Etna

Schematic geological map of Etna volcano. Modified after Gillot et al.(1994).

Etna is an active volcano reaching an altitude of about 3315 m, and covering an area of about 1260 km². It has developed at the intersection between various tectonic lineaments, the most important being the NNW-SSE trending Tindari-Letojanni-Malta and the NNE-SSW trending Messina-Giardini fault systems. Etna shows an asymmetrical conical shape. The flank inclination is low (5-10 degrees) up to 1800 m altitude, but increases sharply (about 25 degrees) in the upper part of the volcano. This morphology results from the coalescence and superimposition of several distinct cones, and from flank eruptions which built up a large number of monogenetic cones concentrated along zones of weakness, especially in the NE sector.

Four main evolution stages have been distinguished for Mount Etna activity:

The first stage (580 to 225 ka) was characterised by emplacement of tholeiitic basalts, which were erupted over a wide area from the Iblean Plateau in the south to the Peloritani mountains in the north, and presently crop out as pillow-lavas, hyaloclastites and sills along the Ionian Sea coast north of Catania, between Acicastello and Acitrezza, and along the south-western margin of the volcano.

Starting from about 220 ka, the volcanic activity concentrated in the Ionian coast and changed from tholeiitic to Na-alkalin. A number of central volcanoes (Ancient Alkaline Centres or Timpe Volcanoes) were constructed over a time span of about 100 ka (172 to 96 ka), and their remnants mainly crop out along the present-day margin of Etna. Successively, various cones (Tifoglietto, Cavigghiuni, Vavalaci etc.) making up the so-called Trifoglietto unit were built up by effusive and explosive eruptions between about 80 to 60 ka.

Finally, the Mongibello stratovolcano was constructed between about 60 ka to present. Older Mongibello activity built up the so-called Ellittico volcano, consisting of prevailing benmoreitic to trachytic lavas and pyroclastics, and was closed by a caldera collapse (at about 15 ka). Recent Mongibello activity (14 ka to present) has been characterized by dominant effusive eruptions and strombolian explosions, giving lava flows and scoria cones, which cover extensively the flanks of the Etna volcano. Minor collapses (e.g. the summit Piano caldera, about 2000 years before present) occurred during recent Mongibello activity.

Etna is marked by several important volcano-tectonic structural features, in addition to the calderas mentioned above. The Valle del Bove is one of the largest and best known. It is a horse-shoe shaped depression occurring on the eastern flank of the volcano. It formed about 8000-5000 years before present, but its origin is controversial. Some authors suggest it is related to coalescing collapses widened by rapid erosion; others propose flank sliding along fault planes. Other important volcano-tectonic features are represented by fractures radiating from the central crater area and forming three main rift zones along the flanks. Rift zones have been preferential sites of flank eruptions and are marked by a large number of cinder cones, a few meters to more than 200 m high. The relations between these structures and the regional stress field are debated, although most authors agree that the structural features and extensional stress regime of the eastern flank of Etna are related to proximity of the Tindari-Letojanni-Malta strike-slip fault system.

The eruptive style of Etna has varied, although effusive activity and strombolian explosions have dominated. High energy explosive eruptions have also occurred, especially during the activity of older Mongibello, when magma evolution produced benmoreitic-trachytic melts which were erupted as pumice fall and ignimbrites. Historical eruptions have occurred both at the summit vents and along the flanks. Activity at summit vents has consisted of quiet steam emission, strombolian to vulcanian and subplinian explosions and lava fountaining, sometimes accompanied by lava effusions, lasting from a few hours to months or sometimes years. As a consequence, the morphology of the summit crater area has been continuously modified. At present it consists of a slightly N-S elongated circular platform representing a small collapsed area (Caldera del Piano), containing a major cone with two summit large craters (Bocca Nuova and Voragine) and two lateral cones (NE Crater, SE Crater). Flank activity has been prevailingly effusive and strombolian; it has often taken place at low elevations (down to 300 m a.s.l.) close to inhabited areas, raising severe problems for civil defence.

The Etna rocks have tholeiitic to Na-alkaline affinity, with a few products exhibiting a potassic alkaline tendency (e.g. Monte Maletto). Compositions range from basalt to hawaiite, mugearite, benmoreite and trachy. Ol-hy normative tholeiites are the lowest exposed products. They show ophitic to poorly porphyritic textures. Phenocrysts phases include MgO-rich olivine, plagioclase and diopside to augite pyroxene set in a groundmass formed of the same phases plus Fe-Ti oxides and glass. Alkali basalt are aphyric to weakly porphyritic with phenocrysts of clinopyroxene, plagioclase and olivine (sometimes with chromite inclusions) set in a groundmass of the same phases plus glass. Hawaiites (trachybasalts) are porphyritic with phenocrysts of plagioclase, MgO-rich olivine, clinopyroxene, Fe-Ti oxides and sometimes kaersutite. Groundmass contains the same phases and in some cases nepheline, sodalite, and phlogopite.

Younger hawaiites have a typical phenocryst paragenesis consisting of plagioclase, clinopyroxene and olivine with some Fe-Ti oxides. Locally, these rocks are referred to as etnaites. Mugearites and benmoreites are porphyritic with phenocrysts of dominant plagioclase, salite to augite clinopyroxene, olivine, and some amphibole and Ti-magnetite set in a groundmass containing the same minerals, plus some alkali-feldspar, nepheline, sodalite, phlogopite and apatite.
Trachytes are aphyric to weakly porphyritic with microphenocrysts of plagioclase, augitic clinopyroxene, Fe-Ti oxides, and some olivine and kaersutite, set in a groundmass containing alkali-feldspar, clinopyroxene, olivine, Ti-magnetite, biotite and sporadic sodalite and nepheline.

Bibliography

• Peccerillo. A. Plio-Quaternary Volcanism in Italy. (2005)

Etna Basalts
Olivine, clinopyroxene and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene, magnetite and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)
Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)
Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)
Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)	Olivine, clinopyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)
Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Pyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine, Pyroxene and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)
Olivine and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)	Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Pyroxene and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)
Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Pyroxene and plagioclase in a Basalt from Etna. PPL image, 2x (Field of view = 7mm)
Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Olivine and plagioclase in a Basalt from Etna. XPL image, 2x (Field of view = 7mm)	Plagioclase crystals in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)
Olivine and Plagioclase crystals in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)	Plagioclase crystals in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)	Pyroxene and Plagioclase crystals in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)
Pyroxene and Plagioclase crystals in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)	Olivine crystal in a Basaltic Scoria. PPL image, 2x (Field of view = 7mm)	Plagioclase and pyroxene glomerocryst. PPL image, 2x (Field of view = 7mm)

Sandstone Xenolith
Contact between a sandstone xenolith (top left) and basaltic lava (lower right). PPL image, 2x (Field of view = 7mm)	Contact between a sandstone xenolith and basaltic lava. PPL image, 2x (Field of view = 7mm)	Contact between a sandstone xenolith and basaltic lava. PPL image, 2x (Field of view = 7mm)