La Morfologia
dell'Etna
I crateri e le aree sommitali (di Boris Behncke, traduzione di Andrea Fiore)
Main summit cone (left) and Southeast Crater,
with small black 1971 cone in foreground (photographed from south in August
1991)
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 as well as spatter ramparts along eruptive fissures scattered over its
flanks. The number of individual flank eruptive centers is 250-300, with new
such structures added in almost each flank eruption, most recently in the summer
of 2001.
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.
Composite
photographs of the view of Etna's western flank as seen from the summit
of Monte Minardo, a large prehistoric flank cone (1304 m high). Left is
the view to east-northeast, right the view to east-southeast. Note the
numerous cones from flank eruptions in the middle ground, and the enormous
main volcanic edifice in the background. Light-colored lava flow in left
image is of the 1763 eruption, the conspicuous cone at left is Monte Ruvolo.
Dark flow at upper right side of 1763 flow is of the January-March 1974
eruption from the Monti de Fiore twin cones. Photos were taken on 17 May
1999.
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 single, 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.
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.
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.
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).
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
cause destruction beyond imagination. Photo mosaic taken on 23 September
1989.
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, 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.
Monti
De 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 De 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.
Eruptive
apparatus of March 1985 at Piccolo Rifugio
Contrasting with the
sizeable cone on the preceding image, 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 lies at about 2300 m elevation on the
south flank of Etna.
Eruption
vents and pahoehoe lava of the March 1985 eruption
Close-up view of the
March 1985 eruption site, 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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
