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Mount
Etna is a highly complex volcano whose history has been characterized
by the construction of numerous volcanic edifices at shifting locations
and repeated catastrophic collapse of these structures. The recent and
currently active volcanic edifice (Recent Mongibello) has a summit cone
complex and hundreds of cinder and spatter cones, pit craters, and spatter
ramparts along eruptive fissures scattered over its flanks. The number
of individual flank eruptive centers is more than 300, with new such structures
added in almost each flank eruption, most recently in the winter of 2002-2003.
Summit activity occurs at four craters: the Voragine and the Bocca Nuova,
which lie in the area of the former Central Crater, and the Northeast
(NE) and Southeast (SE) Craters. The latter have built sizeable cones
- referred to as the Northeast and Southeast Crater cones - leaning against
the bulky main summit cone that hosts the Voragine and the Bocca Nuova.
Among the most remarkable features of Etna's eruptive behavior is the
capability of the volcano to erupt from many places from its flanks, while
activity in the summit area is essentially concentrated at the four summit
craters, which remain in stable positions since decades and are frequently
active.
In the following,
first some remarks are made about flank vents before a longer section
deals with the summit craters and their evolution and behavior. Eruptive
histories of these craters as well as flank eruptions are treated in the
chronologic section of the Etna pages.
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Photo
mosaic of Etna's western flank as seen from the summit of Monte
Minardo, a large prehistoric flank cone (1304 m high). The upper
western flank of the volcano is seen in the background; only a few
months after these photographs were taken (17 May 1999), lava flows
emitted from the Bocca Nuova summit crater streamed down this flank
to the tree limit and remain strongly visible today. Numerous cones
from flank eruptions of different ages lie in the middle ground.
Light-colored lava flow in center left is of the 1763 eruption with
its source cone Monte Nuovo showing a slight reddish hue. The conspicuous
cone at left, and partly surrounded by the 1763 lava flow is Monte
Ruvolo. Behind it to the right is Monte Lepre. Dark flow at upper
right side of 1763 flow is of the January-March 1974 eruption from
the Monti de Fiore twin cones, behind which rises the cone of Monte
Rosso. In center right part of the image (immediately behind the
yellowish spot), lies the larger of the two Monti Tre Frati, with
the smaller cone lying behind it. The conspituous cone at the left
skyline is Monte Maletto |
Flank
eruptive centers. The
size of the flank cones varies significantly, as illustrated in the photos
below, depending on the explosivity and duration of an eruption. Eruptions
farther downslope tend to build larger and more isolated cones (such as
the conspicuous Monte Rossi near Nicolosi, formed in the 1669 eruption)
while rows or chains of cones, often of small dimensions, are constructed
at eruptive sites at higher locations. One of the most impressive chains
of eruptive cones is Monti Silvestri at the 1892 eruption fissure, close
to the tourist facilities near Rifugio Sapienza, on the southern flank of
Etna.
Many
of Etna's flank cones have horseshoe-shaped craters, which are commonly
open on the downslope side. While such a morphology can be due to the
preferential accumulation of ejected pyroclastics on the leeward side
- which will occur primarily during a relatively short-lived eruption
that occurs during strong wind - most of the horseshoe-shaped cones owe
their morphology to the outflow of lava from their vents on the downslope
side. The more or less continuous outflow of lava prevents the accumulation
of pyroclastics to build a high rim on this side. However, quite a significant
proportion of the flank cones are symmetrical and not open on one side;
in these cases lava effusion occurred from vents located on the lower
flanks or at the base of the cones, as has also been observed at some
of the cones formed during the 2001 and 2002-2003 flank eruptions. The
large cones of Monte Minardo and Monte Ruvolo on the western flank and
Monte Ilice and Monte Gorna on the southeastern flank are examples of
this type. But even most of the more regularly shaped flank cones have
several summit vents, which has led to the formation of more or less deep
notches in the crater rims.
Etna's
flank cones are not distributed regularly over the flanks of the volcano,
but they tend to be clustered in certain areas pertaining to more or less
defined rift zones (see map). The most conspicuous
rift zone lies on the northeastern flank and is marked by the presence
of numerous linear crater groups and gaping fractures. A less defined
rift zone lies on the south flank between 2600 and 1600 m elevation; below
1600 m, this rift zone extends into a fan-shaped sector (on the lower
southeastern flank of the mountain) with numerous large and isolated cones.
