Fire and Brimstone

NW Rota-1 is Active!

Kathy Cashman
University of Oregon

The Volcano is active! Large plumes continuously billow up from Brimstone Pit. Plumes carry CO2 bubbles and ash particles. (click image for full size)

As the time approached for our first Jason dive, the anticipation in the air became p
alpable. We had been preparing for this dive for two years, and during this time had been monitoring the volcano’s activity through glimpses provided by other cruises in the area. In February 2008, a hydrophone and plume sensor were deployed on a mooring at the summit of NW Rota-1 volcano, with the help of the US Coast Guard ship Sequoia. The mooring stayed down for a year and was retrieved in February 2009. The data from both the hydrophone and plume sensor clearly showed that the volcano had been very active from February through August 2008 and less active from August through February 2009.

Ash from plume after a large burst of activity, covers the basket of instruments in front of ROV Jason.
Just after the hydrophone was recovered, the Japanese research vessel Natsushima sent their ROV Hyperdolphin to have a quick look, and they found the volcano in a state of low level eruptive activity. But volcanoes are notoriously fickle and unpredictable. What would we find when Jason reached the bottom on this visit?

Our first Jason dive confirmed the previous day’s evidence from the CTD survey that Brimstone Pit, the active eruptive vent, is still active and well named. It sits southwest of, and slightly below, the volcano’s summit and is currently at a depth of 522 m below sea level, almost 40 m higher than it was when we last visited it three years ago. The vent is about 2 m across and formed by a loose pile of lava blocks. It appears to be continuously active, emitting billowing ‘clouds’, large bubbles of gas, and an episodic rain of small lava fragments. Unlike volcanic plumes on land, however, the ‘clouds’ are not formed of steam but instead are composed of tiny dispersed droplets of molten sulfur, or brimstone, an ancient name for sulfur. The bub
bles are filled with CO2, all that remains of the magmatic gas after the original water and sulfur have condensed to liquid (water) or solid (sulfur) phases. The lava fragments provide hints of what is happening in the vent, behind the curtain of sulfur clouds.

My personal reaction to my first glimpse of Brimstone Pit was not only excitement but also amazement and awe. I am a volcanologist who has spent my career traveling the world to study active volcanoes. But this was my first experience with a volcano erupting under the ocean – an ocean that on the surface, looks unremarkable, with no hint of what is happening below. My reaction was to try to understand the activity that I was seeing on the video monitors by comparing it with volcanoes that I have studied on land, particularly the volcano Stromboli in Italy, which is also perpetually active.

'Within the 'smoke', many gas bubbles of CO2 can be seen. Typically after bubbles appear, activity at Brimstone would increase, actually shaking the ROV. (click image for full size)
Volcanic eruptions occur when magma rises from depth to the surface. On land, eruptions are explosive when magma rises fast enough for the dissolved gases to form bubbles within the magma, and then expand violently to erupt a mixture of gas (from the bubbles) and magma (which cools to form solid fragments). A common analogy for this process is the explosive opening of a pressurized bottle of champagne. In contrast, when magma rises slowly, the bubbles can escape freely and magma erupts passively to form lava flows and domes. Underwater, conditions that produce explosive eruptions are probably similar to those that operate in terrestrial environments, as evidenced by the presence of large, submarine, pumice-filled calderas, such as those south of Japan. However, when magma ascent is slow, as is the case at Brimstone Pit, the seawater alters volcanic activity in important ways, particularly because of the capacity of cold seawater to absorb heat. This has the dual effect of causing rapid condensation of many gas species (as described above) and rapid quenching of lava, which causes the magma to shatter into the small, angular fragments that we have seen raining down from the volcanic plume. The weight of the overlying water also helps to limit the energy of the activity, which allows Jason to view and sample the vent activity at close range (from only 1 or 2 meters away), closer than we will ever be able to approach explosive volcanic vents on land!

Thus, our first glimpse of Brimstone Pit on this trip shows a volcano in continuous activity, with a vent that has built to a height almost equal to the volcano’s summit and producing a eruption plume composed primarily of particulate sulfur, CO2 bubbles and lava fragments. How representative is this snapshot of the volcano’s normal range of eruptive activity? We’ll find out over the next few weeks!

Volcano Observation Videos:
All video copyright by Advanced Imaging and Visualization Lab WHOI

White billowing eruptive plume full of sulfur and ash coming out of Brimstone vent (no audio).

Clear bubbles rising from the eruptive vent are mainly CO2, while the white cloud is dominated by sulfur from SO2 (no audio).