Kathy Cashman
University of Oregon
 As we watched from the Jason ROV, a spire of lava (to the right of the sulfurous clouds) was extruded from the Brimstone vent. |
But first to define some terms. Geologists use the word “magma” to describe molten rock beneath the Earth’s surface, where it consists of melt with dissolved gases (think of it as champagne in the bottle, before the cork is popped). The word “lava” is used when the magma has reached the surface, by which time it has lost most of its gases. Lava may be either very fluid, and form long thin flows (as in Hawaii), or very viscous (sticky) and form short thick lava flows and domes (as at Mount St. Helens, WA). The rate at which lava comes out of the vent also affects the flow behavior, with lower rates usually leading to shorter flows.
At Brimstone Peak, the magma is moderately viscous and it is coming out of the vent very slowly. Additionally, the seawater cools the lava rapidly. The result is that the lava solidifies as it emerges from the vent to form odd-shaped pillars and bulbous protrusions. Although from our viewpoint the emerging lava is usually veiled in plumes of sulfur ‘smoke’, we see evidence of its presence. New lava degasses vigorously for several minutes, thus the location of active lava extrusion can be tracked by watching the regions of vigorous degassing. Additionally, as the rapidly solidified magma comes out of the vent, it pushes on the surrounding lava, which causes these slightly older and cooler lava blocks to jostle together, move aside, and eventually break into pieces. In this way the ‘new’ lava quickly transforms to rubble that forms a growing apron around the active vent.
Is this how NW Rota grows? The answer to that question appears to be yes, at least over the past several years. We just completed a new high-resolution bathymetric survey of the volcano’s summit, to compare with a similar survey made in 2004. It appears that the vent, formerly known as Brimstone Pit, should now be called Brimstone Peak, because it is now a cone that stands almost 40 m (about 130 feet) high and about 100 m (330 feet) across. This growth has not been steady, however, as observations in 2006 showed that the growing cone had collapsed to produce a slide of loose debris down slope. As we continue to monitor and revisit NW Rota over time, we hope to assemble a more complete picture of the dynamic growth of this underwater volcano.
This time-lapse movie shows lava rising and pushing previously erupted blocks to the side in the vent over a time period of an hour and a half. The areas of the most intense degassing are where new lava is actively erupting (no audio). |
Watch closely as recently erupted (and already degassed) lava on the right side of the vent is shoved aside as new lava rises behind it and vigorously degasses. |