This latter sector is the most densely populated areas on Etna, and the
presence of several historical eruptive centers amidst this growing urban
agglomerate is of great significance for volcanic hazard considerations
at Etna.
More
detail and a list of the more conspicuous flank eruptive centers is given
on a new page created in December 2003.
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Monte
Barca, a flank cone near Bronte
Lying south
of the town of Bronte on Etna's western flank, Monte Barca is a
remote flank eruptive center of the volcano, built on early Pleistocene
marine sediments. The cone whose shape is reminiscent of a small
boat is prehistoric (or, if it formed during historic times, its
eruption and growth were not observed due to its remote location);
however, its good preservation indicates that it is relatively young.
The photo was taken on 2 March 1998. Note the cemetery of Bronte
in the foreground |
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Monte
Moio, Etna's most remote flank cone
View from
the Passopisciaro-Solicchiata area on Etna's northern flank towards
the village of Moio and the eccentric cone Monte Moio standing behind
it, in February 1998. Monte Moio is composed of at least two overlapping
cones with three craters. Although its age is not known precisely,
the youthful shape of the cone indicates that it formed during the
last few thousand years. A voluminous lava flow that extends almost
20 km eastwards to the Ionian Sea is tentatively attributed to the
Monte Moio eruption, but more precise correlation is necessary.
Although often described as a volcano on its own, it is just one
of hundreds of Etnean flank cones, even though it is one of the
most remote of them. It lies on the northern side of the Alcantara
valley and is one of the few spots of Etna that belongs to the province
of Messina (the bulk of Etna lies in the province of Catania) |
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Monti
Rossi, the cone of the 1669 eruption
Panoramic
view of Monti Rossi, the large cone formed at the main eruptive
vent of the 1669 eruption on Etna's south flank, seen from about
1 km north. The cone is now a public park. Buildings of Nicolosi,
including the complex of Hotel Gemmellaro are extending up the flank
of Etna on the left side of Monti Rossi. The 1669 eruption devastated
the town of Nicolosi completely, covering the site with tephra deposits
several meters thick. A similar eruption today would cause destruction
beyond imagination. Photo taken in May 2000 |
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Monti
Silvestri, Monti Calcarazzi and Montagnola
The middle
southern flank of Etna is has some classic examples of Etnean fissure
eruptions with eruptive cones aligned on it, due to frequent eruptions
on what is considered the "south rift zone" of the volcano.
Montagnola, the peak forming the skyline, is a large pyroclastic
cone formed during the summer 1763 eruption; its lava flows formed
a peculiar ridge visible in the upperr left of the photo. Three
years later, another eruption built the Monti Calcarazzi crater
row whose uppermost cones are visible in the right center. In 1892,
a six-months long eruption led to the formation of yet another crater
row, the Monti Silvestri, from the largest of which the photo was
taken on 24 May 1998. Another 1892 crater is visible in the left
center. Nonwithstanding the frequent eruptions in this area (another
one occurred in 2001), the largest complex of tourist facilities
(including the Rifugio Sapienza and the base station of the cable
car) has developed immediately to the west of the Monti Silvestri |
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Monti
Di Fiore, cones of the 1974 eruption
The western
flank of Etna is another area of more pronounced volcanism, although
eruptions occur much less frequently than on the southern and northeastern
flanks and in the Valle del Bove. The most recent eruption prior
to 1974 occurred in 1843, but the vents lay further north. An eruption
in February 1763 (a few months before the Montagnola eruption) built
two cones, Monte Nuovo and (possibly) Monte Mezzaluna. In January-March
1974, a similar eruption built two new cones, named Monti Di Fiore.
