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Ulawun volcano (Papua New Guinea): ash cloud from yesterday's major explosion dissipated, but SO2 plume over area

So, 4. Aug 2019, 18:09
18:09 PM | VON: T
SO2 plume from Ulawun over Papua New Guinea (image: NOAA)
SO2 plume from Ulawun over Papua New Guinea (image: NOAA)
SO2 plume over the area (image: http://sacs.aeronomie.be)
SO2 plume over the area (image: http://sacs.aeronomie.be)
Satellite image of Ulawun today (image: Copernicus EMS @CopernicusEMS / twitter)
Satellite image of Ulawun today (image: Copernicus EMS @CopernicusEMS / twitter)
No new significant ash emissions have been observed from the volcano today, but satellite images show strong heat and intense steaming from the summit crater.
As well, fresh deposits of pyroclastic flows from yesterday's eruption can be recognized on satellite imagery.
It seems that these flows fortunately did not reach great distance beyond the base of the steeper summit cone. There are no reports of significant damage, partly this could be because most of the local residents near the volcano are still in evacuation centers.
Although no ash is visible on satellite data, VAAC Darwin continues to warn that some remaining ash plume could be drifting in up to 45,000 ft altitude towards the SW, but suggests that the concentration of residual volcanic ash is by now very low. The latest VVAC report expected that the ash is likely to have dissipated by now or will disappear within a few hours.
In contrast, a large SO2 plume from the eruption continues to be drifting and expanding over the area (SO2 aerosols stay suspended much longer in the atmosphere).

Why did the ash plume dissipate so quickly?
If the erupted ash is very fine-grained (micrometer range or below), it can stay suspended a long time, but if it is not (millimeter range or only little below), it will fall down much more quickly even from great heights.
This seems to have been the case, just as during the previous paroxysm on 26 June. Also this time, as images on social media show, a lava fountain was observed from the summit vent during the eruption.
This means that the lava was relatively fluid, as typical for Ulawun, comparably hot and fluid, probably close to basaltic or basaltic-andesitic composition.
The low viscosity of the erupted magma explains why the eruption did not produce extremely fine-grained ash: fragmentation occurs when gasses separate from the liquid phase (magma). If it is fluid, this process requires little energy and results in relatively large particles: spatter bombs, lapilli, and sand-sized ash, all of which can fall back rather quickly.

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