Pleistocene subglacial volcanism in Iceland: tectonic implications

At several stages during the last 700 kyr, tectonic and volcanic activity d ue to Lithospheric spreading occurred beneath a 1000-1500 m thick ice cap i n Iceland. Magmatic activity has been recorded by the emplacement of subgla cial Volcanic edifices. Table volcanoes are the subglacial equivalent of ae rial shield volcanoes. Hyaloclastite ridges are the subglacial equivalent o f aerial eruptive fissures. Some hyaloclastite ridges are located in curren tly inactive areas, whereas they are nearly absent in some parts of the cur rently active Neovolcanic Zone. A part of this discrepancy can be attribute d to glacial erosion. A manual reconstruction of the flowing pattern of the ice cap, based on glacial landforms, shows that some parts of the Neovolca nic Zone were occupied by fast flowing ice streams. In these areas, most hy aloclastite ridges have been removed as eruptions proceeded: fast ice flow and water/debris flows triggered by volcanic eruptions have transported sub glacial volcanic products to the sea. Subglacial volcanic products have bee n preserved beneath ice divides, where ice motion was slower, and in some t able volcanoes, where magma supply was sufficient to counteract removal by ice how Once the effect of glacial removal has been subtracted, the arrange ment of the subglacial volcanic edifices appears clearly. Similarly to the post-glacial eruptive fissures, the hyaloclastite ridges are gathered in sw arms associated with central volcanoes located in the Neovolcanic Zone. How ever, the area covered by hyaloclastite ridges is wider than the extent of the currently active fissure swarms. This discrepancy suggests either conti nuous wandering of the volcanic activity from one fissure swarm to another for the last 700 kyr, or narrowing of the active rift zone at the end of th e last glaciation. (C) 1998 Elsevier Science B.V. All rights reserved.