The larger of these cones, lying at about 1650 m elevation, is seen
from the east in this photo taken by Marco Fulle in October 1998 |
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Eruptive
fissure of March 1985 at Piccolo Rifugio
Contrasting
with the sizeable cones in the preceding images, the March 1985
eruption center did not develop any significant cones. Only a line
of "hornitos" or driblet cones grew on the eruptive fissure, reaching
about 3 m maximum height. The fissure cut right through the building
of the "Piccolo Rifugio", already damaged by a fracture formed two
years earlier, and lava issued from below the building. The 1985
eruption site was at about 2300 m elevation on the south flank of
Etna and was completely buried by lavas of the 2001 and 2002-2003
eruptions. Photo taken in September 1989 |
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Eruption
vents and pahoehoe lava of the March 1985 eruption
Close-up view
of the March 1985 eruption site (taken in September 1989), looking
west. The eruptive vents developed several spiny hornitos and small
lava shields. Note the abundant presence of pahoehoe lava at the
vents. The main lava flow field is aa lava. During the eruption
which lasted from 8 March until 13 July 1985, lava flowed on top
of the much larger 1983 lava field and initially appeared to threaten
the hotels and tourist facilities around Rifugio Sapienza, severely
threatened and partially destroyed in 1983 |
The
summit craters. Etna's
summit is occupied by a large cone complex including the original Central
Crater (now nearly substituted by the two craters Bocca Nuova and Voragine),
the NE Crater and the SE Crater. The latter two craters are relatively
recent features: the NE Crater first formed in 1911 and the SE Crater
was born in 1971. While previously being considered "subterminal"
vents by many authors, these two craters have become very large and high
structures in recent years, the summit of the NE crater being the highest
point of Etna between 1978 and 1986 and possibly again since 1996, so
they are now rather considered genuine summit craters.
The Central
Crater, the Voragine and Bocca Nuova. Before 1911, the central summit
cone was a broad structure 250 m high whose top was truncated by the 500
m-diameter Central Crater. Historical records indicate that this cone
formed within about one century following the collapse of a preceding
summit cone during the catastrophic 1669 eruption. Lava overflows from
the Central Crater or from vents on the flanks of the central summit cone
occurred frequently until 1869, and an eruption from the Central Crater
in July 1787 was among the most violent documented eruptions of Etna in
historical times, generating lava fountains up to 3000 m high and flows
that extended up to 5 km to SW and W.
During
the late 19th and early 20th centuries, the Central Crater was a single
deep pit with near vertical walls, with its depth varying between 150
and more than 200 m. Between 1916 and 1922 more or less continuous eruptive
activity occurred at a cluster of vents in the NE part of the crater,
building a large cone which became known locally as the "cono avventizio".
The central portion of the crater was gradually filled with pyroclastics
and lava flows, which formed a flat terrace. In mid-1922 this period of
crater filling ended and a new collapse pit formed in the center of the
terrace; collapse also removed nearly all of the "cono avventizio".
A new period of rapid crater filling began in 1939, the main focus of
eruptive activity once more lying at the site of the former "cono
avventizio". Intermittent Strombolian and effusive activity built
new cones and the central pit was filled with lava, so that no trace of
it was visible by mid-1940. Renewed activity in the first half of 1942
added more lava and pyroclastics before a violent paroxysmal eruption
on 5 July 1942 completely overwhelmed the topography of the Central Crater.
A 30 m thick sheet of lava covered nearly all of the crater floor and
obliterated any trace of the pyroclastic cone in its NE part. These conditions
are clearly shown in an impressive aerial photo taken in 1943 which was
published in Chester et al. (1985). A small collapse pit formed in the
northeastern part of the Central Crater platform in late-October 1945,
which became soon known as the Voragine, or "the Chasm".
Vigorous
eruptions in the 1950's and early 1960's built large cones in the central
and southern parts of the summit crater platform while the Voragine collapse
pit in the NE part of the crater gradually enlarged. The culminating event,
in 1964, was the growth of a two large cones - one around the Voragine,
and another around a new vent (the 1964 crater) on its southern side-
to several tens of meters above the summit platform. This cone raised
Etna's summit to 3330 m, an elevation only to be surpassed by the NE Crater
in the late 1970's. The Central Crater was filled completely during this
period, and voluminous lava overflows in 1956, 1961 and most notably in
1964 extended up to 4 km towards N, NE, NW, W, SSW and SE.
In
1968, a small pit - the Bocca Nuova, the "new mouth" - opened on the W
flank of the large cone formed in 1964 around the Voragine. While its
initial diameter was only 8 m, collapse of its walls in the almost 30
years since have enlarged this pit to a gaping crater some 350 m across
(as of late 1997). Most of the cones formed between 1955 and 1964 have
been engulfed by the collapse of the walls of Boca Nuova, reducing the
height of what remains of the main 1964 cone to about 3310 m elevation.
