The petrology of basalts from Surtla (Surtsey), Iceland

Surtla is the site of a short-lived submarine vent which built basaltic elastic deposits almost to sea level, in 1963, early in the eruption of Surtsey. Since then wave and current activity have eroded the volcanic pile such that in July 1981 its top was a fairly level plateau 45 m below sea level, and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules. This winnowed layer was underlain by a nonreworked, poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles, steam explosions and quench brecciation. During the eruption, the explosion violence and associated comminution increased as the pile built up to shallower water depths. It is argued that at times of continuous effusion a cupola of steam was situated over the vent, as indicated by scoriaceous spatter which shows agglutination and “bread-crust” features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption.