Pleistocene subglacial volcanism in Iceland: tectonic implications

At several stages during the last 700 kyr, tectonic and volcanic activity due to lithospheric spreading occurred beneath a 1000–1500 m thick ice cap in Iceland. Magmatic activity has been recorded by the emplacement of subglacial volcanic edifices. Table volcanoes are the subglacial equivalent of aerial shield volcanoes. Hyaloclastite ridges are the subglacial equivalent of aerial eruptive fissures. Some hyaloclastite ridges are located in currently inactive areas, whereas they are nearly absent in some parts of the currently active Neovolcanic Zone. A part of this discrepancy can be attributed to glacial erosion. A manual reconstruction of the flowing pattern of the ice cap, based on glacial landforms, shows that some parts of the Neovolcanic Zone were occupied by fast flowing ice streams. In these areas, most hyaloclastite ridges have been removed as eruptions proceeded: fast ice flow and water/debris flows triggered by volcanic eruptions have transported subglacial volcanic products to the sea. Subglacial volcanic products have been preserved beneath ice divides, where ice motion was slower, and in some table volcanoes, where magma supply was sufficient to counteract removal by ice flow. Once the effect of glacial removal has been subtracted, the arrangement of the subglacial volcanic edifices appears clearly. Similarly to the post-glacial eruptive fissures, the hyaloclastite ridges are gathered in swarms associated with central volcanoes located in the Neovolcanic Zone. However, the area covered by hyaloclastite ridges is wider than the extent of the currently active fissure swarms. This discrepancy suggests either continuous 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 the last glaciation.