Italy's Volcanoes: The Cradle of Volcanology

Etna Decade Volcano, Italy
Eruption update:
20 May 1999

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19 May 1999

The narrow path leading to the ephemeral effusive vents across recent lavas on the steep Valle del Bove slope. Photo taken on 19 May 1999.

20 May 1999

WARNING: Access to the summit craters can be DANGEROUS. Weather conditions are often unstable, and there is always a risk of sudden explosive activity from the summit craters. As effusive activity from the fissure near the southeastern base of the SE Cone is gradually decreasing, renewed activity may be expected from the craters soon after the cessation of the lava outflow. Any person who enters the area beyond the Torre del Filosofo mountain hut (2900 m elevation) goes at his/her own risk and is not covered by any insurance in case a rescue operation (e.g., with helicopters) is necessary. The same is true for those who try to get close to the still-active effusive vents on the Valle del Bove slope.

To the great delight of both mountain guides and tourists, effusive activity has resumed after about one month in an easily accessible area, about 100 m downslope from the hornitos which formed during the first weeks of the effusive activity initiated on 4 February.
A luminous spot was sighted on Saturday (15 May) evening by Giuseppe Scarpinati (Italian delegate of the Association Volcanologique Europèenne, Paris) in the area of the hornitos when he observed the area with binoculars from his home in Acireale. Earlier that day, Harry Pinkerton (Environmental Sciences Division, University of Lancaster) and Sonia Calvari (Istituto Internazionale di Vulcanologia, Catania) had worked in the same area and noted no active lava. The next day (16 May), lava was extruded from three small vents at low effusion rate. The rate of lava emission increased during the following two days, and a larger effusive vent became active about 20 m upslope from the original three vents, feeding sluggish flows that advanced across older lavas in the northern part of the flow-field formed since 4 February. It was noted that the effusion rate at the ephemeral vent on the western slope of the Valle del Bove apparently decreased somewhat in response to the new effusive activity near the original eruptive fissure of 4 February.
On 19 May, Boris Behncke, Antonella Lentini, Mariangela Porravecchio (IGGUC), Valentina Giambarresi (Catania University), Massimiliano Rindone of Torino, and two groups from France and the Netherlands visited Etna's summit area and the effusive vents at the base of the SE Cone and on the western slope of the Valle del Bove. A part of this group climbed to the summit of the SE Cone from its southeastern base. Since the previous visit by Behncke on 12 May, no significant changes had occurred at the crater of the SE Cone except that all emissions of vapor from the obstructed crater floor had ceased. The crater floor was found to lie at a depth of about 70 m below the western rim, and the crater walls are vertical in most places. Some large blocks on the southern crater rim have detached from the outer rim and are prone to fall into the crater in the future; however, these blocks are in the same position as one week earlier. Fumarolic activity is occurring from cracks between these blocks and the outer crater rim, and from numerous smaller fissures mostly in the southern crater rim and below the western lip of the crater, as well as from a collapse pit formed on the southeastern crater rim some time after the 4 February paroxysmal episode.
While the group stayed on the summit of the SE Cone, ash emissions occurred from Bocca Nuova and NE Crater, producing dilute plumes that rose about 100-150 m above the respective crater rims. Rumbling sounds coming from the direction of the Voragine or (less likely) NE Crater. Those of the group who had remained in the area of the hornitos at the base of the SE Cone heard the noise as well. No visible emissions were associated with the noise, and no further were heard when everybody had returned from the summit of the SE Cone to the area of the hornitos.
The newly formed effusive vent some 80-100 m below the hornitos produced a lava flow about 0.8 m wide at a distance of several meters from the vent, which curved eastward and then northeastward. The vent lay in a much larger, drained channel about 3 m wide in its central portion, which apparently had been active at the beginning of the effusive activity in this place. The shift of the currently active vent from the head of the larger channel (where lava is coming out of a tube) downslope was evident in the form of a ridge which had been built by a series of crusted-over effusive vents, a feature visible at many effusive vents and one of the dominant processes of lava tube formation (growth by downflow accretion of crust at the effusive vent). The emission of lava was frequently accompanied by strong degassing producing a hissing noise, indicating that the lava here was more gas-rich than the lava issuing about 200 m downslope from the ephemeral vent on the Valle del Bove slope. Some 20 m downslope from this active effusive vent, one of the initial three vents (of 16 May) was still extruding small amounts of bulbuous lava, forming a tumulus. The tumulus grew by lava pushing from below and being squeezed through cracks in the tumulus surface - similar to the so-called "inflation cracks" (an expression introduced recently on the Kilauea updates page of the Hawaiian Volcano Observatory). Inactive "inflation cracks" partly filled with sausage-like, bulbuous lava were present around the active tumulus.

