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The
1955-1971 summit eruptions, Part 4 |
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The period of summit eruptions of 1955-1971 is exceptional in the recent history of Etna for its duration and for the intensity of the eruptive activity, although the summit eruptions initiated in 1995 and continuing as of early 2000 are even more vigorous. While the Strombolian activity of the NE Crater was comparatively modest, enormous volumes of lava were emitted from it at low effusion rates, but almost continuously over many years, amounting to at least 220 x 106 m3. It is notable that this crater was active during at least 50% of the entire 16-year period. On the other hand, activity at the Central Crater occupied only 2% of the whole period, but it was considerably more violent. Furthermore it is worth remarking that there was rarely any simultaneous activity at both the Central Crater and the NE Crater, but on a number of occasions the activity shifted from the NE Crater to the Central Crater and then back to the NE Crater. A similar behavior was observed during the 1970's by Chester et al. (1985): the NE Crater was quiet when lava rose within the Voragine or the Bocca Nuova (or both), and collapse occurred in these two craters when there was activity at the NE Crater. The shifting of activity from the NE Crater to the Central
Crater was at the time explained as a consequence of the blockage of the
NE Crater conduit, forcing magma into the conduit of the Central Crater.
It is not clear why then the magma did return to the NE Crater after a
brief period, rather than continue to erupt from the Central Crater, if
the blockage of the NE Crater was effective enough to force the magma
into the Central Crater. In any case the factors controlling the sensitive
hydrostatic equilibrium between the two craters at that time have never
been quite well understood.
One observation, however, may cast some light onto these enigmatic processes. During the period 1955-1971, activity at the NE Crater was always relatively mild and highly regular, with only minor fluctuations over long periods (months to years). In contrast, most activity from the Central Crater consisted of short-lived, violent eruptive episode with high mass eruption rates. It can be speculated that activity at the Central Crater occurred when more voluminous, gas-rich batches of magma rose up within the volcano, which for some reason bypassed the NE Crater. Maybe the conduit of the NE Crater was too narrow to accomodate these magma batches which then rose through the wider conduit below the Central Crater, producing more vigorous activity. In the cases of the isolated and brief eruptive episodes at the Central Crater in 1955, 1956, 1960 and 1961, only the "excess magma" was evacuated from the top of the magma column, after which the magma, once it was rising at a lower and more regular rate again, returned to the NE Crater conduit. The comparatively long-lived Central Crater activity of 1964 led to more complete emptying of the central conduit system, so that ater the end of the 1964 activity it took 18 months until magma returned to the surface at the NE Crater. Equally enigmatic, the three episodes of lava emission from fissures high on the E flank in 1956, 1964 and 1968 represent significant deviations from the regular activity at the NE Crater. The earlier two of these episodes were more or less contemporaneous with vigorous activity at the Central Crater, and may confirm the assumption that the magma supply to the summit craters temporarily increased, which not only caused the magma to be diverted from the conduit of the NE Crater to that of the Central Crater, but it also led to the fracturing of the conduit walls on the side which is structurally the weakest - to the E. In the case of the 1968 event a new effusive fissure formed on the upper E flank while the regular persistent activity at the NE Crater continued without being sensibly affected by the activity of the fissure. This reflected either a very minor increase in the magma supply rate, or the fracturing of the E flank was caused by increased hydrostatic pressure within the conduit; none of these two hypotheses, however, appears to be fully satisfying. Return
to the introduction
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Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology" |
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set up on 14 January 2000, last modified on 19 March 2000
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