Mount Etna
From Wikipedia, the free encyclopedia.
| Mount Etna | |
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 Etna's 2002 eruption, photographed from the ISS |
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| Elevation: | 3,325 metres (10,908 feet) (varies) |
| Location: | Sicily, Italy |
| Coordinates: | 37°44′3″ N 15°0′16″ E |
| Type: | Stratovolcano |
| Age of rock: | 500,000 years |
| Last eruption: | 2005 |
- "Etna" redirects here. For other uses, see Etna (disambiguation).
Mount Etna (also known locally as Mongibello or simply 'a muntagna (the mountain) in Sicilian) is an active volcano on the east coast of Sicily, close to Messina and Catania. It is the largest volcano in Europe, currently standing about 3,320 m (10,900 ft) high, though it should be noted that this varies with eruptions; the mountain is 170 ft (51 m) lower now than it was in 1865. It is the highest mountain in Italy south of the Alps. Etna covers an area of 460 square miles (1190 km²) with a basal circumference of 140 km. This makes it by far the largest of the three active volcanos in Italy, being nearly three times the height of the next largest, Mount Vesuvius.
It is one of the most active volcanoes in the world and is in an almost constant state of eruption. Although it can occasionally be very destructive, it is not generally regarded as being particularly dangerous, and thousands of people live on its slopes and in the surrounding areas. The fertile volcanic soils support extensive agriculture, with vineyards and orchards spread across the lower slopes of the mountain.
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Name and legends
Etna was known in Roman times as Aetna, a name thought to have derived either from the Greek word aitho ("to burn") or the earlier Phoenician word attano. The Arabs called the mountain Gibel Utlamat ("the mountain of fire"); this name was later corrupted into Mons Gibel (translating from its Roman and Arab parts as 'Mount Mountain') and subsequently Etna's current local name Mongibello.
The mountain's regular and often dramatic eruptions made it a major subject of interest for Classical mythologists and their later successors, who sought to explain its behaviour in terms of the various gods and giants of Roman and Greek legend. Aeolus, the king of the winds, was said to have imprisoned the winds in caves below Etna. The giant Typhon was confined under Etna, according to the poet Aeschylus, and was the cause of the mountain's eruptions. Another giant, Enceladus, rebelled against the gods, was killed and was buried under Etna. Hephaestus or Vulcan, the god of fire and the forge, was said to have had his forge under Etna and drove the fire-demon Adranus out from the mountain, while the Cyclopes maintained a smithy there where they fashioned lightning bolts for Zeus to use as a weapon. The Greek underworld, Tartarus, was supposed to be situated beneath Etna.
Empedocles, a major pre-Socratic philosopher and Greek statesman of the 5th century BC, was said to have met his death in the volcano's crater. Etna supposedly erupted in sympathy with the martyrdom of Saint Agatha in 251 AD, prompting Christians thereafter to invoke her name against fire and lightning.
Morphology
Etna is an isolated peak about 18 miles (29 km) from Catania which dominates the eastern side of Sicily. Its shape is that of a truncated cone with a ragged top, which is actually a complex of large volcanic cones hosting four summit craters. Around 260 smaller craters, formed by flank eruptions, occupy the slopes. On the southeastern side of Etna lies an immense gully, the Valle del Bove, which is between 2000-4000 ft (600-1200 m) deep and over 3 miles (5 km) wide. Many of Etna's subsidiary craters reside within this cleft, which is thought to have been created around 3,500 years ago by the collapse of an ancient caldera. The height of the mountain varies with its eruptions; until 1911, there was only one large cone and crater at the summit, but subsequent eruptions have created new craters and cones.
The slopes of Etna form three distinct zones. The lower zone, extending up to about 4000 ft (1200 m) are densely populated and planted with vineyards, citrus fruits, and groves of olives, figs and almonds. The middle zone (up to about 6900 ft / 2100 m) is heavily wooded, mostly with pine and chestnut trees. At the top of the mountain is a volcanic wasteland, dominated by old lava flows, screes and volcanic ash. Few plants grow there and it is covered by snow for much of the year.
Etna is an extremely complex volcano, presenting considerable difficulties in classication. It has features of both a shield volcano and a stratovolcano, and displays behaviour typical of both plinian and strombolian volcanoes. It stands at the convergent boundary where the African Plate is being subducted beneath the Eurasian Plate, deforming the latter and forcing plumes of magma upwards into weak points in the crust such as under Etna. It is perhaps most accurate to describe Etna as being a mixture of overlapping shield and strato volcanoes partially destroyed by repeated collapses and partly buried under subsequent volcanic edifices.
