Soufriere Hills volcano, on the island of Montserrat (16.7 N, 62.2 W, 2000 m) has been in a state of eruption since 18 July, 1995.
20 May 2021
21 May 2006
By Fred Prata (NILU), HansPeter Roesli and Jochen Kerkmann (EUMETSAT)
On the morning of 20 May 2006, after several months of growing lava dome, the eastern flank of the volcano suffered a major collapse with an estimated volume of 90 million m3 of lava, much of it flowing into the ocean. The highly buoyant, phreamagmatic eruption caused a column of ash and gas to rise to heights of at least 17 km and the gases penetrated the tropopause and entered the stratosphere, reaching as high as 20 km.
The initial phase of the eruption was not captured well by the polar orbiting satellite instruments due to mismatch between the timing of the satellite passes and the onset of the eruption. However, sensors on board MSG SEVIRI were able to image the eruption cloud within 15 minutes of the eruption.
The SEVIRI data suggest that in the early phase of the eruption the ice cloud masked the SO2 signature, either because the radiative effects of the ice particles prohibited retrieval of SO2 or because much of the SO2 gas was absorbed onto the ice particles, only to sublimate at a later stage as the ice evaporated. Lowest temperatures at the centre of the eruption cloud were 200 K, while there is evidence of warmer temperatures (210 K) near the cold core suggesting overshooting and penetration into the stratosphere.
The change of the character of the volcanic cloud, from ice cloud (masking the SO2) in the initial phase to a more pure SO2 cloud at a later stage (on 21 May), can be seen by the change in the colour of the cloud, both in the Dust and in the Airmass RGB composites (see link to the animations below the images).
For instance in the Dust RGB, initially the volcanic cloud appears with a red colour (indicating a thick ice cloud) with dark blue borders (indicating a thin ice cloud). Later on, at about 18:00 UTC on 20 May, the entire cloud appears like a thin ice cloud. In the Dust RGB, the SO2 starts to be visible (by a greenish colour) after 19:00 UTC on 20 May to become the dominating feature on 21 May.
AIRS SO2 retrieval (21 May 06:35 UTC, Credit: Fred Prata)
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