Steinunn Jakobsdottir, Matthew Roberts, Kristin Vogfjord, Ragnar Stefansson,
Icelandic Meteorological Office, Reykjavik, Iceland.
The eruption that started at the subglacial Grimsvotn volcano in the Vatnajokull ice cap, Iceland, on November 1, around 22 GMT (Sigmundsson et al., 2004) is now declining. Volcanic tremor decayed rapidly between November 2 and 4 and the eruption plume is greatly reduced, consisting now mostly of steam. Intermittent phreatomagmatic explosions were observed around noon on November 4, sending jets of ash and fragmented ice about 1 km above the crater. The eruption plume rose to a height of 2-4 km. The ash fall is limited well within the Vatnajokull ice cap.
The eruption takes place inside the Grimsvotn caldera near its SW boundary. The eruption was initially under 150-200 m thick ice and melted its way through the ice cap in about 1 hour. An eruption plume was detected by weather radar around midnight, and reached an altitude of 13 km during the night of November 2. The initial inspection of the eruption from an aeroplane around 8 GMT on November 2 confirmed that a phreatomagmatic eruption was in progress from a short (less than 1-km-long) eruptive fissure at 64.40N, 17.23W. At that time a continuous plume rose to an elevation of about 9 km. Observations throughout the day revealed periods of high explosive activity, with maximum plume heights of 12-14 km. The strength of the eruption correlated with the seismically recorded volcanic tremor. Some explosive activity had occurred in a second ice cauldron near the SE edge of Grimsvotn, 8-km to the east of the main crater. This cauldron issued steam when first detected in the afternoon of November 2.
The main tephra sector formed November 1-3 trends to north-northeast as a result of strong southerly winds. The sector is about 30 km wide near the north edge of Vatnajokull at a distance of 50 km from the eruptive site. Tephra fell in inhabited areas in north and northeast Iceland, but only in small quantities. The eruption plume was seen on satellite images and ash drifted over large parts of the North Atlantic and reached Scandinavia. Air traffic was disrupted; an area of 311 thousand square kilometres was closed for flights from the beginning of the eruption until the morning of November 4. Farmers sheltered grazing animals in North Iceland to prevent them from consuming soluble fluorine adhering to ash grains. On November 4, winds changed to north-westerly direction and in the afternoon tephra had been dispersed over the ice cap east of Grimsvotn.
After the onset of the eruption insignificant earthquake activity occurred at the eruptive site, but continuous low-frequency tremor has been recorded during the eruption. The tremor was steady for the initial 15-hours of the eruption. After that it was pulsating and declining.
The jokulhlaup, the glacial outburst flood that preceded the eruption by few days and triggered the eruption (Sigmundsson et al., 2004), reached a maximum in the afternoon of November 2. At that time the peak discharge in the rivers on Skeidararsandur was 3000-4000 m3/s (based on information from the Icelandic Hydrological Service). Discharge declined fast after the peak. No damage has occurred to roads or bridges. The total volume of the jokulhlaup is about 0.5 km3.
The eruption follows a pattern similar to previous eruptions in 1983 and 1998, with probably less than 0.1 km3 of magma erupted. These eruptions, together with the 1996 Gjalp eruption north of Grimsvotn reveal much higher activity at Grimsvotn than during the middle part of last century, and may indicate that Grimsvotn is entering into a new period of high volcanic activity that may last for decades. Such a high activity period has been predicted on the basis of the observed cyclic volcanic activity in the area in the preceding millennium (Larsen et al., 1998).
Larsen, G., Gudmundsson, M. T., and H. Bjornsson, Eight centuries of period volcanism at the center of the Iceland hotspot revealed by glacier tephrostratigraphy, Geology, 26, 943-946, 1998.