A raft of pumice in the Pacific

A part of the pumice raft
Photo from CNN.com

Driving to work this morning, I heard a headline on NPR about a floating raft of rocks out in the Pacific and, sure enough, Google worked and I found the pictures! On Thursday, pilots and sailors from the New Zealand navy discovered pumice, sometimes inches high and looking like floating ice, covering an area estimated to be more than 7,500 square miles. It appears to be an oblong area 250 x 30 nautical miles in dimension.  Volcanologists love to try to reconstruct the "equivalent magma volume" that it would take to produce this--was it a cubic mile? a fraction of a cubic mile? However, unless we know how much of the surface it covers (1%? 50%?), what it's average thickness is (a few inches?), and especially, what the density of individual pieces is, we can't make that calculation. We do know for sure that the density is less than 1 g/cm3 (the density of water), and some older studies of pumice from a Japanese caldera eruption showed that the density varied between 0.211 and 0.777, with an average of 0.51. My guess (with no measurements at all) is that the pumice in the above photo covers about 5% of the frame, and this is probably a photo selected because it has lots of pumice. Let's assume that it covers 1% of the 7500 square miles, to an average depth of 6". If I did my math right, this is a depth of 0.0001 mile, and so the volume of the pumice would be 7500*0.0001 = 0.75 cubic miles. If the average density of the pumice is about 0.5, then the erupted volume would have been a few tenths of a cubic mile.  A pretty healthy burp.

***Note added on August 11: Scientists initially (as of yesterday) thought that the eruption was from Monowai, a volcano that I posted about an eruption a year ago. However, as of today, they have determined that the raft was spotted first on July 19, whereas Monowai didn't erupt until August 3. They now believe that the eruption was from an unknown volcano about half way between New Zealand and Tonga. There were more than 157 earthquakes between magnitude 3 and 4.8 in this area between July 17 and 18. This information is from Stuff.co.nz. There is also a 36 second video from the air of the raft on this site. My guess from looking at the video is that my estimate above of an aerial coverage of 1% may be high by as much as an order of magnitude because the still photo in the upper left is definitely one of the highest density areas. So, perhaps a few hundreths of a cubic mile equivalent magma volume, with a few tenths as an upper bound.

Pumice rafts have been observed before. There has been speculation (sorry, I don't have a reference) that they could have been a platform on which life originated on earth--a platform on which a soup of organic chemicals from hydrothermal vents in the ocean could have accreted, been zapped by lightning to form the complex organic molecules leading to life. It has also been proposed that life could have migrated from island to island in the Pacific, or from continent to island, e.g., South America to the Galapagos, on such rafts.

More recently, Scot Bryan of Queensland University of Technology, has studied pumice rafts from the 2001 and 2006 eruptions in Tonga, and suggested that they could have been crucial in the formation, evolution, and future of the Great Barrier Reef off of northeast Australia. Bryan's studies showed that the initial 440 square kilometer floating mass of pumice broke into streaks on which "millions to billions" of marine organisms such as cyanobacteria, barnacles, molluscs, corals, anemones, and crabs, started hitchhiking. As the fragmented raft wandered more than 5000 kilometers over 8 months, 80 species of plants and animals journeyed along with it. When the corals, coralline algae, anemones, and other reef dwellers that were hitchhiking got the the Reef, they simply decided to call it home (well, a bit of an anthropomorphism!!).

Bryan views this as a positive thing because it may indicate that volcanic activity in the Pacific can replenish the Reef, which is stressed and dying in the warming waters around it.  On the downside, however, there are some marine pests such as sponges and mussels that journey along as well.

Bryan's research is featured in this Phys.org article from which I obtained this information, and in the technical article by Bryan et al., "Rapid, Long-Distance Dispersal by Pumice Rafting," in PLOS-ONE, a peer-reviewed, open access journal.

Monowai volcano: fast outpouring of magma observed over 5 days

Bathymetry of summit of Monowai Cone May/June 2011.

How fast can a volcano spew out magma? This is a question that has perplexed volcanologist for a very long time.  Recently a team of scientists, led by A.B. Watts of Oxford University,  conducting a routine bathymetric survey onboard the research vessel  SONNE, were fortunate enough to be in the proximity of Monowai seamount volcano when a dramatic eruption occurred.**  Monowai is located along the 2,500 km-long Tonga-Kermadec Arc, where a submarine volcano can be found approximately every 50 km.

Eruptive volume versus duration of magmatism for submarine volcanoes.

Monowai is not small. It is a 10-12 km-wide strato-volcanoe approximately 1 km high with a 7-10 km wide caldera in its summit, approximately 0.6 km deep. Since it was first discovered in 1944 it has had a history of activity (discoloured water emanations and seismic ity). On May 14, 2011, the scientific team observed gassy discoloured water on the summit of the Monowai Cone. Three days later, a swarm of seismic events began and lasted for 5 days. On June 1, after the seismic activity subsided, the ship returned and carried out a second survey of the cone. They found significant changes in depth at the volcano. The main differences were (1) an increase in depth of up to 18.8 meters attributed to collapse structures on the flanks of the volcano, and (2) a decrease in depth of up to 71.9 m due to growth by eruption of new lava flows. The most striking feature of the surveys was a new cone near the summit. From its dimensions (100 m diameter at its base, at least 40 m high), it appears that about 0.00875 cubic kilometres of magma was erupted, most likely during the 5-day swarm of seismic activity. Extrapolating these rates to annual output, Monowai joins Kilauea, Iceland, Montserrat, the Azores, Hawaii, and the Canary Islands withoutput on the order of 0.1 cubic kilometres per year.

**Reported in A.B. Watts, et al., Rapid rates of growth and collapse of Monowai submarine volcano in the Kermadec Arc, Nature Geoscience, Advance publication, doi: 10.1038/NGE01473, posted May 13, 2012. Featured on Geology.com on May 15, 2012.