Since
the growth of the last major structure on the summit crater platform in
1964, collapse of the Bocca Nuova and the Voragine have alternated with
the partial infilling of both pits. In several cases magma rose within
the Voragine pit almost to its lip (such as in the spring of 1980), and
small cones and intracrater lava fields grew within the Bocca Nuova at
various times. During the summer of 1997, a period of rapid filling of
the Bocca Nuova and the Voragine began that continued, with interruptions,
until late summer 1998. Meanwhile the septum separating both pits (the
so-called "diaframma") partially collapsed, mostly during the
22 July 1998 eruption from the Voragine. Further collapse occurred there
during another strong Voragine eruption on 4 September 1999. The latest
chapter in the saga of these two craters is the nearly complete filling
of the Bocca Nuova in October-November 1999, which led to the first overflows
of lava from this crater onto the external slopes of Etna, and the burial
of the "diaframma" under a large pyroclastic cone. Since the
end of that eruption, two major pits are present within the Bocca Nuova,
in the NW and SE of the crater. Activity within these pits in late-2000
and early-2001 repeated the history of the Bocca Nuova, with periods of
filling alternating with collapse.
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The
Central Crater and the NE Crater in the 1920's
Spectacular
aerial view of Etna's summit, taken from the northeast at an unknown
date, probably during the 1920's when a small cone grew in the initial
collapse pit of the NE Crater (see also below) and lava spilled
over its rim. The vapor-filled, 450 m-wide Central Crater is visible
behind the plume from the NE Crater. The photo was published on
postcards which are still for sale |
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The
Central Crater from the air in 1934
Aerial view
from the northwest of Etna's Central Crater, taken possibly in 1934
(this is one of a series of photos published in the December 1934
issue of "Le vie d'Italia"). The relatively flat crater
floor lies a few tens of meters below the rim and is perforated
in its NE part by a fuming pit, site of all activity in the crater
during the 1920's to 1940's. The Voragine formed in 1945 in the
same area. The southern wall of the Valle del Bove is visible in
the background |
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Intracrater
cones growing within the Central Crater, 1940
Steaming twin
cones growing in the northeastern part of the Central Crater in
1940. Lava is issuing from vents at the base of the cones at right.
The filling of Central Crater was proceeding rapidly in those years
due to near continuous mild Strombolian and effusive activity, interrupted
at times by more vigorous activity. By 1943, the Central Crater
was almost completely filled with lava and pyroclastics. Photo is
from a postcard no longer for sale |
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Initial
stage of the Voragine, 1947
A small collapse
pit formed in the northeastern part of the Central Crater in October
1945; this pit was the site of frequent activity during the following
decades, and gradually enlarged by collapse and/or explosive activity.
It is now known as the "Voragine" or "Chasm",
but many locals name it "Centrale". Note the flat crater
floor around the new pit, and the notch in the northeastern crater
rim (the NE Crater stood below that notch but was still too small
to be visible from the photographer's position at the time this
photo was taken by G. Cumin of the former Istituto di Vulcanologia
of Catania University) |
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The
Voragine in January 1999
View across
the Voragine towards northeast, from the crest of the "diaframma"
(the septum separating the Voragine from the Bocca Nuova), on 21
January 1999, with the NE Crater in the left background. The vigorous
activity of 1998 left the Voragine almost brimful with pyroclastics
and lava. Some of the craters active in 1998 can be seen in the
center and left parts of the photo. Activity declined in the autumn
of 1998, permitting access to the crater, and the excursion to the
"diaframma" rendered a sense of awe, mixed with the consciousness
that this area had been one of the least accessible areas on Earth
only six months before |
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The
Bocca Nuova in January 1999
Eruption of
a cone in the northwestern part of the Bocca Nuova seen from the
"diaframma" on the afternoon of 21 January 1999. The depth
of the crater has decreased to about 60 m, compared to 150 m in
1995 when the current period of filling began. Initially a hole
only about 8 m wide, the Bocca Nuova grew by collapse to a vast
pit at least 350 m in diameter. The maximum depth was more than
700 m, observed in the mid-1970's |
The
NE and SE Craters.
