Lava Cones and Shields on Intermediate-Rate Mid-Ocean Ridges
Wednesday, 17 December 2014
Most eruptions of basalt along mid-ocean ridges produce either sheet flows or pillow mounds and ridges. Rare eruptions on the Juan de Fuca and Gorda Ridges and on the Alarcon Rise (northern East Pacific Rise) produce volcanic cones or shields from point sources. Bathymetric maps at 1-m resolution from an autonomous underwater vehicle enabled classification of these ~circular features. The most common are 290-510m across and <50m tall cones with craters or tumulus-like inflated flows on their summits. There are 8 of these on the upper south rift, caldera floor, and southwest caldera rim on Axial Seamount; one on North Cleft segment near the 1986 pillow mounds; and one in the axial graben on northern Endeavour Segment. Hundreds of smaller pillow mounds lack craters or tumulus-like inflated flows at their summits. Three 660-1300m across circular cones have either a crater or an inflated tumulus-like structure at their flat to slightly domed summits. One in the axial graben on the northern Endeavour Segment is dissected by extensional faulting. Cage Seamount on the Coaxial Segment south of the 1993 pillow ridge is the most voluminous at 1100m across and >200m tall. Two flat-topped cones are located near the center of Alarcon Rise. A low-relief shield volcano on the northern Alarcon Rise is ~1700m across and only ~45m tall, and is cut by numerous faults and fissures. Two other shields, 860m and 1700m across and 50-70m tall, occur south of the 1996 North Gorda pillow mounds. These shields are decorated with small pillow mounds. Five 100-250m across and 15-30m deep pits collapsed on the northern shield. These constructional cones and shields form during eruptions where the initial fissure consolidated to a point, indicative of long duration activity. They are constructed during uncommon eruptions with flux larger than produces pillow mounds and smaller than produces sheet flows. They are a submarine equivalent of subaerial shield-building eruptions.