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Effusive vents and lava tumulus on the western slope of Valle del Bove on 12 May 1999. See photos below for more detail.

13 May 1999

During the past 12 days (since 30 April), little significant change has affected the effusive activity initiated on 4 February at the SE base of the SE Cone. Lava is still flowing from the area of the 4 February fissure through a lava tube and appears at the surface only at about 2600 m elevation on the western wall of Valle del Bove. Active lava fronts do not extend below 2000 m elevation.
On 12 May, Boris Behncke (IGGUC) and Giuseppe Scarpinati (Italian delegate of the Association Volcanologique Europèenne, Paris) visited the summit area of Etna, including Bocca Nuova and SE Crater, and entered the Valle del Bove to get a close view of the active effusive vents. The summit craters were exceptionally quiet, apart from near continuous but essentially passive emissions of light brown ash from the northwestern vent of Bocca Nuova. This activity, which was most likely caused by internal collapse related to the slow sinking of the magma column in the conduit, was entirely noiseless and ash plumes barely rose above the crater rim. A deposit several centimeters in thickness covered the southern, southeastern and eastern sides of the main summit cone.
After descending from the main summit cone to the saddle which separates it from the SE Cone, Behncke climbed to the summit of the SE Cone whose western flank was found to be extraordinarily steep (up to 50 degrees, much more than is common on volcanic cones) and climbing was only possible because it consisted of agglutinated bombs. The crater was practically gas-free and its interior was perfectly visible, so that it could be stated that its floor had collapsed and the conduit was no longer open. There was, however, some gas and vapor emission from the upper part of the fracture which had split the southeastern flank of the cone on 4 February.
There was no visible activity anywhere above the Valle del Bove rim in the lava field of the eruption initiated on 4 February, the only surface flows appearing about 200-250 m below the original eruptive fissure, in Valle del Bove. Behncke and Scarpinati reached the main effusive vent area - there is now only one active lava tube through which lava is transported to the effusive vents - which did not appear to have changed significantly since Behncke's previous visit on 30 April. At the time of their arrival, lava was issuing from two ephemeral vents (ephemeral, because such vents frequently change in configuration and location) on the northern and eastern sides of a large "tumulus". Tumuli of this kind form by the continuous pressure of lava pushing from below towards the surface, and by lava oozing upwards through cracks in older lavas at the surface (see the fourth of the photos of the 14 April visit to the active lava flows). The tumulus was about 10-15 m across and consisted of uplifted, tilted, craggy blocks of older lava and minor volumes of more recently extruded lava, much of the latter being very smooth-surfaced pahoehoe. The northern effusive vent fed a well-channelized flow about 0.3-0.4 m wide while from the vent on the eastern side of the tumulus lava was squeezed out like toothpaste, which then descended the steep eastern face of the tumulus.
During investigation of the tumulus and of the effusive vents, Behncke and Scarpinati continuously heard ominous cracking and knocking sounds from below, and small rockfalls from the sides of the tumulus were frequently observed. At the same time, the rocks at the surface of the tumulus were seen to be slowly fracturing. It was evident that magma was forcefully pushing from below, causing uplift and lateral spreading of the tumulus, and lava was seen rising slowly within cracks between the blocks at the surface. After about 15 minutes of observation, Behncke and Scarpinati left the unstable tumulus area and continued their observation from a point some 15 m upslope. For another 20-30 minutes, the tumulus gradually extended in all directions, while portions of its southern and southeastern margin appeared to be subsiding, as magma was instruded into it at a depth of a few meters. Fracturing was also observed on the slope above the tumulus, indicating that a larger volume of magma was arriving at the end of the feeder tube and nearing the surface.
As time passed, the large blocks of older lava showed more and more cracks and began to fall to pieces while ever larger rockfalls and collapses occurred on the flanks of the tumulus. After this, the entire area of the tumulus became highly mobile, and its eastern and southeastern sides which were precariously perched above the steep Valle del Bove slope began to slide downhill, producing spectacular cascades of more or less incandescent blocks and exposing the fluid, highly incandescent heart of the tumulus. The most dramatic phase which lasted no longer than 5 minutes saw the virtual unfolding of the whole structure as older blocks of lava were slipping from the overwhelmingly mobile lava now exposed at the surface, and crashed down the steep slope into the Valle del Bove. Where the two observers had walked and photographed only half an hour before, an incandescent chasm 15 m wide and 5-6 m deep opened, and lava slowly flowed from draining lava tubes which had until shortly before fed the two ephemeral vents seen at the beginning of the visit. Fresh lava welled up at the western end of the collapse depression, rapidly filling it and spilling over its southeastern rim, causing further spectacular rockfalls. Some rocks at least 20 cubic meters in volume were seen to fall, with fresh incandescent lava being attached to them like glue. The overflowing lava appeared to be more voluminous than that which had previously issued from the two ephemeral vents, and it is probable that the arrival of this larger volume had caused the spectacular collapse of the tumulus.
The previously active flows, cut off from their supply, soon stagnated, and small lava tubes with still-incandescent walls which had fed the effusive vents became visible; unfortunately it was difficult to approach these places due to the enormous heat radiated from the area. Fresh lava spilled down in a southeasterly direction, forming two branches which travelled 100-150 m in about 30 minutes.
The destruction of the tumulus, spectacular though it was, was not a geologically significant event and does not mean that the physical conditions of the present effusive activity have changed, although it might be the result of a minor, temporary increase in the effusion rate. It has to be remembered that the effusive vents at the Valle del Bove slope are no true eruptive vents, but they are instead the point where the lava leaves a feeder tube, about 1 km away from the area of the original eruptive fissure.
A more significant change observed during the 12 May visit is the obstruction of the SE Crater which had until recently served as a degassing valve for the magma erupting from its base. It is possible, though, that degassing is equally occurring from Bocca Nuova, which is clearly connected to the present effusive activity, as evidenced by the cessation of magmatic activity in this crater immediately after the beginning of the effusive activity on 4 February.
This effusive activity which continues since 98 days without interruption has diminished significantly during the past two months, but it may continue for weeks to come.