Both the NE and the SE Craters started in a very similar manner as collapse
pits on the flanks of the main summit cone. In the case of the latter,
the initial stages of its formation are well documented. The SE Crater
started as a degassing pit during the late stages of the peculiar 1971
eruption and remained inactive thereafter until spring 1978. Since then,
it has been the site of six major eruptions, some of which were closely
related to eruptions on the upper flanks of Etna. The most significant
events during the brief history of the SE Crater were the episodes of
vigorous lava fountaining preceding and following the 1989 flank eruption,
culminating in one of the most violent eruptions of Etna in recent decades,
on 5 January 1990. Between late 1996 and July 1998, the crater showed
continuous Strombolian and effusive activity characterized by relatively
low eruption rates; from September 1998 until February 1999 it was again
the site of 22 powerful eruptive episodes that built the cone to an unprecedented
height of about 3260 m. Between 26 January and 29 August 2000, the SE
Crater was the site of 66 (sixty-six) violent eruptive episodes, and sixteen
further paroxysms occurred between 9 May and 17 July 2001. These events
led to further growth of the SE Crater cone, which by early 2001 had reached
3300 m elevation.
The
growth of the NE Crater was interrupted by repeated collapse throughout
the 1950's, and it was only during that decade that a large cone began
to develop within the collapse structure. This was accompanied by voluminous
outflows of lava which created a vast lava apron around the base of the
cone except on its S side, and lava extended far down the NW, N and NE
flanks. Vigorous growth of the cone and the surrounding lava field occurred
again from 1966 until 1971, a period characterized by what became famous
as "persistent activity" with continuous Strombolian bursts and slow lava
effusion, mostly from vents on the flanks or at the base of the pyroclastic
cone. The largest of these formed in 1970 and was nicknamed "Nordestino"
(little Northeast) by the local mountain guides.
In
another eruptive period lasting from 1974 until early 1977, similar persistent
activity was accompanied by effusion from boccas farther downslope on
the N flank, but still related to NE Crater. In the summer of 1977, the
crater changed its behavior, producing a series of short-lived but violent
eruptions that ended in late March 1978. These were characterized by high
lava fountains and voluminous lava effusion from the main vent with flows
extending as far as 7 km downslope; the cone was eventually breached on
its NW side during one of these episodes. Similar episodes of activity
occurred in September 1980 and February 1981. The most violent eruption
of NE Crater so far occurred in September 1986 and ended with the partial
destruction of the cone after which there was little eruptive activity
for the next nine years. Collapse of the internal parts of the crater
occurred frequently during and after the large 1991-1993 eruption in Valle
del Bove.
In
late 1995, a new series of paroxysmal eruptions with high lava fountains
started, followed by Strombolian and effusive activity in the summer of
1996. This activity and ensuing collapse completely altered the morphology
of the crater. Another paroxysmal eruption on 27 March 1998 and mild Strombolian
activity deep in the collapse pit in the summer of 1998 did not result
in further significant morphological changes, but the powerful Voragine
eruption on 22 July 1998 caused the collapse of the southern flank of
the NE Crater cone and deposited several meters of pyroclastics on its
summit. A new period of Strombolian activity within the central pit of
the Northeast Crater between June and September 2002 led to the temporary
filling of the pit to about 50 m below its rim, but subsequent collapse
(related to the flank eruption in October-November 2002 on the Northeast
Rift) enlarged the pit to the west rim of the crater and re-transformed
it into a bottomless abyss.
The
Southeast Crater has experienced a story very similar to that of the NE
Crater which has already been summarized above. Its life began during
the latest stage of the fascinating 1971 eruption when eruptive fractures
opened across the upper western and northwestern part of the Valle del
Bove, giving lava flows that caused serious damage to forests, fruit gardens,
and isolated buildings uncomfortably close to the village of Fornazzo.
While lava flowed quietly from the lowermost vents, outside the northern
rim of Valle del Bove, a degassing pit formed on the southeastern base
of the main summit cone, erupting steam and ash. This pit, later named
Southeast Crater, remained silent for seven years after the 1971 eruption
and then became the site of frequent eruptions that continue to the present
day.
