by
Freysteinn Sigmundsson, Pall Einarsson, Magnus Tumi Gudmundsson, Niels
Oskarsson, Thordis Hognadottir, Anette Mortensen,
Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland
Steinunn Jakobsdottir, Matthew Roberts, Kristin Vogfjord, Ragnar Stefansson,
Icelandic Meteorological Office, Reykjavik, Iceland
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The eruption of Grimsvotn volcano, Iceland, which started on November 1
(Sigmundsson et al., 2004a) came to an end on November 6, with no
low-frequency volcanic tremor above background values recorded after 12
GMT. A minor steam plume emanated from the eruptive site after the end of
the eruption.
The eruption was phreatomagmatic throughout its duration. No lava was formed; all magma fragmented into pyroclasts that accumulated at the eruptive site or was carried by the eruption plume as tephra. The composition of the magma is quartz tholeiite, similar to earlier Grimsvotn eruptions. The eruptive fissure melted a circular 1 km diameter void in the ice cap. At the end of the eruption, steep about 100 m high ice walls surrounded the void. The whole 1 km long E-W trending eruptive fissure was active during the first days of the eruption, but only one vent was active on November 4 and 5. Eruptive material piled up at the main crater, forming a low relief island in a meltwater lake in the ice void at the end of the eruption. Although the eruptive site is located inside the Grimsvotn caldera, it is outside the Grimsvotn subglacial caldera lake. Preliminary estimates put the ice melting at 0.2 km3 at the eruptive site. This meltwater flowed towards the Grimsvotn subglacial lake which stored about 0.7 km3 of water prior to the onset of the jokulhlaup near the end of October. Some ice melting occurred in areas separate from the main eruptive site, including at ice depressions above the subglacial water flow path from Grimsvotn, but the melted ice volumes are small. The total volume of the jokulhlaup, the glacial outburst flood that had started prior to the eruption and which may have triggered it (Sigmundsson et al., 2004b) was only about 0.5-0.6 km3 (based on information from the Icelandic Hydrological Service). After the eruption, water continues to leak from Grimsvotn. However, a large increase in the flow or another jokulhlaup is not anticipated in relation to the eruption. Even though air traffic was disrupted in a large area, the tephra sector was thin outside of the Vatnajokull ice cap. At the northern edge of the ice cap the tephra sector was about 1 cm thick. In inhabited areas further north its thickness was less than 1 mm and ash particles did not form a continuous layer. The November 1-6, 2004 eruption of the Grimsvotn volcano was of similar duration as an eruption in 1983, but somewhat shorter than its 1998 eruption that lasted 10 days.
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