Morphology and distribution of lava flows on mid-ocean ridges: a review

Abstract

Both pillow lavas and sheet flows are important features of oceanic volcanism at mid-ocean ridges. Submersible surveys of the neovolcanic zone and off-axis areas of these complex seafloor volcanic terrains conducted during the past two decades have uncovered a wide array of basaltic surface forms, with pillow basalts displaying bulbous, spherical, and elongate tubular patterns and sheet flows exhibiting smooth, lobate, rippled, wrinkled, ropy, whorly, hackly, and jumbled configurations. The genesis of pillow lavas and sheet flows appears to be coupled to variations of spreading rates within the plate boundary zone. While pillow lavas predominate at slow spreading centers (i.e., Mid-Atlantic Ridge), sheet flows prevail along the active volcanic zones at intermediate (i.e., Galapagos Rift and Juan de Fuca Ridge), fast and superfast (i.e., East Pacific Rise) spreading centers. Yet, both types of flows are present at each setting. The distribution of pillow lavas and sheet flows along a ridge segment is related to the topographic gradient of the ridge axis. Hence, the formation of sheet flows tends to be favored by the development of topographic highs between ridge segment boundaries where the crustal lid is thinnest and rifting directly taps the underlying magma chamber. Fissure eruptions here release copious amounts of surface-fed fluid lavas which decrease in volume down the topographic gradient. Away from the bathymetric high of the ridge segment, lava flows become more distal and channelized, thereby fostering greater concentrations of tube-fed pillow flows toward the segment boundaries. The type of lava flow morphologies encountered at any specific site on the seafloor is contingent upon the effusion rate, cooling rate, and slope, as demonstrated by laboratory simulations.