19 May 1999 photos
19 May 1999 19 May 1999
Left: The SE Cone seen from south, seen from Torre del Filosofo at a distance of about 1 km. Part of the people in the foreground are the group who is mentioned in the text.
Right: View of the bottom of the SE Crater from southwestern rim, showing complete absence of gas emission.
19 May 1999   19 May 1999
19 May 1999   19 May 1999
Left: View towards the newly active ephemeral vent. Arrow points to the lava tube above this vent (the area of the lava tube has been contrast-enhanced for better visibility.
Right: View from the western rim of Valle del Bove onto the valley floor (compare to similar 12 May image). Click here for an annotated version of the enlarged image.
19 May 1999   19 May 1999
Left: Eastern side of the tumulus collapsed on 12 May. This narrow (about 0.3 m) lava flow has built a rampart instead of flowing in a channel.
Right: View of vent 1 on the northern side of the collapsed tumulus. Emptied flow channel is visible in lower part of the photo while the tube which fed the vent is visible at upper left.
19 May 1999   19 May 1999
These two photos show enigmatic lava structures at vent 2 on the eastern side of the collapsed tumulus (arrow in left image points to lense cap about 3 cm in diameter). Two types of lava are visible: brown, relatively dense lava with a ripply texture to which paches of dark, scoriaceous, fresh-looking lava is attached. Brown lava may have been subjected to heat as newer lava flowed across it, and the surface texture may be a result of shearing. Similar structures can be found on the sides of lava channels.
19 May 1999   19 May 1999
Left: This is what remains of vent 2 (the one which extruded lava like toothpaste on 12 May, shortly before the tumulus collapse). Note that part of the lava "bulge" at the vent is still present; evidently a crust formed on it before its inner part drained away.
Right: A closer look at vent 2 and the drained lava channel below it.
19 May 1999   19 May 1999
Left: Side view of vent 2, showing part of the vent's interior and remains of the lava skirting the vent.
Right: View into vent 2 from upper part of the lava channel below it.
19 May 1999   19 May 1999
Left: The emptied cavity below vent 2, and a small mass of lava extruded on the cavity floor just before the vent was entirely cut off from lava supply as the tumulus behind it collapsed.
Right: A view from the western side of the collapsed tumulus into the collapse depression formed on 12 May, showing the complex evolution of the ephemeral effusive vent within it. At an early stage, lava issued from the vent at the bottom of the photo, but the vent then shifted away from its original location, leaving a trace of partially collapsed vent remainders and bulbuous lava masses. Width of the area in the photo is about 3-5 m.
19 May 1999   19 May 1999
Left: Full view of the tumulus collapsed on 12 May, and lava flowing from the depression left by the collapse; view is to the east.
Right: Ribbon of lava flowing from 12 May collapse depression. Note smooth (pahoehoe) lava forming a ribbon about 1 m wide at right.
19 May 1999   19 May 1999
Left: View from eastern side of the 12 May collapse depression, and the active lava coming out of it. Persons in upper part of the photo give scale.
Right: Small secondary effusive vent (marked by the letter "x") giving a minor flow that joins the main flow coming from the collapse depression.

