Post-eruptive gravity changes from 1990 to 1996 at Krafla volcano, Iceland

The 1975-1984 Krafla lifting episode was a major lava- and dyke-producing e vent associated with the release of extensional strain accumulated over mor e than 200 years at the divergent plate boundary in North Iceland. The pres ent work provides a unique example of gravity decreases and increases susta ined over a long period following a major eruptive episode at a rift volcan o. After height correction, persistent net gravity decreases over the sourc e of observed Mogi-type deflation occur with gravity increases occurring fu rther away from this centre of deformation, Gravity decreases are interpret ed in terms of drainage from a shallow magma chamber. The net gravity decre ases require that at least 4 X 10(10) kg of magma must have been drained du ring the 6-year observation period, Assuming a density of 2700 kg m(-3), th is magma would occupy 1.5 X 10(7) m(3) and by analogy with results obtained for Kilauea, this implies a magma chamber volume change of 4.1 x 10(6) m(3 ), This is consistent with the chamber volume change deduced from ground de formation data assuming a Poisson's ratio of 0.25 and a Mogi source, Net gr avity 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, contr acting and draining magma body causes subsidence at the surface. These resu lts contrast with observations from the Askja caldera, Iceland, where post- eruptive deflation has been shown to be accompanied by negligible net gravi ty changes above the Mogi-type source in the caldera. Long-term post-erupti ve deflation and magma drainage have not been observed at subduction-relate d volcanoes; this may be a function of a difference in magma viscosity. (C) 1998 Elsevier Science B.V. All rights reserved.