Abstract

Eruptions from partly ice-covered and ice-capped volcanic systems constitute nearly 60% of all known historical (i.e. in the past 11 centuries) eruptions in Iceland. Since the fourteenth century such eruptions have been reported in contemporary or near-contemporary documents. At least 120 historical eruptions have broken through the ice on the glaciated parts of five volcanic systems and have left tephra layers in ice and soil, or been recorded at the time. An unknown number of eruptions did not breach the overlying ice and left no record at all. Beginning as subglacial eruptions, most eruptions break through the ice in minutes, hours or days and can last from a few days to several months. A single vent or the whole length of a fissure may then emerge to emit highly-fragmented tephra in hydromagmatic explosions of varying strength. The volume of airborne tephra varies by at least four orders of magnitude with dispersal range varying from near-vent to transatlantic. In most of the eruptions the magma was of basaltic composition. Eruption frequency is highest within the Grímsvötn system where up to seven eruptions every 40 years have occurred during peaks of activity and at least 70 eruptions over historical time. The observed pattern of temporally and spatially close eruptions, separated by periods of low or no activity, leaves open the question whether pre-Holocene deposits of several closely spaced subglacial eruptions can be securely distinguished from those formed in a single subglacial eruption.