Post-Eruptive Gravity Changes From 1990 To 1996 At Krafla Volcano, Iceland

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Journal Article: Post-Eruptive Gravity Changes From 1990 To 1996 At Krafla Volcano, Iceland

The 1975-1984 Krafla rifting episode was a major lava- and dyke-producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in North Iceland. The present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano. After height correction, persistent net gravity decreases over the source of observed Mogi-type deflation occur with gravity increases occurring further away from this centre of deformation. Gravity decreases are interpreted in terms of drainage from a shallow magma chamber. The net gravity decreases require that at least 4_1010 kg of magma must have been drained during the 6-year observation period. Assuming a density of 2700 kg m-3, this magma would occupy 1.5_107 m3 and by analogy with results obtained for Kilauea, this implies a magma chamber volume change of 4.1_106 m3. This is consistent with the chamber volume change deduced from ground deformation data assuming a Poisson's ratio of 0.25 and a Mogi source. Net gravity increases are more spatially extensive and are most likely caused by migration of the steam-water interface and/or closure of micro-fractures in lavas above the magma chamber during post-eruptive cooling and contraction. We present a model for the Krafla magma chamber in which a cooling, contracting and draining magma body causes subsidence at the surface. These results contrast with observations from the Askja caldera, Iceland, where post-eruptive deflation has been shown to be accompanied by negligible net gravity changes above the Mogi-type source in the caldera. Long-term post-eruptive deflation and magma drainage have not been observed at subduction-related volcanoes; this may be a function of a difference in magma viscosity.

Hazel Rymer, John Cassidy, Corinne A. Locke and Freysteinn Sigmundsson

Published Journal 
Journal of Volcanology and Geothermal Research, 1998

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Hazel Rymer,John Cassidy,Corinne A. Locke,Freysteinn Sigmundsson. 1998. Post-Eruptive Gravity Changes From 1990 To 1996 At Krafla Volcano, Iceland. Journal of Volcanology and Geothermal Research. (!) .