Detailed studies of mid-ocean ridge volcanism at the Mid-Atlantic Ridge (45°N) and elsewhere

Abstract

This thesis provides a comprehensive study of the Axial Volcanic Ridge (AVR) at 45˚30 N on the Mid-Atlantic Ridge. A number of datasets were collected over the area, including: EM120 ship based bathymetry, TOBI sidescan sonar, Isis high-resolution bathymetry, Isis video and sampling dives and crustal magnetisation surveys. In this thesis I seek to explore the questions of the volcanic building blocks of AVRs and their spatial and temporal evolution in a number of ways.

Very detailed volcanological mapping of the seafloor is used to provide semi-quantitative estimates of the relative proportions of different lava morphologies on and off the AVR and within the upper oceanic crust. I find that the AVR is characterised by predominantly pillow lavas while the flatter areas of seafloor around the AVR are covered by higher effusion rate lava morphologies. These observations are combined with the bathymetry and sidescan sonar datasets to elucidate the detailed nature of the building blocks of AVRs, which I find to be volcanic hummocks, composed predominantly of pillow lavas. These hummocks are morphologically the same as pillow mounds described at intermediate-spreading rate ridges. From these observations we identify common collapse scarps and associated talus deposits, which if buried may contribute significantly to increased porosity and lower seismic velocity in seismic layer 2A. Sediment cover is used as a proxy for seafloor age, and suggests that both the AVR and the flat seafloor around it are a similar age.

Statistical analysis of the distribution and size of volcanic hummocks on the AVR finds their numbers to have been vastly underestimated in previous studies. I also show that hummock density is very variable across the AVR, possibly corresponding to many discrete melt sources.

Evidence from 45˚N does not support either a uniform, long period life cycle model as has been proposed, or a steady state AVR. Instead I suggest that the AVR is the surface representation of robust magma supply, and irregular nature of this melt supply will control the surface appearance of the AVR