1891 AD submarine eruptive processes and geochemical studies of floating scoria at Foerstner volcano, Pantelleria

Joshua Kelly, University of Rhode Island

Abstract

On October 17, 1891 a submarine eruption occurred at Foerstner volcano in the Straits of Sicily 4 km northwest of the island of Pantelleria, Italy. The eruption produced floating scoria bombs, or balloons, that discharged gas at the surface and eventually sank to the seafloor. Activity occurred for a period of one week from an eruptive vent located within the Pantelleria Rift at a water depth of 250 m. ^ Remotely Operated Vehicle (ROV) video footage and high resolution multibeam mapping of the Foerstner vent site was used to create a geologic map of the 1891 AD deposits and conduct the first detailed study of the source area associated with this unusual type of submarine volcanism. The main Foerstner vent consists of two overlapping circular mounds with a total volume of 6.3 x 10 5 m3 and relief of 60 m. It is dominantly constructed of clastic scoriaceous deposits with some interbedded effusive pillow flow deposits. Petrographic and geochemical analyses of Foerstner samples by X-ray fluorescence and inductively coupled plasma mass spectrometry reveal that the majority of the deposits are highly to extremely vesicular, hypocrystalline tephrite basanite scoria that display porphyritic, hyaloophitic, and vitrophyric textures. An intact scoria balloon recovered from the seafloor consists of an interior gas cavity surrounded by a thin lava shell comprised of two distinct layers; a thin, oxidized quenched crust surrounding the exterior of the balloon and a dark grey, tachylite layer lying beneath it. Ostwald ripening is determined to be the dominant bubble growth mechanism of four representative Foerstner scoria samples as determined by vesicle size distributions. ^ Characterization of the diversity of deposit facies observed at Foerstner in conjunction with quantitative rock texture analysis indicates that Strombolian-like activity is the most likely mechanism for the formation of buoyant scoria bombs. The deposit facies observed at the main Foerstner vent are very similar to those produced by other known submarine Strombolian eruptions (short pillow flow lobes, large scoriaceous clasts, spatter-like vent facies). Balloons were likely formed from the rapid cooling of extremely vesicular magma fragments as a result of a gas-rich frothy magma source. The exterior of these fragments hyperquenched forming a vesicular glassy shell that acted as an insulating layer preventing magmatic gas in its interior from escaping and thus allowing flotation as densities reached less than 1000 kg/m3. We believe that lava balloon eruptions are more common than previously thought, as the eruptive conditions required to generate these products are likely to be present in a variety of submarine volcanic environments. Additionally, the facies relationships observed at Foerstner may be used as a paleoenvironmental indicator for modern and ancient basaltic shallow submarine eruptions because of the relatively narrow depth range over which they likely occur (200-400 m).^

Subject Area

Geology|Geochemistry

Recommended Citation

Joshua Kelly, "1891 AD submarine eruptive processes and geochemical studies of floating scoria at Foerstner volcano, Pantelleria" (2013). Dissertations and Master's Theses (Campus Access). Paper AAI1535821.
http://digitalcommons.uri.edu/dissertations/AAI1535821

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