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In the Gjálp eruption in 1996, a subglacial hyaloclastite ridge was formed over a volcanic fissure beneath the Vatnajökull ice cap in Iceland. The initial ice thickness along the 6 km-long fissure varied from 550 m to 750 m greatest in the northern part but least in the central part where a subaerial crater was active during the eruption. The shape of the subglacial ridge has been mapped, using direct observations of the top of the edifice in 1997, radio echo soundings and gravity surveying. The subglacial edifice is remarkably varied in shape and height. The southern part is low and narrow whereas the central part is the highest, rising 450 m above the pre-eruption bedrock. In the northern part the ridge is only 150–200 m high but up to 2 km wide, suggesting that lateral spreading of the erupted material occurred during the latter stages of the eruption. The total volume of erupted material in Gjálp was about 0.8 km3, mainly volcanic glass. The edifice has a volume of about 0.7 km3 and a volume of 0.07 km3 was transported with the meltwater from Gjálp and accumulated in the Grímsvötn caldera, where the subglacial lake acted as a trap for the sediments. This meltwater-transported material was removed from the southern part of the edifice during the eruption. Variations in basal water pressure may explain differences in edifice form along the fissure. Partial floating of the overlying ice in the northern part is likely to have occurred due to high water pressures, reducing confinement by the ice and allowing lateral spreading of the edifice. The overall shape of the Gjálp ridge is similar to that of many Pleistocene hyaloclastite ridges in Iceland. Future preservation of the Gjálp ridge will depend on the rate of glacial erosion it will suffer. Besides being related to future ice flow velocities, the erosion rate will depend on the rate of consolidation due to palagonitization and shielding from glacial erosion while depressions in the ice are gradually filled by ice flow directed towards the Gjálp hyaloclastite ridge.

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