Mid-ocean ridge eruptive vent morphology and structure: evidence for dike widths, eruption rates, and evolution of eruptions and axial volcanic ridges.

High-resolution side scan sonar data of Mid-Atlantic Ridge (MAR) inner valley floor axial volcanic ridges reveal details of their architectural elements. We develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features. Inhibition of gas exsolution, lack of magma disruption, and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling, leading to more rapid centralization of eruptions along the widest places in the dike. Dikes are predicted to initially feed eruptions from fissure vents, producing lines of hummocky ridges; centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts. The widest dikes are predicted to produce smooth flows up to several kilometers in length, which should pond in adjacent lows. Edifice sizes predicted on the basis of volume fluxes, and flow lengths implied by the widths of dikes observed on the seafloor, in Iceland and in ophiolite complexes, are in quantitative agreement with the dimensions of observed features. Each ridge is made up of the products of a variety of these individual dike-emplacement and extrusive events involving various dike widths, cooling times, and eruption durations. On the basis of typical MAR spreading rates, about one dike emplacement event would be expected every 40 years; since not all dikes reach the surface, the actual mean interval between eruptions will exceed this. MAR dikes should solidify between average dike emplacement events. Dike emplacement events are more frequent on the East Pacific Rise (EPR) and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes. Differences between the morphology of the MAR axial volcanic ridge and the sheet flow dominated EPR are attributed to on average wider dikes erupting with greater frequency along the EPR