Mt. Etna - Volcanological Evolution

Mount Etna, one of the largest active volcano in the Mediterranan area and the largest in Europe, covers an area of about 1260 km² reaching at its top an elevation of about 3350 m (a.s.l.). The volcanic activity is at the base of myths and legends from classic times. The most ancient known records on Mount Etna, one of the "workshops" of the Latin god Vulcanus, and on its eruptive activity date back to several centuries B.C., being clearly reported in chronicles by historians (Tucydides and Diodorus) or recalled in poems (Pyndarus).

Etnean activity started around 600 ka BP (Gillot et al., 1994), after the end of Upper Pliocene to Pleistocene, with subaqueous and subaerial eruptive activity at the north-western edge of the Iblean Plateau. Mt. Etna is a multiple stratovolcano, consisting of various edifices centred on distinct eruptive axes, of whom the most recent ones may be still recognized. Since the 19^th century two main centres of activity were recognized by Sartorius von Waltershausen (1880), and C. Gemmellaro (1860): the younger of the edifices was named as Mongibello (a combination of the latin - mons - and arab - gebel - words used as the local name of the Mountain), and the older one, related to a distinct magma uprise system, as Trifoglietto. In the last decades, the Etnean volcanism has been related to the activity of many different eruptive centres, grouped into five major volcano-stratigraphic units (Romano, 1982; Cristofolini et al., 1991; Gillot et al., 1994), namely defined as Units of the Basal Subalkaline (tholeiitic to transitional) lavas (~600-250 ka), Ancient Alkaline Centres (~225-100 ka), Trifoglietto (~80-40 ka), Ancient Mongibello (also known as Ellittico; ~35-14 ka) and Recent Mongibello (~14 ka to Present).

Mount Etna is located at the edge of a major crustal discontinuity; the subaerial edifice is actually placed over the still uprising, uplifted footwall (Monaco et al., 1997) of a Late Quaternary crustal-scale belt of normal faults (Hirn et al., 1997; Laigle, 2000; Bianca et al., 1999), which partially reactivated the Malta Escarpment, a NNW-SSE Mesozoic discontinuity separating the domains of continental crust of the Pelagian Block (Iblean Plateau; Ben Avraham et al., 1995; Torelli et al., 1998) and of thinned crust of the Ionian Sea (Makris et al., 1986). This belt intersects an other regional fault system (NE-SW oriented), parallel to the coast-line between Taormina and Messina, next to the front of the south-verging overthrust pile of the Apennine-Maghrebian mountain range. The volcano appears then placed in an anomalous position, and its tholeiitic to Na-alkaline geochemistry and regional geophysical data suggest magma sources unrelated to deep crustal slabs. The relationship between the foreland monocline at the front of the Apennines in the Ionian Sea than and in Sicily, and the extension of the Apennines arc should produce a right-lateral transtension and a sort of vertical “slab window” which might explain (1) the Plio-Pleistocene uprise of mantle generated magmas in eastern Sicily and (2) the late Pliocene to present right lateral transtensional tectonics and seismicity of eastern Sicily. The area of transfer of different dip and rollback occurs along the inherited Mesozoic continental margin between Sicily and the Ionian Sea (i.e., the Malta escarpment; Gvirtzman and Nur, 1999; Doglioni et al., 2001). The volcano is an upper Quaternary structure, the bulk of which is composed of lavas and tephra erupted during the last 225 ka (Gillot et al., 1994). From North to South, the volcanic cover lies above a sedimentary substratum at the front of the Maghrebian thrust belt, upper Miocene terrigenous levels, and Early-Middle Pleistocene foredeep sedimentary clayey successions, deposited on the flexured margin of the Pelagian block.

The oldest volcanics (~600 ka BP) are submarine tholeiitic to transitional basalts, erupted at depths around 500 m b.s.l. (Corsaro and Cristofolini, 2000), in a wide gulf  extending between the northern mountain chain and the Iblean Plateau to the south. These are found now, associated with upper Pleistocene marly levels, as very limited outcrops of pillow-lavas, hyaloclastites and sills (Corsaro and Cristofolini, 1997) along the Ionian coast, next to Acitrezza and Acicastello. Similar lavas and minor Na-alkaline products, however, are found, at the downthrow side of the NNW-SSE fault belt, on the Ionian sea floor at depths between 800 and 1300 m b.s.l. offshore of Acireale (Coltelli and Pompilio, personal communications; Cristofolini, unpublished data). Subaerial tholeiites ~300 ka old, erupted after a strong regional uplift, are found on the oldest river terrace of the Simeto River, ~300 m higher than the present-day valley-bottom: they currently outcrop at the south-western periphery (Adrano – Paternò) of the volcanic cover, but probably spread over much wider areas, and are presently buried under later volcanics, as shown by drill-hole samples (Cristofolini et al., 1991) and by the diffuse presence of tholeiite boulders in the oldest river terraces deposits. These earlier levels are most commonly aphyric to oligophyric basalts, whereas the later volcanics, generally varieties related to Na-alkaline basalt magmas, mostly are markedly porphyritic hawaiites to mugearites, often joined by more differentiated benmoreites to trachytes.

