Two-thirds of Earth's surface is formed at mid-ocean ridges, yet sea-floor spreading events are poorly understood because they occur far beneath the ocean surface. At 9 degrees 50'N on the East Pacific Rise, ocean-bottom seismometers recently recorded the microearthquake character of a mid-ocean ridge eruption, including precursory activity. A gradual ramp-up in activity rates since seismic monitoring began at this site in October 2003 suggests that eruptions may be forecast in the fast-spreading environment. The pattern culminates in an intense but brief (approximately 6-hour) inferred diking event on 22 January 2006, followed by rapid tapering to markedly decreased levels of seismicity.
Digital seafl oor imagery collected on 37 camera tows and Alvin dives, in which we identify 186 contacts between new and old lava, are used to create the most detailed map of a mid-ocean ridge (MOR) eruption to date. Lava fl ows erupted in 2005-2006 at the East Pacifi c Rise (EPR) covered an area of 14.6 km 2 along ~18 km of the EPR crest between 9°46′ and 9°56′N. The 2005-2006 lava is characterized by infl ated lobate and sheet morphologies in the fl ow interiors and pillow forms at terminal fl ow fronts. Numerous lava channels ~10-50 m wide and 1-5 m deep trending approximately east-west served as distributory pathways. Eruptions were sourced from fi ssures within the EPR axial summit trough as well as fi ssures located on an off-axis fi ssure mound ~600 m east of the EPR axis between 9°52′ and 9°56′N. Portions of the lava fl ow reached as far as ~2 km east of the axis near 9°51.2′N. Using a conservative estimate of 1.5 m for the average fl ow thickness implies that the 2005-2006 eruptions produced ~22 × 10 6 m 3 of lava, 4-5 times larger than estimated volumes of 1991-1992 EPR lava fl ows. Estimated lava volume for the 2005-2006 eruptions represents <15% of the magma available in the axial magma chamber.
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