As reported by Sistema Poseidon, activity at Etna during 9 April-13 May 2001 was chiefly characterized by typical episodic Strombolian blasts, ash emissions, and modest lava flows. The larger lava flows that emerged from new vents and grew during June and July will be discussed in later reports.

Activity during mid- to late-April 2001. During this time interval ash escaped at the Bocca Nuova (BN) vent. The weather thwarted direct observations of summit activity; however, later information was obtained through outings to intermediate elevations and from La Montagnola surveillance camera.

Lava continued to flow from a vent low on the NNE flank of the Southeast Crater (SEC) cone, as it has since approximately 20 January 2001. This lava flowed down the SEC's NE flank. During the nights of 18 and 21 April observers noted that the SEC produced flashing, denoting effusive activity. The SEC also continued to give off gray-colored gas from both the fumarole on the crater's edge and from the pit-crater in the crater's interior. Later in April the SEC's N flank vent continued to emit lava variably, but generally weakly, and beginning 26 April, the flow became visible principally from the volcano's NE quadrant. During 26-28 April degassing increased at SEC, yielding abundant clouds of white steam that diminished on 29 April.

Observations on 27 April revealed two hornitos (at 3,085 m, ~3 m high, and aligned N-S). They produced steady emissions, sounds of pressurized gas, and discontinuous expulsion of vitreous and blistering lava fragments which fell within a few meters of the vents. The more northerly hornito produced a lava flow within a confined channel. At about 3,000 m elevation, this lava river divided into two branches before rejoining just above 2,900 m. In late April, the flow rate was estimated at 2-3 m³/s.

A party viewing the base of BN's crater saw two prominent, steep-sided fissures that were ~100 m in length and at least 30-50 m deep. At a shelf inside the N fissure a small pyroclastic cone gave off dense brown and reddish clouds visible from the slopes of the volcano. The fissure in the SW quadrant also degassed intensely, and both fissures gave off almost continuous noise associated with magma inferred to reside at depth. A field of semi-circular fissures was observed nearby running S and W from this depression. Observers also noted fumaroles emitting bluish gas. Until at least early May, Voragine and Northeast craters continued weak degassing.

When seen on 3 May SEC's N hornitos had grown by almost 1.5 m compared with the preceding week. The lava canal had also widened to about 2 m, corresponding to a significantly increased flow rate, 5-10 m³/s. Two small lava flows developed on the E and W sides of the hornitos.

Strombolian eruptions starting on 7 May. Strombolian activity began again at the SEC late on the morning of 7 May. When seen on 9 May these eruptions were almost continuous, as frequent as about 45-50 explosions per minute, including some strong ones that sent lava fragments 20-30 m above the crater. Lava fragments as big as a meter in diameter were thrown up to 50 m above the crater rim.

Beginning at 1400, along with a new increase in tremor, the Strombolian activity evolved into a more violent phase at 1520-1540. Ballistics landed at elevations as low as ~3,000 m, reaching the spatter rampart at the S base of the cone. At about 1630 modest lava fountaining was observed from the fracture on the N flank of the SEC. Jets of magma reached ~100 m high. The fragments emitted from the lava fountain fell mostly in the SW sector of the volcano.

At the same time, the Montagnola camera began to register frequent ash emissions from the cone's summit; Strombolian activity and ash emissions continued until midnight in a discontinuous manner and with variable intensity. Observations on 10 May showed a substantial decrease in the activity at the SEC summit. Weak explosive activity was observed from the N fracture.

The lava emission from the fracture cutting the N flank of SEC continued with more or less intense phases. On 9 May, the cessation of lava fountaining was followed by a repeat of effusive activity, still within the same area of emission, which gave rise to finger-like flows ~1.5-2 km long. On 10 and 13 May, short lengths of the active branches of the flows were observed. The outburst led to a considerable plume that impacted local air traffic.

Bocca Nuova continued to issue brown-reddish ash emissions, presumably ongoing ash-bearing eruptions from one of the fissures described above. On 9 May a new fumarolic field was seen in the S part of the Bocca Nuova, extending from the rim to half way down the cone.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Sistema Poseidon, a cooperative project supported by both the Italian and the Sicilian regional governments, and operated by several scientific institutions (URL: http://www.ct.ingv.it/en/chi-siamo/la-sezione.html).