Gudmundsson, Magnus Tumi; (1992) The crustal structure of the subglacial Grimsvotn Volcano, Vatnajokull, Iceland, from multiparameter geophysical surveys. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The crustal structure of the subglacial Grímsvötn volcano, Vatnajökull Ice Cap, Iceland, has been studied with shallow seismic reflection, magnetics, gravity, seismic refraction and recent data on bedrock topography. Grímsvötn has been highly active in recent times and has developed three calderas, the east (18 km2), north (11 km2) and the main caldera (20 km2). The subglacial topography in the area is characterized by hyaloclastite ridges, formed by subglacial volcanism. These ridges are made of low density nonmagnetic hyaloclastite tuffs with abundant pillows in places. It is inferred that hydrothermal alteration has reduced the magnetization of the rocks in places, especially within the calderas. The floor of the 300-600 m deep main caldera dips gently from south to north and parts of it are covered with lava flows. The caldera fill is at least 100 m thick, made of volcanoclastics interbedded with lava flows and sills. The uppermost 2.5 km of the crust are a region of rapidly increasing P-wave velocity and density. A Bouguer anomaly high over the volcano is caused by a basic intrusive complex below 2 km depth. This complex has a minimum volume of 500 km3 Semicircular Bouguer anomaly and magnetic highs may be the expression of a cone sheet swarm, 8-9 km in diameter. On the basis of the bedrock morphology, the east caldera is considered to be the oldest while the other two appear to be recent formations. Within the main caldera the number of subglacial mounds created by eruptions is roughly equal to that of recorded eruptions over the last 200 years. It is proposed that the main and northern calderas formed when magma was drained laterally some 50 km to the southwest at the time of the eruption of the 12 km3 Laki lava in 1783. The very high heat flow (about 5000 MW) observed in Grímsvötn over the last 100-200 years may be a consequence of the caldera collapse. It is thus considered to be a transient feature and the decline in heat flow observed over the second part of this century is believed to be due to decreased volcanic activity.

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