J÷kull 41, 21-45, 1993.
Magn˙s T. Gudmundsson and Helgi Bj÷rnssonAbstract
University of Iceland,
Dunhaga 5, 107 ReykjavÝk.
During the period 1934 to 1991 evidence has only been found for three or four volcanic eruptions within the GrÝmsv÷tn volcanic centre, i.e. the directly observed eruptions in 1934, 1938, 1983, and probably a small eruption in 1984, deduced from seismic tremors. Tephra layers observed by visitors in the northwestern part of the GrÝmsv÷tn depression in the period 1934 to the 1960's have been misinterpreted as signs of eruptions; the very same ash cover was observed throughout the period. This ash cover dates back to the eruption of 1934, but earlier GrÝmsv÷tn eruptions may have contributed to its formation. Reported openings in the ice shelf (1945, 1954, 1960) are considered not to be signs of eruptions but could be explained by either steam explosions of hydrothermal reservoirs sealed by impermeable caprock or by increased upwelling of hydrothermal fluid in reservoirs of high permeability due to pressure release during lowering of the GrÝmsv÷tn lake level in j÷kulhlaups. Frequent j÷kulhlaups in the period 1938-1948 can be adequately explained by high melting rate at the site of the eruption of 1938.
The eruptions of 1934 and 1983 produced hyaloclastites of volume 30-40 m³ and 10 m³, respectively. The eruption of 1938, on the other hand, produced volcanic material of the order of 400 m³ and may have been the third largest eruption in Iceland this century, after Hekla in 1947 and Surtsey in 1963-1967.