A morphometric analysis of the submarine volcanic ridge south-east of Pico Island, Azores

Abstract

A region of crustal extension, the Azores Plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the Pico Ridge. Using bathymetric data and Towed Ocean Bottom Instrument (TOBI) side-scan sonar imagery, we measured the dimensions (diameter, height, slopes), shape, and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge. Our analysis and interpretation of the measurement and texture data suggest the following: (1) the various edifice types do not correlate with depth ranges, suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed; (2) the cones have a mean diameter of 948 m, height of 152 m, and slope of 15.6°, and are peaked rather than displaying flat-topped summits; and (3) while hummocky-textured cones also occur, smooth-textured cones predominate. The cones seem to develop preferentially outwards, then upwards, with only a weak correlation between diameter and height, suggesting that the cone population does not evolve self-similarly. Although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth, the hummocky cones have a significantly greater mean diameter than the smooth cones. We suggest that hummocky-textured cones (probably involving eruption of pillow lavas) are formed after voluminous smooth textured flows. Nearest-neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not. We interpret this finding to suggest subaerial cones being masked by over-covering flows, which tend to flow further than submarine lava flows. Furthermore, the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions, because of a lack of consistent pathway down gradient. Fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation, consistent with the regional tectonic trend of this part of the Azores Plateau. Cones form as each feeding dyke intrusion cools and the eruption becomes localized along point-source vents along the fissure. The ridge seems to be predominantly formed from fissure eruptions along the ridge axis, with subordinate transport of lava down its flanks.