Volcanoes Erupt Under Ice and Water
Imaging system reveals surprises in the ocean.
Under the Arctic ice, a group of scientists has made a startling discovery, courtesy of a new camera. Studying an ultraslow-spreading midocean ridge, the team found evidence of explosive volcanic eruptions on the seafloor. Such eruptions were believed impossible at these depths because scientists had thought the ridges did not have the needed carbon dioxide, said team member and Woods Hole Oceanographic Institution geophysicist Robert A Sohn.
One reason for the eruptions may be that ultraslow-spreading ridges are especially conducive to CO2 storage and explosive discharge, an explanation that the researchers think may be the case. Alternatively, it could be that scientists have systematically underestimated the carbon dioxide process at all midocean ridges.
“There are various ways to state it, but for the science community, it means we have to go back to the drawing board in terms of thinking about CO2 flux and processes at midocean ridges,” Sohn said.
Besides Woods Hole, researchers represented the University of Texas at Austin; the Swedish Polar Secretariat in Stockholm; Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany; Johns Hopkins University in Baltimore; Göteborg University and Stockholm University, both in Sweden; AIST in Higashi, Japan; and the University of Tokyo.
A detailed map created by bouncing sound waves off the seafloor shows the topography of the Gakkel Ridge in the Arctic Ocean. The axial valley contains many distinctive, cratered volcanoes. Black lines show where the bottom was surveyed by a camera; white dots show where seafloor samples were taken. The inset map shows the location of the ridge. The scale at the bottom indicates depth below sea level. Map courtesy of Martin Jakobsson, Robert Reves-Sohn, Adam Soule and the AGAVE science team.
The midocean ridge is the largest volcanic chain in the world, dwarfing anything on land. It plays host to a number of phenomena that are important or bizarre, such as the strange biological communities that cluster around deep-sea hydrothermal vents. The problem for researchers is that getting to the ridges is not easy and staying there for an extended time is even harder. The challenge is particularly acute in the Arctic, where the ridges are covered by both deep water and an ice pack.
These glassy, granular fragments of seafloor basalt were a key piece of evidence that volcanoes along the Gakkel Ridge have exploded violently. Photo by Adam Soule and Claire Willis, Woods Hole Oceanographic Institution.
The scientists developed their own equipment to overcome these problems, including an imaging system that they mounted on unmanned underwater vehicles. Thanks to an optical fiber connection between the vehicles and a surface ship, researchers aboard an icebreaker maneuvered the vehicle 2 to 5 m above the seafloor, capturing still and high-definition video imagery of the terrain.
Approximately 10 cm of pyroclastic deposits – fine shards of rock blasted from a volcanic eruption – cover high-standing pillow lava on the south side of Duque’s Hill along the Gakkel Ridge. The exoskeleton of an unidentified species of sponge is visible in the foreground. Photo courtesy of Robert Reves-Sohn, Hanumant Singh, Timothy M. Shank, Susan Humphris, William Lange and the AGAVE science team.
Using a retractable cable, they brought samples of the Gakkel Ridge – a segment of the midocean ridge near the North Pole – to the surface for additional testing. The images and samples showed the seafloor near the ridge blanketed with pyroclastic deposits, geologic formations created by an explosive eruption brought about by the catastrophic discharge of carbon dioxide. The deposits were at depths of 4000 m, far below what had been thought to be the depth limit for such activity. As Sohn noted, the explanation for these findings is unknown.
It may be that researchers would discover many such explosive deposits and hydrothermal vent fields if they had the technology that allowed them to extensively explore the Arctic midocean ridges. For its part, the group already is working on its own technological improvements.
“The next steps are to completely automate the survey process so that both surveying and sampling can be done with robots,” Sohn said.
Nature, June 26, 2008, pp. 1236-1238.
- A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
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