We reported last week that a huge pumice raft was discovered in South Pacific, north of New Zealand, and today we might have the answer to what might have caused it.
Small volcanic rocks or pumice covered the area of 250 nautical miles (463 km) in length and 30 nautical miles (55 km) wide. Pumice is typically created when super-heated, highly pressurized rock is violently ejected from a volcano. Alternatively, it can be formed when lava and water are mixed. Most pumice is light enough to float on water.
Though the pumice was spread over a vast area of the South Pacific, the origin was a mystery. An undersea volcano several hundred kilometers to the north of the pumice—Monowai—had erupted on August 3, but an airline pilot reported seeing pumice as early as August 1. Two data sources provided clues to pinpoint the volcano: earthquake records and satellite imagery. After reports of the pumice rafts surfaced, scientists from Tahiti and New Zealand’s GNS Science connected the eruption with a cluster of earthquakes in the Kermadec Islands on July 17 and 18.
Working independently of GNS, volcanologist Erik Klemetti (Eruptions Blog) and NASA visualizer Robert Simmon examined a month’s worth of satellite imagery from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS). They discovered the first signs of the eruption—ash-stained water, gray pumice, and a volcanic plume—in imagery from 9:50 a.m. and 2:10 p.m. (local time) on July 19, 2012.
Klemetti matched the satellite imagery with ocean floor bathymetry to identify Havre Seamount as the likely source. The eruption was strong enough to breach the ocean surface from a depth of 1,100 meters (3,600 feet). On his blog he writes:
The hard part was trying to identify a source. I took the georeferenced Terra image and overlaid it on Google Earth to find the location relative to Macauley and W and then checked out the bathmetry … and lo and behold, the source of the plume seems to be a U-shaped edifice. This edifice had no label in the Global Volcanism Program Google Earth layer, but after finding a map of the Kermadec Islands, it appears that this is Havre seamount (near Havre Rock), a volcano that we don’t know very much about at all. In fact, Havre doesn’t even have an entry in the GVP database or really any information about it – heck, it doesn’t even show up on many maps of the active Kermadec volcanoes.
Here is how we determined that it is likely from Havre Seamount – comparing a Google Earth overlay of the Terra image from July 18th with maps of the Kermadec arc volcanoes.
The GNS Science press release (which came out after we have convinced ourselves of the location/timing of the initial eruption) on the activity in the Kermadec Islands appears to back up this conclusion based on work done by Alain Bernard at the Laboratoire de Volcanologie. Their approximate latitude/longitude of the eruption matching up with the location of Havre and the plume spotted on the Terra/Aqua images (see above and below), however they don’t specifically identify Havre. GNS Science volcanologist Brad Scott noted:
A short seismic swarm with hydroacoustic and seismic phases on the Polynesian Network on July 17 and 18. The earthquake swarm began on 17 July, 0732h UTC and ended on 18 July, 14:05 with more than 157 events, with magnitude between 3.0 and 4.8., probably earthquakes associated with the rise of magma in the eruptive edifice.
To confirm Havre as the source, research vessels will need to head out there and try to find evidence on the seafloor for the eruption, so confirmation might take months to occur, Klemetti writes.
Featured image credit: NASA/MODIS