olelog

Gakkel Ridge is the continuation of the Mid-Atlantic spreading ridge in the Arctic Ocean, and thus part of the North American - Eurasian plate boundary. The Gakkel Ridge is the slowest spreading ridge on earth with a rate of less than one centimetre per year. It is also the deepest ocean ridge, ranging from 3-5 km deep. Rifting started 55 million years ago, when the Eurasian Basin opened, and now The Gakkel Ridge separates the approximately 4000 m deep Eurasian Basin into the Amundsen Basin and the Nansen Basin.


The Gakkel Ridge is running 1,800 kilometres from the northern tip of Greenland to Siberia. In 1999 a series of 300 strong earthquakes over a period of eight months indicated an eruption at 85° N 85° E in 4 km water depth. The eruption was as big as the one that buried Pompei in 79.

A research team led by the American Woods Hole Oceanographic Institution (WHOI) went to the area on the Swedish icebreaker Oden in 2007 where they, with a specially developed camera, discovered extensive layers of volcanic ash on the seafloor, which indicates a gigantic volcanic eruption. Such violent eruptions were not thought possible at great ocean depths because of the intense weight and pressure of water and because of the composition of seafloor magma and rock.

They found jagged, glassy rock fragments spread out over a 10 km² area around a series of small volcanic craters about 4,000 m below the sea surface along the Gakkel Ridge. A tremendous blast of CO₂ must have been released into the water column during the explosive eruption. Seafloor volcanoes usually emit lobes and sheets of lava during an eruption, rather than explosive plumes of gas, steam, and rock that are ejected from land-based volcanoes. Because of the hydrostatic pressure of seawater, ocean eruptions are more likely to resemble those of Kilauea than Mount Saint Helens or Mount Pinatubo.

The researchers used a combination of survey instruments, cameras, and a seafloor sampling platform to collect samples of rock and sediment, as well as dozens of hours of high-definition video. They saw rough shards and bits of basalt blanketing the seafloor and spread out in all directions from the volcanic craters they discovered and named Loke, Oden, and Thor.

Closer analysis has shown that some of the tiny fragments are angular bits of quenched glass known to volcanologists as limu o Pele, or "Pele's seaweed." These fragments are formed when lava is stretched thin around expanding gas bubbles during an explosion.

Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m).

The findings were reported in the 26 June 2008 issue of the journal Nature - Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean.


• http://www.nature.com/nature/journal/v453/n7199/edsumm/e080626-08.html
• http://www.nature.com/nature/journal/v453/n7199/abs/nature07075.html
• http://www.eurekalert.org/pub_releases/2008-06/whoi-gds062508.php




PS of 21 July 2008:
My attention has been drawn to a post with much info and many figures and bathymetric maps concering the Gakkel Ridge here.