1. Bullard Laboratories; Department of Earth Sciences; Madingley Road, Cambridge CB3 0EZ UK
2. Nordic Volcanological Institute; Reykjavik Iceland
3. Department of Geology and Geophysics; Woods Hole Oceanographic Institution; Woods Hole Massachusetts 02543 USA
4. Division of Geological and Planetary Sciences; California Institute of Technology; Pasadena California 91125 USA