A degassing event at Brimstone started releasing an escalating number of bubbles (probably CO2) as the plume cloud increased in volume. Notice the pieces of sulfur at the base of the cloud.
NOAA
2006-04
<a rel="nofollow" class="external free" href="http://oceanexplorer.noaa.gov/explorations/06fire/logs/hires/bubbles_hires.jpg">http://oceanexplorer.noaa.gov/explorations/06fire/logs/hires/bubbles_hires.jpg</a>