Volcanoes which erupt subglacially have a distinct form, with a flat or slightly domed top. Herdubreid in northern Iceland erupted beneath the ice sheet which once covered most of the island during the last glaciation. The thickness of the ice sheet is indicated by the height of the steep cliffs. The meltwater torrent in the foreground is an outflow from Vatnajökull, the largest of the remaining icecaps in Iceland JA