Detailed distribution and rapid degradation of small seamounts on the MAR axial volcanic ridge, 45°30'N

In May-June 2008, James Cook cruise 24 conducted a detailed geophysical survey and geological sampling of a single axial volcanic ridge on the Mid-Atlantic Ridge axis at 45°30'N using the TOBI deep-towed instrument and Isis ROV. TOBI sidescan and bathymetry define the AVR morphology in unprecedented detail, while TOBI and Isis magnetic field measurements help define the younger parts of the ridge (all within the 0.8 Ma Brunhes chron). Over 180 samples were recovered on 11 Isis dives for geochemical analysis and radiometric dating. Here we concentrate on a detailed 2 km-square survey of part of the AVR crest and flank. This area contains ~100 identifiable volcanic cones ranging from 250 m to ~20 m diameter, both conical and flat-topped. There is no very clear relationship between neighbouring cones, though they tend to follow broad trends both parallel and oblique to the AVR axis that may reflect deep-seated tectonic control. Inferred crustal magnetisation peaks follow similar trends. Magnetisation varies significantly within the AVR flank, does not fall monotonically away from the axis, and displays the highest values 1800 m off- axis, corresponding to an age of 164 ka if accretion was linear at the regional accretion rate of ll km/Ma. Many of the volcanic cones, including some on the AVR axis, have already undergone significant degradation by flank failure and mass wasting, leaving scars that cut deep into the cones and are extremely steep (often vertical). This evidence suggests that the volcanic landscape of the oceanic crust is susceptible to significant tectonic degradation virtually from the time it is formed.