Eruption of Grímsvötn volcano (Iceland), May 2011
Image of the eruption column of Grímsvötn and of the ash cloud moving counterclockwise around the southern and eastern sectors of Iceland (the outlines of the island are shown as a thin black line). Photo was acquired by the spectroradiometer MODIS on NASA's Aqua satellite at 05:15 GMT on 22 May 2011. (Source: NASA MODIS Rapidfire)
On the late afternoon of 21 May 2011, a new eruption started at the most active volcano of Iceland, Grímsvötn, whose summit caldera lies in the western portion of the huge Vatnajökull glacier, in the southeastern portion of the island. Grímsvötn had previously erupted in late 2004.
The eruption was preceded by about 1 hour of strongly increased seismicity, but inflation of the volcano had been observed for several years, which lately had been accompanied by numerous earthquakes and a few episode of increased volcanic tremor. The onset of the eruption was quite sudden and violent: around 18 to 19 GMT, an eruption column shot up, rising to at least 17 km height in a short time, and causing heavy ash falls toward south and east. Both in terms of eruption column height and mass eruption rates, the initial phase of this eruption was far more intense and significant than that of the May-April 2010 eruption of Eyjafjallajökull, in the south of Iceland.
The great eruption cloud rising from Grímsvötn on the evening of 21 May 2011, photographed by Ólafur Sigurjónsson. (Source: Veðurstofa Íslands, Icelandic Meteorological Office)
On 22 May, the eruption continued at elevated intensity, causing ash falls over a vast area of Iceland; the ash cloud produced by the initial phase of the eruption was meanwhile moving toward the British Isles, where interruptions in the air traffic were announced. Ash emission continued, though at diminishing eruption rates, through 23 and 24 May, while air traffic in the British Isles and in Germany suffered interruptions. On the morning of 25 May, the eruption was observed to have all but ceased, and the volcanic tremor amplitude showed a marked reduction.
Map showing the projected distribution of volcanic ash, released with Advisory 2011/016 by the Volcanic Ash Advisory Centre (VAAC) of London, 25 May 2011, indicanding the presence of volcanic ash in the skies over Germany and over the Baltic states. The graphic also announces that "That eruption has ceased". (Source: VAAC London)
The composition of the ash emitted during this new eruption is basaltic, as in all recent eruptions of Grímsvötn; the strong explosivity of the activity was entirely caused by interaction of magma with a subglacial caldera lake and glacial ice.
Internet sources:
Institute of Earth Sciences (University of Iceland)
The Global Volcanism Program (Smithsonian Institution) entry for Grímsvötn