Do you plan to visit Etna
in the near future?
Check the weather
forecasts for the Etnean area!
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
A tumulus collapses
of photos taken with a 50 mm lens shows the gradual collapse of
a lava tumulus on the late afternoon of 12 May 1999 on the western
slope of Valle del Bove. As large blocks on the tumulus subside
and fall down the sides of the tumulus (see frame #7), lava in
the interior of the tumulus is exposed.
series of photos showing the collapse of the tumulus and transformation
into a large effusive vent. Frame #1 shows slabs of older lava
on the tumulus surface before the collapse while the other frames
show successive stages of collapse; frame #5 shows huge incandescent
blocks rolling down the flank of the tumulus.
more photos of 12 May 1999, see the bottom of this page.
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
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
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.
May 1999 photos
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
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
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.
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.
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.
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.
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).
Page set up on 26 May 1999