Update 14th August – Source found – see below
Reports have appeared in the press over the last few days of a huge white raft of pumice encountered by the Air Force and a navy ship about a 1000 miles from New Zealand. From Stuff.co.nz:
The navy said the raft – 463 kilometres by 55 kilometres – was spotted by an RNZAF Orion returning on patrol from Samoa.
Lieutenant Tim Oscar, a Royal Australian Navy officer on an exchange with the NZ Navy, saw what he described as “the wierdest thing I’ve seen in 18 years at sea.”
“The lookout reported a shadow on the ocean ahead of us so I ordered the ship’s spotlight to be trained on the area. As far ahead as I could observe was a raft of pumice moving up and down with the swell,” he said.
“The rock looked to be sitting two feet above the surface of the waves, and lit up a brilliant white colour in the spotlight. It looked exactly like the edge of an ice shelf.”
The ship took samples of the pumice and at first it was thought to be linked to the submarine volcano Monowai off the New Zealand coast.
Monowai started erupting on August 3, whereas scientists have determined the pumice raft was first spotted on July 19. An Air New Zealand pilot also took a photograph of it on August 1.
Tahitian vulcanologists have determined that the raft became visible on July 19 and was caused by a volcanic eruption associated with a series of earthquakes in the days prior.
They have pin-pointed the origin of the raft to 72 kilometres south west of Curtis Island, one of the Kermadec islands, halfway between New Zealand and Tonga.
More than 157 earthquakes between magnitude 3 and 4.8 occurred in the area between July 17 and 18, the Laboratoire de Geophysique told their colleagues at GNS Science.
More information at Stuff.co.nz: scientists-rock-theory-on-pumice-raft where a clear video of the raft from the air is also available.
Note the scale bar on the map above; the raft is reported to extend more than half this length.Erik Klemetti covers it well on his Wired Eruptions Blog and has the following image, which also shows the scale of the raft:
Update 14th Aug (h/t Adam Gallon) – Source of eruption found – from NASA Earth Observatory:
Working independently of GNS, volcanologist Erik Klemetti and NASA visualizer Robert Simmon examined a month’s worth of satellite imageryfrom 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. (Although the Kermadec Islands are east of the International Date Line, they follow New Zealand time.)
Hidden by clouds in the morning image (above, top), the site of the eruption is clearly visible in the afternoon image (lower). 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).
Reports of pumice are not unusual in this part of the world as this comment from Willis Eschenbach attests:
I was the navigator on a tramp steamer in that area near Tonga, and I want to point out another mechanism bringing the heat to the surface.
This is the effect of the massive quantities of pumice that is released by these Tongan Trench volcanoes. I’ve sailed across huge (a couple km across) patches of pumice in the area. It is quite common to see rafts of pumice drifting onto the Fijian beaches, driven by the ceaseless south-east trade winds.
In the same area around Tonga I’ve also seen (once) pumice, bubbles, and discolored water rising together to the surface. It put on a very good imitation of an uncharted reef, and gave me some very uneasy minutes double-checking my sextant sights and scrutinizing the charts (which said deep water everywhere) until I realized what it was.
The following photos were taken by the crew of the yacht Maiken in 2006 and appear on the crew’s blog. The new island and raft were found in satellite photos at the time; it is now identified as Home Reef an ephemeral island whose top has repeatedly breached the surface and is subsequently eroded by wave action again.