March 28, 2010

A Brief History of NW Rota

When I began my career in oceanography 45 years ago I never dreamed I would ever be where I am today – sitting over an erupting volcano in the western Pacific.
Styx Vent does its part in supplying material for the growth of NW Rota-1.
Our long term study of this volcano began with the discovery of its gaseous volcanic plume in 2003 followed by the first dive to Brimstone Pit in 2004 when we realized we were the first human witnesses to a submarine eruption. At that time Brimstone was spewing gas clouds laden with molten sulfur and a small amount of ash. During subsequent dives in 2005 with the Japanese ROV Hyper-Dolphin and in 2006 using Jason, the activity increased with semi-continuous volcanic ash bursts and intense degassing with an occasional glimpse of red hot magma through the swirling sulfurous cloud. Last year Brimstone was slowly extruding spines of lava. This year we have found multiple vents with highly variable activity - no doubt due to the disturbance to the upper “plumbing system” resulting from the large landslide that occurred over the past year.
Rota Summit: Hydrophone, biologic experiment and marker precariously hold onto what’s left of the summit of NW Rota-1 at 518 meters. Black area is where Jason triggered small sediment slide when deploying instruments.

NW Rota is more or less a “typical” composite submarine volcano found along the Mariana volcanic arc. During its early growth in deep water eruptions produced primarily lava flows. Several smaller deeper submarine cones composed primarily of lava flows are found ~12 nautical miles (~20 km) east of NW Rota. As it grew shallower eruptive activity became more explosive as the pressure holding the gas dissolved in the magma lessened. This transition led to a gradual change in the form of the volcano because volcanic fragments such as ash and larger pieces can only build out to the “angle of repose” after which the material sloughs away into deeper water building out the flank of the volcano. We see this process on a small scale when Jason triggers slides in the newly deposited volcanic sand near the eruption sites. Yesterday we saw small scale mass movements at Styx vent when it suddenly became very active. In fact, the Jason was “shoved” away from the vent as material cascaded downslope in front of us. The large landslide that occurred here during the past year is part of this long term process of volcano construction.
Southern flank of NW Rota-1: White stained angular blocks on top of debris flow from summit litter slope near where we “heard” one of the lost moorings.

Some arc volcanoes undergo much more catastrophic events. A volcano called West Rota lying ~ 16 nautical miles (~30 km) SE of NW Rota had a very large eruption ~40,000 years ago when the entire top of the volcano was removed during an explosive eruption that formed a caldera (large crater) similar in size to Crater Lake in southern Oregon.

We have to appreciate that we are only looking at a short chapter in the life history of NW Rota. How long this “growth spurt” will last is anyone’s guess.

Bob Embley
NOAA Pacific Marine Environmental Laboratory
Vents Program