ASHDRIFTSLayers of volcanic ash (samples shown in inset) blanket the Arctic seafloor 4,000 meters down. The ash is evidence of an explosive eruption, long thought impossible at those depths.
A two-week cruise on an icebreaker to the top of the world last summer gave scientists a look at the aftermath of an event once thought impossible: a violent volcanic eruption on the deep-sea floor.
In 1999, a global network of seismic instruments detected the largest swarm of earthquakes ever to occur along the planet’s system of mid-ocean ridges, where tectonic plates spread to form new ocean crust. Several aspects of the recorded vibrations suggested that the quakes were generated by volcanic activity, says Robert A. Reves-Sohn, a geophysicist at the Woods Hole Oceanographic Institution in Massachusetts.
However, he notes, many scientists have doubted that explosive volcanism can take place at the 4,000-plus-meter depth where these quakes occurred because the immense pressure of overlying water prevents seawater from flashing into steam, a major driving force for such eruptions.
The source of the quakes was the Gakkel Ridge, a mid-ocean ridge that runs along the bottom of the Arctic Ocean. Sonar scans at a stretch of the ridge about 500 kilometers from the North Pole revealed several distinctive volcanic features, says Reves-Sohn. The largest of these undersea features, which usually have flat tops scarred with prominent central craters, are about 2 kilometers across and a few hundred meters tall.