Evidence of Recent Volcanic Activity Found
Along the Slow-Spreading Gakkel Ridge
By Kristen Watson
Palisades, N.Y--Researchers working under the ice canopy in the Arctic Basin, the last of Earth's oceanic frontiers, have confirmed that volcanoes and other tectonic processes often accompany seafloor spreading along the global mid-ocean ridge (MOR) system.
A team of researchers from Columbia, the University of Hawaii at Manoa and Tulane collected data along the Gakkel Ridge, the Earth's slowest spreading MOR, which was thought to be non-volcanic. Data was acquired using mapping sonars developed by Columbia researchers attached to a nuclear-powered submarine. Bathymetric data and sidescan images of the ultra-slow spreading (approximately one centimeter per year) eastern Gakkel Ridge depict two young volcanoes covering approximately 447 square miles of the seafloor. The location of the western volcano is the site of close to 250 teleseismic events detected in 1999.
The data demonstrate that eruptions along the ridge are larger and more frequent than previously theorized. "I first noticed the existence of an unusual seismic swarm on the Gakkel Ridge back in the summer of 1999," said Maya Tolstoy, an associate research scientist at Columbia's Lamont-Doherty Earth Observatory. "It looked classically volcanic."
Tolstoy researched how to confirm the presence of volcanism in this area. Eventually, she realized she did not have to go far to validate her theory. Dale Chayes, one of Tolstoy's Lamont-Doherty colleagues, had collected bathymetry and sidescan image data of the Gakkel Ridge using an underwater mapping system he developed for science ice exercises (SCICEX), a program run by the U.S. Navy and National Science Foundation.
The program used nuclear-powered Sturgeon class submarines to study the ice canopy, oceanography, biology and geology of the Arctic Basin. With permanent pack ice covering the Arctic Ocean, scientists cannot depend on the usual sea-going vessels or satellites to map the ocean floor, making nuclear submarines the optimal tools for Arctic research.
Columbia Equipment Shines Light on Darkened Seas
For SCICEX-98 and SCICEX-99, the U.S. Navy's submarine USS Hawkbill (SSN 666) was equipped with seafloor characterization and mapping pods (SCAMP), the geophysical mapping system built by Chayes' team of engineers to create the first high-quality, systematic three-dimensional maps of the Arctic seafloor. "SCAMP is composed of a sidescan swath bathymetric sonar (SSBS), a high-resolution subbottom profiler (HRSP), a Bell BGM-3 gravity meter and a data acquisi tion and quality control system (DAQCS)," said Chayes. "The SSBS collects data across a swath of seafloor as much as seven times the water depth perpendicular to the submarine's track.
This data can be used to construct a digital terrain model of the shape of the seafloor and the water depth, or to construct an image of the reflectivity of the seafloor that is similar to an aerial photograph. The HRSP emits sound that penetrates as much as 600 feet into the seafloor below the submarine and from this data images of the sediment layers can be constructed."
The data for this study was collected during the SCICEX-99 cruise. "The timing was just right in that it turned out they had mapped directly over the area of seismic activity toward the end of the swarm," said Tolstoy.
When researchers first looked at the sidescan data for this area, they discovered a large high-reflective peak indicative of recent volcanic activity. As they processed the data further they found a second similar feature further to the east, which appeared older due to greater faulting. These findings prove that volcanoes occur at ultra-slow spreading rates and that when eruptions do occur they are very large events.
"The seismicity associated with this eruption was unprecedented in its magnitude and duration when compared with previously documented mid-ocean ridge eruptions," said Tolstoy.
G. Michael Purdy, director of Lamont-Doherty, said, "A recent eruption on such a slow spreading ridge reaffirms the fact that we live in an era of discovery in ocean sciences. There remains much that is unknown, and even more that is not understood about the sea floor of our own planet. Lamont scientists continue to lead in these ongoing efforts to collect the observations of natural phenomena that are needed to understand our planet's active processes."
The presence of linear magnetic abnormalities over the entire ridge also suggests the ridge is volcanic. Three profiles across the axis of Gakkel Ridge depict a central high on the axial valley floor that may be a constructional ridge similar to those observed on the slow-spreading Mid-Atlantic Ridge.
Tolstoy and Del Bohnenstiehl have also performed interpretive research of the seismicity, which indicates that the magma for this eruption was tapped directly from the mantle.
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