe Scarpinati
The birth of Etna's fourth and youngest summit crater, the Southeast Crater. On 14 May 1971, one week into the second (flank) phase of the eruption, a collapse pit opens at the southeastern base of the central summit cone (whose flank is seen at left). From this pit, clouds of vapor and lithic ash (composed of pulverized old rock, not fresh magmatic particles) are ejected, forming a low plume. This photo was taken by Carmelo Sturiale, volcanologist at the University of Catania, from a position near the Torre del Filosofo mountain hut
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