In the early morning of Monday 1 November 2004, after a run of of seismic activity that started in mid 2003, there was an earthquake beneath Grímsvötn, Iceland (see map). On the same day, an eruption warning was sent to the Civil Defence shortly before tremors indicated that the eruption of Grimsvötn had started (at 21:50 UTC). The eruption plume was first detected by weather radar around midnight. It reached an altitude of 13 km. However, as reported by the Nordic Volcanological Institute, there were eruptions pulsed resulting in a changing eruption column height from 8-9 km up to 13-14 km. On 3 November 2004, the ash plume reached Norway, Finland and Sweden (see chart showing significant weather) causing the diversion of trans-Atlantic flights to the south of Iceland to avoid the ash cloud, and the canellation of domestic flights to the northeast of Iceland. The Dutch airline KLM had to cancel 59 flights, stranding hundreds of passengers at Amsterdam's Schiphol Airport because of the cloud of ash "hanging above Europe" (suspended ash particles can cause abrasion inside the engine that can result in stalling and shutdown). From 3 to 5 November 2004, the intensity of the eruption decreased substantially. Finally, as reported by the Institute of Earth Sciences at the University of Iceland, the eruption of Grimsvötn volcano ended on 6 November 2004.
Looking at the Meteosat-8 images below, the volcanic cloud is best visible in the high-resolution visible (HRV) channel, which shows a highly-reflective cloud stationary over the Grimsvötn area. At 09:30 UTC, with the sun shining at low elevation angle from the south-east, the volcanic cloud produces a distinct shadow on the lower- level clouds that surround the volcano. From the length of that shadow and the sun zenith angle, one could calculate the height of the volcanic cloud above the low-level clouds.
According to observations (see also the pictures of the eruption), the volcanic plume was mainly formed of steam and ash, but it also contained sulfur dioxide. The existance of sulfur dioxide within the plume is confirmed by the WV7.3 - IR13.4 and the IR10.8 - IR8.7 brightness temperature difference images at 14:00 UTC that clearly show the volcanic plume. The IR10.8 - IR8.7 difference shows values of about -5 K, which indicates that the plume at this time reached the lower stratosphere (about 13-14 km) where the temperature increases with height. Earlier, at 11:00 UTC, the volcanic plume is also well visible in the difference images, but the difference IR10.8 - IR8.7 is positive (about + 6 K), indicating that the volcanic plume was below the tropopause (about 8-9 km) where the temperature decreases with height.
Animation of the IR10.8 channel data confirms the pulsating character of the Grimsvötn eruption on 2 November 2004: between 12:00 and 13:30 UTC the top of the volcanic plume cooled down from about -20°C to -55°C. Also the HRV channel shows that between 12:00 and 13:30 UTC an intensive eruption took place that resulted in an increase of the height of the volcanic cloud.
As regards the ash, from the IR10.8 - IR12.0 difference images it is difficult to confirm the presense of ash within the volcanic plume. Normally, this difference should be negative for ash clouds. An explanation could be that there was too much water vapour in the volcanic cloud and/or that the ash particles were too big/heavy so that most of them dropped down in the vicinity of the volcano. The latter is confirmed by the fact that the ash fall deposit was limited to the areas close to the volcano (see MODIS image).
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