12 May 1999 photos
Left: The SE Cone seen from south, at a distance of about 600 m. Note the fissure that ruptured the southeastern flank of the cone on 4 February, steaming in its upper part. Right: The SE Cone seen from the southeastern rim of the former Central Crater, its summit being at about the same elevation (3250 m).
Left: A view of the SE Cone from the ESE side of the former Central Crater. Note meter-sized blocks ejected during the cone's latest eruptive episodes in January-February 1999 which have accumulated at its base. Height of the cone from base to summit is about 60 m. Right: Summit crater of the SE Cone as viewed from its southwestern rim. The notch in the northern crater rim through which lava flowed onto the north flank during the latest eruptive episodes is visible at left; the white dot in visible at distance through the notch is the Etna Observatory.
Left: View into the SE Crater from its WSW rim, showing only a little white vapor plume rising from the crater floor where intense degassing had occurred until about one week earlier. Right: View from the summit of the SE Cone of the area of the ongoing effusive activity. Flows erupted since 4 February 1999 are outlined by a red line, the 4 February eruptive fissure is indicated by a yellow broken line, the asterisk indicates the location of the hornitos formed at the upper end of the eruptive fissure that became active after the end of the paroxysmal activity of 4 February. The broken line in brown color indicates the rim of Valle del Bove as it was before 4 February, highlighting the large volume of lava accumulated on the Valle del Bove rim.
Left: Looking down into the Valle del Bove from the southern margin of the new lava flow-field on the Valle del Bove rim. Dark tongues of lava erupted since 4 February have arrived at the base of the Serra Giannicola (the craggy crest at right) and come close to the Monti Centenari (visible in upper center of photograph). Right: Effusive vent on western Valle del Bove slope from which lava is squeezed out like toothpaste.
Left: A closer look onto the "toothpaste" vent, showing detail of the peculiar surface surrounding this vent (right bottom of photo), and southern part of Valle del Bove in the distance. Right: The lava flow channel at the other effusive vent seen in activity shortly before the tumulus collapsed. The width of the channel is about 0.5 m.
Left: About half an hour before, this was a large lava tumulus with a highly complex surface and the two effusive vents shown in the two previous photos. The collapse has transformed it into a depression from whose walls lava is slowly oozing back onto its floor while fresh lava is welling up from the end of the main lava tube at the lower left. Right: Close-up view of the new effusive vent formed in the core of the collapsed tumulus (flow is to the right).

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