Magma mixing at the El Hierro submarine eruption
Floating scoria above the submarine vent when observed for the first time on Oct 15, 2011 (photo: IGN)
The Spanish Instituto Geográfico Nacional (IGN) just published an interesting report "INFORME PETROLÓGICO DE LA ERUPCIÓN DE LA ISLA DE EL HIERRO" - a petrographic analysis of the juvenile magma erupted during the (ongoing) submarine eruption south of El Hierro Island: The report (in Spanish) is found at: www.01.ign.es/ign/resources/volcanologia/pdf/Informe_petrologico_erupci%C3%B3n_Hierro.pdf
Cross-section of one of the bombs - the mechanical mixing of the 2 different magmas is well visible: the silica-rich white trachytic and the silica-poor black basanitic magma. It is evident that both were fluid at the time of mixing. (image: IGN)
The eruption started from a submarine fissure system on 10 October extending in a N-S direction in the submarine prolungation of El Hierro's southern rift zone, beginning at about 6 km distance from La Restinga at depth of 1000 m, until the closest vent at about 2 km distance and depth of ca. 300 m.
Above the vents, floating (still hot) scoria and bombs of up to 30 cm diamenter were first observed and sampled on 15 Oct.
The shape and texture of these scoria, with quenched surfaces due to rapid cooling with sea water, were similar to the scoria composing the cinder cones dotting the rift zone on land. This suggests that the eruption is a submarine equivalent of basaltic strombolian fissure eruptions.
The fragments were mainly black and of basanitic composition, i.e. extremely poor in silica (even less than basalt). They contain 43‐45% de SiO2 and about 2% of volatiles (H2O, CO2, Cl, S, ...) and were erupted at temperatures around 1200 ºC. Their density was about 2,700 kg/m3. A second juvenile magma component was observed as white pumice, often contained INSIDE and mechanically mixed with the black basanite lava: the white lava was analyzed to contain 63-72% silica (i.e. trachyte - rhyolite), 4-5 % volatiles, erupted at 850‐900 ºC and had densities of 2,300 kg/m3.
This felsic (=silica-rich material) was only observed in the first days of the eruption and constituted less than 10% of the total observed magma. The apparent mixing of magma might very well have been a trigger of the eruption.
New research on these floating rocks have shown that these rocks are highly radioactive to their content of Uranium and Thorium. The white part is probably a remain of mesozoic limestone (=skarn?). The white area of the rocks is rich in Uranium. So probably there was a kind of mixture and hydrothermal effects on magma from very deep areas in the earth crust.