Mapping Volcano 'Smoke'

An Eruptive Plume Above the Volcano

Sharon Walker
NOAA Vents Program

Turbidity data from the CTD tow (black sawtooth line) show a cloudy eruption plume over the volcano (reds) and clearer water elsewhere (blues). (click image for full size)

Before our first Jason dive we conducted a detailed survey of the water column with a CTDO (conductivity-temperature-depth-optical) instrument package that includes 20 large bottles for sampling the water. We want to measure the physical properties of the ocean surrounding the volcano, as well as map the extent of any plumes generated by eruptive activity and sample the water for chemical analyses. The optical sensor tells us where the most particle-laden parts of the plume are. We also have sensors that give us a general indication of the chemical variability within the plume (for pH and oxidation-reduction potential), and others that measure particle sizes. We raise and lower the CTD through the water as we tow it to distances up to 4 km away from the

Deployment of CTD on ship. Gray cylinders are water sampling bottles triggered by scientists at specific depths.
(click image for full size)

On our first CTD tow we found an intense plume over the summit, similar to what we've seen during our past visits when the volcano was erupting. This plume is so full of particles that our measurements reach values near the upper limit of our optical sensor each time we've passed near Brimstone Pit, the active eruptive vent. Our chemical sensors respond strongly near the summit too - another indication the volcano is still active. This is the first evidence on this expedition that the volcano is probably still erupting!

In addition to mapping and sampling the eruptive plume over the summit of the volcano, we are interested in finding out what is happen
ing deeper along the flanks. In 2004 and 2006, we observed multiple layers of turbid, particle-laden water surrounding the volcano at depths much deeper than the eruption site. We believe these particles were transported there due to landslides. We know that freshly erupted material accumulates on the steep slopes around the active vent and can become unstable. When it becomes unstable it slides down the flanks of the volcano, sending clouds of finer particles off into the surrounding ocean. It is interesting that we have not seen any deep particle layers so far during our CTDO tows this visit, suggesting that no slides have happened recently.

We don't know how often these slides occur or
what sorts of events trigger them, but we've come this time prepared to try to answer that question. We will be deploying instruments that will stay here for the next year and measure any turbidity layers deep along the flanks that might develop. Current meters will monitor the currents around the volcano and a hydrophone will let us know when eruptions occur and how intense they are. We hope we will catch one or more slide events in the act and be able to find out what triggers them.

View a slide show of the CTD instrument being launched from the R/V Thompson: