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Pantelleria volcano, south of Sicily, Italy

volcano number: 0101-071 (according to Volcanoes of the World, 1994 edition)

summit elevation: 836 m

location: 36.77°N, 12.02°E

Note: this information is not yet complete!!!

The following is summarized from Civetta et al. (1984, 1988), Cornette et al. (1983), Mahood & Hildreth (1983, 1986), Orsi & Sheridan (1984), Wallmann et al. (1988) and Wright (1980); additional information and images from Claudio A. Tranne (Università di Bologna) will soon be supplied.

The volcanic island of Pantelleria with an area of 83 km^2 lies in the Straits of Sicily, about 100 km from the SW coast of Sicily but only 70 km from Africa's north coast. It sits on the Pantelleria Rift which forms the deepest part of the Straits and whose floor consists of continental crust (which has a thickness of only 20-21 km, however). The largest diameter of the island is 13.7 km in NW-SE direction with a minimum diameter of 8 km from NE to SW. Montagna Grande, at 836 m, is the highest point on the island.

In volcanology, Pantelleria is quite famous for its peralkaline volcanism and its geological evolution and eruptive dynamics have been studied by many researchers during the past 100 years or so, culminating during the 1980's with a number of new papers being published, some of them expressing quite conflicting opinions (see citations above).

Geological history

The geological history of Pantelleria is generally subdivided in two major parts, one preceding the large Green Tuff eruption (about 45 ka ago) and the period postdating it.

Pre-Green Tuff volcanic history

The earliest products of the volcano are dome lavas exposed at the SW coast near Scauri dated at about 325 ka. During the following ca. 200 ka, numerous ignimbrites and lava flows were erupted from various eruptive centers. One particular event about 118 ka ago was the formation of the basaltic cinder cone of Cuddia del Cat: particular because basalts make up just a few per cent of the total rocks exposed on the island. About 114 ka ago, volcano-tectonic collapse led to the formation of a caldera named "La Vecchia" by Mahood & Hildreth. The SW, S, SE and N rims of this depression are still discernible (see the geological map of Pantelleria; Mahood & Hildreth 1986). The area of collapse was recalculated at about 42 km^2, that is, more than half of the present area of the island. Collapse probably occurred after the eruption of a large volume ignimbrite (Mahood & Hildreth 1986).

Post "La Vecchia" caldera volcanism occurred mainly from ring fractures and let to the building of numerous small volcanic edifices. Erupted products were lavas, Plinian fall deposits and at least three welded tuff units. These volcanics were dated at between 106 and 79 ka.

Voluminous pantelleritic lava flows erupted about 67 ka ago, building a broad shield (Cuddia Attalora) and several smaller shields. This eruption is thought (by Wallmann et al. 1988) to have been possibly triggered by hydrostatic unloading during a lowstand of the sea level (stage 4 of the delta 18 O record).

Green Tuff eruption and collapse of Cinque Denti caldera

About 45 ka ago, a series of violent explosive eruptions produced large volumes of ignimbrites, surges and airfall tephra, combined known as the Green Tuff of Pantelleria. Mahood & Hildreth (1986) estimate the volume of erupted magma in that event at about 2.5 km^3 DRE (dense-rock equivalent). Many of the ignimbrite units show thorough welding and rhemorphism (that is, flow when still hot immediately after emplacement) and there has been debate whether the deposits are the result of a rapidly emplaced fall deposit or of the welding of an ignimbrite (Mahood & Hildreth 1986, Orsi & Sheridan 1984).
The Green Tuff has been correlated with a prominent tephra layer (Y-6 ash) encountered in sea-floor drill holes to the east of Pantelleria up to several hundred km away from the island (Keller 1981).

The eruption of the Green Tuff was accompanied by collapse of a new caldera 6 x 7 km in diameter in the center of the island, named Cinque Denti caldera by Mahood & Hildreth (1986) and Monastero caldera by Civetta et al. (1984). Collapse probably occurred before the end of Green Tuff eruptions because some late eruptive products unconformably overlie the caldera rim. The volume of the newly formed depression was estimated at 3.4 km^3, significantly more than the volume of erupted magma.

