Never before has a volcanic eruption on a slow- or ultraslow-spreading mid-ocean ridge been both observed seismically and confirmed on the seafloor. During the first half of 1999, a long-lived volcanic-spreading event occurred on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean. The seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption. Sonar images from the U.S.S. Hawkbill, which passed over the area within four months of the start of activity, are consistent with the presence of a large, recently erupted flow and a volcanic peak directly in the area of seismic activity. Seismic activity began in mid-January and continued vigorously for three months; a reduced rate of activity persisted for an additional four months or more. In total, 252 events were large enough to be recorded on global seismic networks. Although a limited number of volcanic-spreading events have been observed globally, the duration and magnitude of the Gakkel Ridge swarm, when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates, suggest that a negative power-law relationship may exist between these parameters and spreading rate. Fault activation, in response to magmatic emplacement, appears to have occurred over a broad region, suggesting that magma may have been tapped from mantle depths. The slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth. In combination with bathymetric setting and sidescan sonar confirmation, the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow-spreading rates.
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