Two eruptions in one
Today we tell you about the first eccentric outbreak that has been scientifically documented. In an eccentric eruption, the magma does not reach the top via the main vent, but seeks another path from the magma chamber to the surface. Since it has to break through hard lava rock from previous eruptions, such eruptions are very rare.
The 2001 eruption was an extremely multi-layered activity that lasted for 24 days, formed 7 fractures and brought magma with two different chemical compositions to the surface:
- magma that came up via the existing main vent,
- magma that rose via a different, new pathway and thus incorporated more foreign material.
Preparatory phase
The new millennium got off to a turbulent start on Etna, with 66 eruptions within 7 months. After a brief period of calm, a new spectacle began in 2001, and this time a huge one. This had been expected, as the activity of the last few years had already shown clear signs of increasing magma supply from the Earth’s mantle, volcanic tremors and earthquakes, as well as a deformation of the volcano’s structure (similar to a large panettone rising and already showing signs that it will soon collapse).
After a spirited year in 2000 or early 2001, when explosions were mainly concentrated at the southeast crater, almost alternating between the opening in the north “Levantino” and the opening in the south “Sudestino”, lava emission started to increase again in May.
Chronicle of the 2001 eruption
The explosions became more and more frequent until 13 July, the day that marked the beginning of a seismic activity in which 2600 earth tremors were recorded in only 4 days (recorded by the monitoring systems of the INGV of Catania, the National Institute of Geophysics and Volcanology). At the same time, several fractures formed in the then flat Piano del Lago area on the southern side of Etna: between the Cisternazza and Montagnola craters, the pressure from the underground was so strong that it could deform the surface at an altitude of over 2600 metres.
On 17 July, three fractures opened, officially marking the beginning of the actual eruption. They opened one after the other, first to the north of South East Crater, then to the south at an altitude of 2950 m, and then at about 2720 m near the so-called “Belvedere”, a viewpoint of the Valle del Bove valley, the latter causing a lava flow that approached menacingly the ski resorts of Rifugio Sapienza.
On 18 July, at an altitude of 2100 m, another fracture occurs, the fourth. While the previous fractures had formed above the Montagnola crater, this one opens about 500 metres below, to the west of the Calcarazzi craters or above the upper Silvestri crater.
The lava coming out of this fissure directly threatens the infrastructure of the Etna South tourist centre. It flowed towards the La Capannina restaurant, which was saved thanks to the intervention of the Civil Defence and the fire brigade and thanks to the favourable position in the shelter of the large cone of the upper Silvestri crater.
During the formation of this fracture, phreatomagmatic explosions and a magnitude 2.7 earthquake occur. Thanks to the careful work of geologists and volcanologists at INGV, the material produced by this fourth fracture is known to contain many xenoliths, which are inclusions of sedimentary rocks and amphibole minerals that also contain water molecules. These are not found in the lava coming from the other three fractures. Based on this composition, it was recognisable that this fracture was an eccentric activity.
On 19 July, a fifthopening opens about 500 metres above Montagnola, again in the Piano del Lago area at 2750 m altitude. Mainly phreatomagmatic explosions take place, forming a 200 to 300 m high column of lapilli and ash that the wind carries to towns kilometres away.
The phreatomagmatic eruptions are caused by the meeting of magma and water. These explosions characterise this fifth opening and also the lava flow at 2100 m (fourth opening), but not the first three, which also occurred in the Piano del Lago area, just a few hundred metres away.
Surprisingly, on 20 July, the southeastern crater again made its presence known with a slight strombolian activity. On its northern side, a series of fissures (the sixthfracture) also forms, heading towards the Valle del Leone and producing a lava flow that runs towards the foot of Pizzi Deneri and then towards Monte Simone. All other openings remain extremely active and productive.
On 22 July, the activity of the opening increases in intensity at an altitude of 2100 m, the lava fountains reach a height of 100 to 200 metres. The large ejection of lava makes the lava flow arrive quickly at an altitude of 1060 m, but fortunately it slows down when it reaches a flat area. On the other hand, the lava flow coming from the fracture at 2720 m destroys some pylons of the cable car and reaches 2070 m altitude.
On 23 July, a seventh fracture opens on the south flank of Southeast Crater, producing a lava tongue about one and a half kilometres long.
On 24 July, the advance of most of the lava flows slows down, but the opening at 2570 m, near the Montagnola crater, changes its activity. Strombolian activity begins, becoming steadily stronger and forming a cinder cone. The eruption remains dominant in the following days, and while the other flows slow down or stop, this one destroys more pylons of the cableway and becomes more threatening. It flows around the top station of the cable car, but without causing any damage, and reaches the bottom station, but is saved by the dams built to protect the cable car and the Rifugio Sapienza.
This spectacular activity continues over the following days until, on 30 July, a newly formed lava flow sets fire to the cable car mountain station. Fortunately, from this moment on, the lava flows of the newborn cone Piano del Lago (today: Laghetto crater or also Monte Escrivà) subside, also thanks to the formation of a small stream at the foot of the crater that pours into the valley Valle del Bove.
On 1 August, the ash ejected by the big new cone once again forces the closure of Catania airport. Dangerous phreatomagmatic explosions are still occurring at 2100 m, but fortunately the lava flow, having slowed its course, overlaps the tongues formed in the past few days. The streams flowing from the opening at an altitude of 2720 m move towards Monte Nero degli Zappini.
Finally, from 3 August onwards, the activity of the last still active streams also decreases noticeably until it stops completely on 6 August. Only the opening at 2100 m remains active until 10 August, the last day of this eruption.
The volume of lava ejected during this huge eruption is 25 million cubic metres (dense-rock equivalent; estimated by Behncke & Neri).
On our tours on the south side of Etna (such as the Etna 2750 tour), we discover the evidence of this multi-layered eruption, or rather of these two enormous eruptions: we have lunch in the Laghetto crater, visit the quarry at 2100 m altitude and walk along the lava flow that was created at that time.