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Dynamic magma mixing revealed by the 2010 Eyjafjallajökull eruption

Author(s): O. Sigmarsson | I. Vlastelic | R. Andreasen | I. Bindeman | J.-L. Devidal | S. Moune | J. K. Keiding | G. Larsen | A. Höskuldsson | Th. Thordarson

Journal: Solid Earth Discussions
ISSN 1869-9537

Volume: 3;
Issue: 2;
Start page: 591;
Date: 2011;
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Injection of basaltic magmas into silicic crustal holding chambers and subsequent mixing of the two components is a process that has been recognised since the late seventies to have resulted in explosive eruptions. Detailed reconstruction and assessment of the mixing process caused by such intrusion is now possible because of the exceptional time-sequence sample suite available from the tephra fallout of the 2010 summit eruption at Eyjafjallajökull volcano in South Iceland. From 14 to 19 April the tephra contains three glass types of basaltic, intermediate, and silicic compositions recording rapid magma mingling without homogenisation, involving evolved FeTi-basalt and dacite with composition identical to that produced by the 1821–1823 AD Eyjafjallajökull summit eruption. The time-dependent change in the magma composition suggests a binary mixing process with changing end-member compositions and proportions, or dynamic magma mixing. Beginning of May, a new injection of deep-derived basalt was recorded by deep seismicity, appearance of magnesium-rich olivine phenocrysts together with high sulphur output and presence of sulphide crystals. Thus the composition of the basaltic injection became more primitive and hotter with time prowoking changes in the silicic mixing end-member from pre-existing melt to the solid carapace of the magma chamber. Decreasing proportions of the mafic end-member with time in the erupted mixed-magma, demonstrate that injections of Mg-rich basalt was the motor of the 2010 Eyjafjallajökull explosive eruption, and that its decreasing inflow terminated the eruption. Significant quantity of silicic magma is thus still present in the interior of the volcano. Our results show that detailed sampling during the entire eruption was essential for deciphering the complex magmatic processes at play, namely the dynamic magma mixing. Finally, the rapid compositional changes in the eruptive products suggest that magma mingling occurs on a timescale of few hours to days whereas the interval between the first detected magma injection and eruption was several months.
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