Small central volcanoes, fed by transitional to prevailingly Na-alkaline magmas, similar to intra-plate oceanic types, started to develop (~200 ka BP) above the earliest subalkaline volcanic levels. Most of these edifices, related to the activity of the Ancient Alkaline Centres and of the following Trifoglietto Unit, are strongly dismantled by erosion and widely covered by younger volcanics; their products chiefly crop out only along fault scarps (locally named Timpe) or uplifted cliffs, and along the inner walls of the Valle del Bove (eastern flank). This is a prominent feature of the Etnean morphology, being a deep horse-shoe shaped valley carving the eastern flank of the volcano. The Valle del Bove origin still is matter of debate: it might have been formed by caldera collapses of ancient edifices, easterly sliding of the seawards volcanic mass, rapid erosion of steep flanks (cf. Guest et al., 1984; Mc Guire and Pullen, 1989; Calvari et al., 1998). Large amounts of detritus derived from the Valle del Bove is now forming an alluvial fan in the vicinity of Giarre-Riposto (cf. Calvari et al., 1996).Within the old volcanic sequence, starting from at least 100 ka ago (Coltelli et. al., 2000), volcaniclastic levels (pyroclastic fall and flow deposits and lahars, largely resulting from debris-flow) are inter-bedded with lava flows, as evidence of effusive to highly explosive subplinian to plinian activity (Busà et al, 1997; Cortesi et al., 1988, Coltelli et al., 1995, 2000; Cristofolini et al., 1991). As a consequence of this behaviour, most of the edifices composing the complex structure of Mount Etna show characters of stratovolcanoes. At about the same time, the main feeding systems tended to take a better defined location, probably at the intersections of faults of the two afore-mentioned major regional belts (the NNW-SSE “Iblean-Maltese” and the NNE-SSW systems), which act as preferred feeding structures of the volcano.

The most recent activity (Mongibello, <35 ka BP) was characterized by recurrent, significant explosive activity until a few thousands of years ago. Paroxysmal eruptions gave origin to calderas, the most recent of which are still recognizable, although largely filled by younger products (“Ellittico caldera", 15 ka, ca. 4.5 km across; "Piano caldera" 122 b.C). During the last centuries, explosive activity of Mongibello was quite mild, almost persistent at the summit vents to sporadic at lateral vents. Intensity of summit vent phenomena is very variable (quiet steam emission to strombolian explosions and lava fountaining), sometimes associated with small lava effusions, lasting few hours up to several months, or even years. At present, there are several vents in the top region; 1) the Chasm; 2) the Bocca Nuova, constituted by two large pits; 3) the South East crater; 4) the North East crater. Each of them behaves independently, thus suggesting a complex system that feeds their activity.

Peripheral vents can open also at low elevations (down to 300 m a.s.l.), even outside the edge of the volcanic cover (Gravina di Catania, Mojo Alcantara). They mostly pour out lava flows, with tephra originating modest spatter ramparts to large cinder cones, either isolated or associated along the feeding fractures, that mark the Etnean landscape. In the last 350 years, ~70 eruptions occurred, irregularly distributed in time and space (cf. Romano and Sturiale, 1982). Recent lavas are mostly aa, and less commonly pahoehoe, or have their surface covered with irregular slabs variously embricated or piled on top of each other. In these flows complicated tube systems may form (cf. Calvari and Pinkerton, 1999), along which the thermally insulated melt can flow over great distances, feeding lava fronts as far as 10 km or more from the vents. Almost 60% of the Etnean region has been covered by at least one lava flow since the 13^th century, including even some densely populated sectors at low elevations down to sea level, among which the south-eastern one is actually the most relevant. Even if the recent activity is moderately hazardous for human lifes, it seriously threatens all human activities in this densely populated area, because of complete destruction in the lava flooded surface, that remains barren for centuries.

Even if attempts of active damage mitigation have been made in 1983, 1992, 2001 and 2002 by damming and/or diverting the natural flow course, they are to be still considered preliminary, and may be tested only under favourable conditions. At present, main hazard reduction measures consist in designing detailed evacuation plans for the populated areas likely to be involved in eruptive episodes.