March 21, 2010

The Landslide

Ash-covered, steep, unstable slopes around the volcano.

We had previously found evidence for the repeated build-up and collapse of the ash and lava around Brimstone vent at NW Rota, leading to landsliding down the steep southern flank. To try to capture these kinds of events, last year we deployed 4 moorings with temperature and turbidity sensors, current meters, a fluid sampler, and a hydrophone to monitor activity at the volcano between then and now.

In fact, we have discovered on this trip that the volcano experienced a major landslide between last year and this year. Such an event is exactly what we had hoped to capture, except that this one was far larger than anything we had anticipated and appears to have caught all four of the instrument moorings. After an extensive search, we have only been able to communicate from one of the moorings. This one was deployed downslope from the summit at a depth of 1500 m, and it was moved about 1 km downslope (to ~1830 m) by the landslide and the acoustic release reports that it is horizontal. We plan to make a Jason dive on this mooring to see what instruments survived and might be recovered. The data it recorded could be fascinating, but it is deeply disappointing that the other moorings appear to be lost, both for the loss of valuable equipment but also for the lost scientific data. We plan to search with Jason for all of them during subsequent dives, so we have not yet given up all hope of finding them.

Map 1:
By comparing the bathymetry we collected last year to data we collected this year, we can see major depth changes (see map 1). The landslide caused up to 100 m of depth change in the slide headwalls near the summit (blue areas on the map). Downslope of this scour are landslide deposits up to 40 m thick (red areas on the map) extending about 8 km from the summit and down to a depth of at least 2800 m. This landslide is at least an order of magnitude larger than anything we've seen in the last 7 years, and perhaps several orders of magnitude larger. It's huge. The red dots on the first map were the original mooring locations and the blue star is where we have located the 1500-m mooring (~1 km downslope). Amazing.

The changes in the summit area are equally staggering. The volcano summit is still in the same place and at the same height (probably a resistant lava plug), but the landslide event eroded back the summit ridge about 100 m to the north on either side, and one of the ridges SE of the summit has been replaced by a large landslide scar (on the 2nd map, the red line is the new summit ridge and the green dots are the mooring locations).
Map 2:
The lava cone that had built up last year at Brimstone vent was removed by the slide, and a new smaller lava cone is building up in its place. Brimstone vent is in the sample location, but is 25 m deeper, and we now we have found two other actively erupting vents within 50 m. Most of the hydrothermal vent sites we visited last year have been wiped out, but a few survived, and new ones have replaced the old. The biological community at the volcano summit has been severely impacted by the landslide. Amazingly, the two species of shrimp are still hanging on, although one species is now much more abundant than the other and there seems to be far more juveniles than adults. The limpets and barnacles (both new species only found here) that we saw last year have almost been wiped out. Also, there are far fewer areas of microbial mat. Life is tough at an active volcano!

-Bill Chadwick, Oregon State University