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 susta...
| Published in: | Journal of Volcanology and Geothermal Research |
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| Main Authors: | , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
1998
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| Subjects: | |
| Online Access: | https://oro.open.ac.uk/59/ https://oro.open.ac.uk/59/1/Rymer_1_-_post-eruptive.pdf http://www.elsevier.com/wps/find/journaldescription.cws_home/503346/description#description |
| Summary: | 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. |
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