Update on activity Grímsvötn


Grímsvötn volcano
Status Report: 17:00 GMT, 23 May 2011
Icelandic Meteorological Office and Institute of Earth Sciences, University of Iceland
Compiled by:
Gunnar B. Guðmundsson, Níels Óskarsson, Einar M. Einarsson, Árni
Sigurðsson, Bergthóra S. Thorbjarnardóttir, Matthew J. Roberts and
Sigrún Hreinsdóttir.
Based on:
IMO seismic monitoring; IES-IMO GPS monitoring; IMO
hydrological data; weather radar; ashfall reports; UK Met Office
ATDnet; MODIS satellite images.
Eruption plume:
Height (a.s.l.):
The ash plume reached heights of 8 to 10 km last night and this
morning. In the last hours, the plume has reached heights of 5 to 9 km,
but northerly winds have been very strong which can effect the height.
Heading:
Most of the ash cloud heads to the south. At altitudes of 8 km and
higher, part of the plume heads to the west.
Colour:

Brown- or grayish and sometimes black close to the eruption site.
Tephra fallout:
The amount of fallout is great from Vík in the west to the east of
Öræfajökull. The amount of ash fall is the greatest close to the village
Kirkjubæjarklaustur. Ash has been detected in several areas throughout
the country, except in the northwest.
A sample from Kirkjubæjarklaustur has been analyzed, which was
taken around 1h on 22 May. The grains are glassy with micro crystals
of plagioclase. Samples well sorted.
Whole rock analysis: Basalt, with 50-51 Wt% SiO2
Leachate results: 5-10 mg/kg of waterdissolvable flour
Grain size distribution: about 10% of the volume of the analyzed
samples is finer than 10 micrometers
Lightning:
From 17-18h yesterday, about 300 lightning strikes were detected but
much less thereafter. The strikes were most frequent south of
Grímsvötn.
Noise:
No noise from the volcano has been reported.
Meltwater:
No changes in water level have been recorded in the rivers Gígja and
Núpsvötn. Since the eruption is practically at the same site inside the
Grímsvötn caldera as the last eruption, ice-melt is not expected to be
great and therefore swelling of rivers in the next few days is not
expected.

Conditions at eruption site: The eruption site is in the southwest corner of the
Grímsvötn caldera, in the same site as the 2004 eruption. The basaltic
magma is fragmented into tephra in violent magma-water interaction.
Very powerful explosions occur at the eruption site.
Seismic tremor:
Seismic tremor at the Grímsfjall station was fairly stable last night.
After midnight andtoday, the tremor levels have been fluctuating and
decreasing slightly.

Earthquakes:
No earthquakes have been recorded in the volcano since yesterday
afternoon.
GPS deformation:
Rapid deformation was detected at the CGPS station Grimsvotn
(GFUM) in the first hours of the eruption. GFUM is located 5 km east
of the eruption site. In the first four hours the site moved ~ 20 cm in
the north direction, 15 cm towards west and subsided 10 cm. The
deformation rate has since slowed down, with the total displacement in
the first two days of the eruption about 50 cm to the northwest, with 25
cm subsidence. These displacements are ~60% larger than comparable
measurements made after the 1998 and 2004 eruptions of Grímsvötn.
Overall assessment: The eruption has abated slightly since yesterday. No effusion of lava has
been observed.

Details in status report issued collectively by the Icelandic Meteorological Office and the Institute of Earth Sciences