Post-Green Tuff volcanic activity

Eruptive activity continued after the Green Tuff eruption although it is a matter of debate whether there was a significant period of volcanic inactivity (Civetta et al. 1988) or no repose period at all (Mahood & Hildreth 1986). During this activity, trachytic magmas were emitted as lavas and pyroclsatic flows in the SE part of the Cinque Denti (or Monastero) caldera, gradually building a major shield-like edifice which eventually filled two thirds of the caldera. The activity of this intracaldera volcano lasted from about 44 until about 35 ka ago, and by its end about 3 km^3 of new magma had been erupted, a figure corresponding more neatly to the volume of the previously formed caldera (see below). The main source vent for these eruptions was Monte Gibele (700 m elevation) which still has a beautifully preserved summit crater. Mahood & Hildreth (1986) believe that these trachytic caldera-filling eruptions occurred in isostatic compension for the caldera collapse, citing as evidence the corresponding volumes of the erupted magma and the preceding caldera collapse. Civetta et al. (1988) dispute such a hypothesis based on their assumption that caldera formation and trachytic volcanism were separated by a longer repose interval.

Uplift of the central-northern part of the caldera floor is estimated to have begun about 18 ka ago. The formation of the highest summit of the island, Montagna Grande, has been discussed in the scientific literature since the earliest geological studies of the island. While some authors held that the feature was a large lava dome (Rittmann 1967, Villari 1974, 1975, Wright 1980, Cornette et al. 1983, Civetta et al. 1984), Mahood & Hildreth (1986) demonstrated that it is actually an uplifted and tilted block of lavas and pyroclastics previously erupted from Monte Gibele, a hypothesis already proposed by early researchers such as Foerstner (1881), Bergeat (1907) and Washington (1913).
Uplift of the central part of Montagna Grande had an amount of at least 275 m. It is not clear whether the northwestward tilting occurred later, by subsidence of a part of the block or whether uplift was greatest on its SE side.

More recent postcaldera eruptions occurred during the period 18-3 ka from tens of eruptive centers located on the caldera rims, around the base of the intracaldera trachyte shield (that is, in the caldera moat), from radial fractures and along the fractures bounding the Montagna Grande tilted block. The products of this activity wer lava flows and fallout pumice, magmas that showed a return to more evolved compositions. Some of these flows are obsidian. Explosive volcanism was common during this period, producing partially welded pumice fall deposits >40 m thick near the vents. Among these are the Cuddia Randazzo fall deposits dated at 5.4-5.5 ka, and another fall unit of which the source vent is not known is as young as 3 ka. A few of the most recent volcanic products are again of basaltic composition.

1891 submarine eruption near Pantelleria

While no eruptions have occurred on the island during the past 3000 years, a shallow submarine eruption occurred in 1891 about 5 km northwest of the NW coast of Pantelleria. This event was preceded by vigorous seismicity and increasing fumarolic activity on the island. Significant uplift (up to 0.8 m) occurred at the NE coast of Pantelleria in mid-October 1891. The eruption began on 17 October, throwing up steam and large scoria (reportedly up to 1 m in diameter) to heights of 20 m. The activity culminated during its second day and then rapidly declined, ending completely on 25 October.

References (not complete)

Civetta L, Cornette Y, Crisci G, Gillot PY, Orsi G & Requejo CS (1984) Geology, geochronology and chemical evolution of the island of Pantelleria. Geological Magazine 121: 541-562.

Cornette Y, Crisci GM, Gillot PY & Orsi G (1983) Recent volcanic history of Pantelleria: A new interpretation. Journal of Volcanology and Geothermal Research 17: 361-373.

Mahood GA & Hildreth W (1983) Nested calderas and trapdoor uplift at Pantelleria, Strait of Sicily. Geology 11: 103-106.

Mahood GA & Hildreth W (1986) Geology of the peralkaline volcano at Pantelleria, Strait of Sicily. Bulletin of Volcanology 48: 143-172.

Orsi G & Sheridan MF (1984) The Green Tuff of Pantelleria: a rheoignimbrite or rheomorphic fall? Bulletin Volcanologique 47: 611-626

Orsi G, Gallo G & Zanchi A (1991) Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia. Journal of Volcanology and Geothermal Research 47: 1-11.

Orsi G, Ruvo L & Scarpati C (1991) The recent explosive volcanism at Pantelleria. Geologische Rundschau 80: 187-200

Wright JV (1980) Stratigraphy and geology of the welded air-fall tuffs of Pantelleria, Italy. Geologische Rundschau 69: 263-291

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