[Comment on “Satellite radar images capture a subglacial volcanic eruption in Iceland”] Comment: Subglacial eruptions and synthetic aperture radar images



In a news piece published in the May 4, 1999, issue of Eos,Garvin et al. present RADARSAT satellite synthetic aperture radar (SAR) images of the subglacial volcanic eruption in the Gnmsvotn caldera within the Vatnajokull ice cap, Iceland, that occurred December 18–28, 1998. As pointed out in this piece, the images show considerable changes in surface backscatter of the radar signal within the caldera during and after the eruption. The authors suggest that the dark patches with an area of approximately 4 km2, observed within Grímsvötn, are open water. On the basis of the size of the dark patches, Garvin et al. estimate the volume of ice melted to have been about 1 km3 and the energy provided for melting, E = 3 × 1017 J, to have been about 30% of that of the 1996 Gjalp eruption, which occurred 12 km north of the 1998 eruption site [Gudmundsson et al., 1997]. Our groundbased and aerial observations at Gnmsvotn, however,show that this estimate of melting and energy release is far too high.