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Conferència: Mafic intrusions triggering eruptions in Iceland and Canary Islands. SEMINARIS DE LA FACULTAT DE GEOLOGIA I L'INSTITUT DE CIÈNCIES DE LA TERRA 'JAUME ALMERA'
Dr. Olgeir SIGMARSSON (Lab. Magmas et Volcans, CNRS-U. Blaise Pascal, Clermont-Ferrand, França; Institute of Earth Sciences, University of Iceland, Reykjavík, Islàndia)
The last two eruptions in Iceland, Eyjafjallajökull 2010 and Grímsvötn 2011, and the submarine eruption off the coast of El Hierro were all provoked by an intrusion of more mafic magma into pre-existing magmatic system. Injection into Grímsvötn volcano, which is located in the main rift-zone of the Iceland, above the presumed centre of the mantle plume and is the most active volcano of Iceland, has been gradual since the last eruption in 2004. In contrast, at Eyjafjallajökull volcano, one of the least active volcano in Iceland and located at the southern part of a propagating rift-zone where extensional tectonics are poorly developed, mafic magma intrusion occurred over less than a year. Similarly, intrusion of deep-derived basanite under El Hierro started several months before the October 2011 eruption.
Beneath Eyjafjallajökull, alkali basalt was injected into residual rhyolite from the penultimate eruption in the years 1821-23 at approximately 6 km dept. The resulting magma mingIing process was highly complex. The time between intrusion and eruption can be shown to be short, leading to the production of very heterogeneous and fine-grained tephra during the first days of explosive eruption. Several recharge events occurred until the end of the eruption. The proportion of basaltic mixing end-member declined during the eruption and its exhaustion caused the eruption to stop.
At Grímsvötn, a magma chamber with a closed-system behaviour, since the Laki eruption in 1783-84, has been inferred from very regular concentration increase of the incompatible element Th in an otherwise homogeneous qz-normative basalt measured in tephra from the last two centuries. The 2011 eruption was of short duration, or a week, but its eruption column rose higher than 20 km during the first hours and therefore penetrated into the stratosphere. Such forceful eruption has not been documented before at Grímsvötn volcano although the tephra record suggests several large explosive eruptions during Postglacial time.
At El Hierro, the basanitic intrusion remobilized small silicic magma pockets presumably at the interface of the old oceanic crust and the volcanic island. The composite pumice that floated on the sea-surface on 15 October is composed of basanite with a rhyolitic foam interior. The rhyolite has identical Nd isotope ratio to those of El Hierro basanites but very different to those of sediments derived from NW Africa. Magma differentiation of basanite with important fractionation of accessory minerals produced a trachyte that upon 10% assimilation of quartz became a rhyolite. The rhyolite formation occurred rapidly under highly oxidized condition with important volatilization of uranium.
Dia: 30 de maig de 2012
Hora:12:00
Lloc: Sala d'Actes de l'Institut Jaume Almera