Net gravity decrease at Askja volcano, Iceland: Constraints on processes responsible for continuous caldera deflation, 1988-2003

Elske de Zeeuw-van Dalfsen*, Hazel Rymer, Freysteinn Sigmundsson, Erik Sturkell

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

47 Citations (Scopus)


Askja caldera in northeast Iceland has been in a state of unrest for decades. Ground-deformation surveys show that the rate of deformation, i.e., deflation, is much higher then observed at any other dormant volcano in Iceland. This work presents the results from microgravity and deformation studies at Askja from 1988 to 2003. The deflation reaches a maximum of -0.46 m in the centre of the caldera, relative to a station outside the caldera, during the study period. The source of deformation is inferred to be at ∼3 km depth and a recent study infers a second deeper source at ∼16 km depth. The deflation is consistent with a subsurface volume change of -0.018 km3. We find a net microgravity decrease of 115 μGal in the centre of the caldera relative to the same station. This corresponds to a subsurface mass decrease of 1.6 × 1011 kg between 1988 and 2003 based on the use of a point source model. A combination of magma drainage and cooling and contraction of the shallow magma reservoir at 3 km depth is our favoured model, consistent with the integrated observations. We suggest that extensional tectonic forces generate space in the ductile part of the crust to accommodate ongoing magma drainage from the shallow magma chamber.

Original languageEnglish
Pages (from-to)227-239
Number of pages13
JournalJournal of Volcanology and Geothermal Research
Issue number3-4
Publication statusPublished - 15 Jan 2005

Bibliographical note

Funding Information:
This work was supported by The Geological Society, London and an Open University studentship. H.R.'s work on this project was supported by the Royal Society. We would like to thank all the people who have participated in collection of data and discussions, especially Halldór Ólafsson, Corinne Locke, John Cassidy and Andrew J. Ball. F.S.'s work on this project has been supported by the Retina project (EVG1-CT-00046). The geodetic work on this project was supported by a grant from the Icelandic Research Council RANNIS. An earlier version of the manuscript was greatly improved by discussion with Glyn Williams-Jones. We thank Maurizio Bonafede and an anonymous referee for useful reviews.

Other keywords

  • Askja
  • Caldera unrest
  • Deformation
  • Magma drainage
  • Microgravity


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