The visit to the ephemeral vent on the Valle del Bove slope was highly instructive regarding the properties of effusive vents and lava channels. Lava was still flowing from two vents on the floor of the depression formed during the collapse of the large tumulus observed on 12 May (see paragraphs and photos of the 13 May update), feeding flows that descended southeastward within the limits of the flow-field emplaced since 4 February. Flow fronts did not extend to the Valle del Bove floor. Four or five well-channelized flows about 1 m wide were moving down the slope; towards their fronts two of these flows were seen to thicken and broaden to 2-3 m height and 5 m width; it was most curious to see one of these flow fronts advance on top of another, well channelized flow, a process that caused frequent collapse of incandescent blocks from the front of the upper flow due to the movement of the lower flow.
The floor of the depression left after the tumulus collapse of 12 May was partially covered with new lava, - most of this being aa with one small lobe of very smooth pahoehoe - and like at the effusive vent near the hornitos, the main effusive vent had shifted, by downslope accretion of lava crust, about 2 m from the original source at the western side of the collapse depression. A second effusive vent some four to five meters downflow produced a small volume of lava that joined the main flow.
Investigation of what remained of the collapsed tumulus revealed that, surprisingly, the two effusive vents that had been observed on 12 May shortly before the tumulus collapsed, had not been completely destroyed by that event. One of these vents, vent 1, lay on the northern side of the tumulus, and while still active, it had fed lava directly into a well-defined channel. The other vent (2) on the eastern side of the tumulus had squeezed out lava much like toothpaste, which then had spilled down the steep eastern face of the tumulus, and no channel was evident in the upper part of this flow. Both vents were instantaneously cut off from lava supply as the tumulus collapsed, and when the area of vent 1 was observed shortly thereafter, lava was still slowly draining from the flow channel. Observation one week later revealed that the channel had completely drained, its depth being about 1.5 m (compared to a width of 0.8-0.9 m). This is much higher depth-width ratio than in most other lava channels on Etna and many other volcanoes, but normally lava channels are not drained as completely as in this case since the supply of lava decreases gradually, allowing some of the lava to remain and "freeze" on the channel floor. The depth of active lava channels has been quite a bit of a mystery until now, and the observations near the collapsed tumulus may provide valuable information about the parameters of lava channels.
Vent 2, one week after the abrupt cessation of its activity, had a completely different aspect. Its mouth was skirted by thin rims of lava which are remainders of the surface "toothpaste" squeezed out of the vent until the tumulus collapse. Different from vent 1 where lava had been fed from a gently sloping tube directly into a channel, vent 2 was a roughly circular hole which was open towards the downslope side, and evolving into a flow channel - this one being throroughly drained as well - that had been hidden under the lava when observed shortly before the tumulus collapse. Like at vent 1, this flow channel was deeper than wide. It has to be noted that the vent 2 channel is on much steeper terrain, but this apparently had no effect on the depth and width of the flow channel. The vent itself, while being about 1.5 m wide at its rim, then widened at depth to a subcircular cavity from whose floor some bulbuous lava had oozed, probably, at some stage of the tumulus collapse. This indicates that there had actually been a kind of a lava pocket at very shallow (that is, about 1 m) depth.
Vent 2 was surrounded by peculiar lava features which are difficult to explain. There was something like a "basal" lava type, of chocolate brown color, and with a very smooth, but ripply surface, onto which patches of black, scoriaceous lava were attached. Much of these features are almost certainly related to the frequent and rapid shifting of the locus of active lava extrusion, and the shearing of actively extruding lava along solidified lava on the vent or flow channel walls. Much analysis is needed to fully understand the genesis of this peculiar association of brown, smooth and black, scoriaceous lava, but it will probably teach much about the dynamics of flow in and near an effusive vent.
To return to a more general perspective, the rate of lava effusion appears to remain fairly stable since at least three weeks; there may have even been a slight increase in the effusion rate, causing the renewed appearance of active lava high up in the flow-field. The effusive activity, 108 days after its beginning, is far from being as vigorous as during its first six weeks, but it does not show any sign of ending soon, so that lava flows may be expected to continue for weeks to come.

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