Some
of the SE Crater eruptions were particularly violent, like those of early
August 1979 and September 1989-February 1990 and especially those seen
between late 1998 and mid-2001, and consisted of brief episodes of high
lava fountaining accompanied by voluminous outflow of lava. Other eruptions,
like one in 1984 and more recently, in 1997-1998, were characterized by
mild Strombolian activity and slow lava effusion; these eruptions lasted
months to years and were type examples of the "persistent summit
activity" observed in the 1950's to 1970's at NE Crater.
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Early
cone growth at the NE Crater, 1923
View from
the northeastern rim of the Central Crater towards a small cone
in mild Strombolian activity, growing in the collapse pit of the
NE Crater, 15 May 1923. Lava is flowing from effusive vents at the
northern base of the cone towards north (the steaming area in the
left background) and east. The early history of the NE Crater is
characterized by the repeated growth and succesive collapse of small
pyroclastic cones, accompanied by lava overflows, but no permanent
cone formed until the mid 1950's |
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The
NE Crater in late 1998
A view across
the eastern part of the Voragine (in the foreground) towards the
cone of the NE Crater on 1 November 1998. The summit of that cone
is now one of the highest points on the volcano. Note the collapsed
southern flank of the cone in left part of the photo, an effect
of the 22 July 1998 eruption from the Voragine. Photo was taken
by Carmelo Monaco |
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Birth
of the SE Crater, 1971
Steam and
ash emission from a degassing pit at the southeast base of the main
summit cone (at left) in mid-May 1971, during the latest stage of
the spectacular eruption from numerous vents near the summit and
on the eastern-northeastern flanks that began on 5 April 1971. This
pit evidently served as a "pressure valve" for the extensive
fissure system that propagated from here to the northeast across
Valle del Bove, with degassed lava issuing from the lowermost vents.
This pit remained inactive for seven years after the end of the
1971 eruption, but in 1978 it reawakened to become known as the
Southeast Crater, site of most of Etna's summit activity since then.
This photo of Carmelo Sturiale, probably taken in late May 1971
from a point about 1 km south of the newly formed crater, shows
lava from the 1964 summit eruption in the foreground |
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The
SE Crater after the 1971 eruption
Another shot
by Carmelo Sturiale, taken in August 1971 when activity at the newly
formed SE Crater and from the lower eruptive fissures had subsided.
This photo shows neatly the location of the new crater on the lower
SE flank of the main summit cone as seen from the "Belvedere"
area on the rim of Valle del Bove. The front of one of the lava
flows erupted in July 1964 from the Central Crater is visible in
the foreground |
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The
growth of the SE Crater - 1978 (?)
Ash emission
and lava flows from the SE Crater are visible in this photo probably
taken in 1978 when a series of eruptions occurred at this crater
and at fissures extending radially from it into various sectors
of Valle del Bove. While the initial collapse structure has enlarged
to engulf a major portion of the SE slope of the main summit cone
(in the background), a low mound is building in its eastern part.
Photo scanned from a postcard |
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The
growth of the SE Crater - February 1998
For twenty
months, between November 1996 and July 1998, mild Strombolian activity
built a small cone within the crater formed during the early 1990
activity of the SE Crater, and lava issuing from vents on the flanks
and at the base of this intracrater cone led to the gradual filling
of the 1990 crater. In this photo, taken on 18 February 1998, the
intracrater cone (at left) has grown tens of meters above the 1990
crater rim, and lava is overflowing onto the southern flank of the
SE Crater cone |
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The
SE Crater in late 1998
View of the
cone of the SE Crater from the southeast rim of the former Central
Crater on 1 November 1998, photographed by Carmelo Monaco. The cone
has grown with striking velocity during the preceding six weeks
and is standing only about 20 m below the observation point, at
about 3230 m elevation, fifty m higher than it had been in July
1997. A part of the pre-1997 crater rim is still visible at the
right side of the cone's steep upper part. After this photo was
taken, growth of the cone continued vigorously into early 1999 |
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The
SE Crater in early 1999
Same view
as in previous photo, but two and a half months later, 21 January
1999. Ten eruptive episodes have caused considerable growth of the
cone, both laterally and vertically. The huge blocks (ejected in
January 1990 from the SE Crater) in the saddle between the SE Cone
and the main summit cone that served as landmarks for a long time
have been buried by new material, and it is no longer possible to
look down into the crater of the growing cone because it is nearly
as high as the viewing point |
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