Geological history
Volcanic activity at Etna began about half a million years ago, with eruptions occurring beneath the sea off the then coastline of Sicily. 300,000 years ago, volcanism began occurring to the southwest of the present-day summit, before activity moved towards the present centre 170,000 years ago. Eruptions at this time built up the first major volcanic edifice, forming a stratovolcano in alternating explosive and effusive eruptions. The growth of the mountain was occasionally interrupted by major eruptions leading to the collapse of the summit to form calderas.
From about 35,000 to 15,000 years ago, Etna experienced some highly explosive eruptions, generating large pyroclastic flows which left extensive ignimbrite deposits. Ash from these eruptions has been found as far away as Rome, 800 km to the north.
Historical eruptions
About 3,500 years ago, the eastern flank of the mountain experienced a catastrophic collapse, generating an enormous landslide in an event similar to that seen at Mount St. Helens in 1980. The eruption which is thought to have caused this collapse was recorded by Diodore of Sicily, the first known record of an eruption at Etna. The landslide left a large depression in the side of the volcano, known as 'Valle del Bove' (Valley of the Oxen). The steep walls of the Valle have suffered subsequent collapse on numerous occasions. The strata exposed in the valley walls provide an important and easily accessible record of Etna's eruptive history.
The most recent collapse event at the summit of Etna is thought to have occurred about 2,000 years ago, forming what is known as the Piano Caldera. This caldera has been almost entirely filled by subsequent lava eruptions, but is still visible as a distinct break in the slope of the mountain near the base of the present-day summit cone.
The Roman poet Virgil gave what was probably a first-hand description of an eruption in the Aeneid:
- The port capacious, and secure from wind,
- Is to the foot of thund'ring Etna joined.
- By turns a pitchy cloud she rolls on high:
- By turns hot embers from her entrails fly,
- And flakes of mounting flames, that lick the sky.
- Oft from her bowels massy rocks are thrown,
- And shivered by the force come piece-meal down.
- Oft liquid lakes of burning sulphur flow,
- Fed from the fiery springs that boil below.
In 396 BC, an eruption of Etna is said to have thwarted the Carthaginians in their attempt to advance on Syracuse during the First Sicilian War.
1669 eruption
Over the last 2000 years, activity at Etna has been generally effusive, with occasional explosive eruptions from the summit. Its most destructive eruption during this time occurred in March 1669, when an estimated 830,000,000 m³ of lava was ejected. The eruption was preceded by two months of increasingly powerful earthquakes centred on the southern slopes of the mountain, which eventually encouraged most villagers there to abandon their homes. On 11 March, a 9 km-long fissure opened up on the southern flank of the mountain, stretching from an elevation of 2800 m down to 1200 m. Activity steadily migrated downslope, and the largest vent eventually opened near the town of Nicolosi. The cinder cone built up at the erupting vent became known as Monti Rossi (red hills), and is still a prominent landmark today.
Nicolosi was quickly destroyed by lava flows, and two nearby villages were also destroyed during the eruption's first day. The eruption was extremely voluminous, and a further four villages were destroyed in the following three days as the lava flowed south. In late March two larger towns were destroyed, and the lava reached the outskirts of Catania in early April.
At first, lava piled up against the city walls, which were strong enough to withstand the pressure of the flow. However, while the city was temporarily protected, lava flowed into its harbour and filled it in. On 30 April, lava flowed over the top of the city walls, which then gave way. Catanians built walls across major roads to halt the flow of the lava, which were fairly effective but did not prevent the destruction of the western side of the city.
During the eruption, Catania residents also attempted to divert the flows much further upstream. According to a possibly apocryphal tale, their efforts were met with armed resistance from the citizens of a town which would have been threatened by the diverted flow [1]. Whether this event really occurred or not, a law was subsequently passed to forbid the artifical diversion of lava flows. This law was only repealed in 1983.
Recent eruptions
Another very large lava flow from an eruption in 1928 led to the first destruction of a town since the 1669 eruption. In this case, the town of Mascali was destroyed in just two days, with the lava destroying every building. The event was used by Mussolini's Fascist regime for propaganda purposes, with the evacuation, aid and rebuilding operations being presented as models of fascist planning. Mascali was rebuilt on a new site, and its church contains the Italian fascist symbol of the torch, placed above the statue of Christ.
Other major 20th century eruptions occurred in 1949, 1971, 1983, 1992, and 2000. The 1992 eruption saw the town of Zafferana threatened by a lava flow, but successful diversion efforts saved the town with the loss of only one building a few hundred metres outside it.
In 2002-2003, the biggest series of eruptions for many years threw up a huge column of ash that could easily be seen from space and fell as far away as Libya, on the far side of the Mediterranean Sea. Seismic activity in this eruption caused the eastern flanks of the volcano to slip by up to two metres, and many houses on the flanks of the volcano experienced structural damage. The eruption also completely destroyed the Rifugio Sapienza, on the southern flank of the volcano. The Rifugio was the site of a cable car station which had previously been destroyed in the 1983 eruption.
