Basaltic and Other Low-Viscosity Lavas

This chapter summarises the eruption processes, flow behaviour, and characteristics of low-viscosity basalt lavas. Basalts are primitive magmas formed in the mantle, and their eruptions are the most frequent and voluminous on Earth, especially in submarine environments. They also form the largest volcanic features on Earth, such as the mid-ocean spreading ridges, flood basalt provinces, oceanic plateaus, and very large shield volcanoes. Here we first discuss the different factors affecting subaerial basalt lava behaviour, such as eruption temperature and effusion rate, which are complexly interrelated to a number of physical factors, principally magma viscosity and yield strength. In turn these control basaltic eruption styles, flow characteristics, and lava morphology. The two main types of subaerial basaltic lava are called pahoehoe and a’a, which are now best viewed as end-member types with a range of transitions in between. Tholeiitic flood basalt lavas are the largest and farthest flowing lava flows known, and consist of extensive sheet pahoehoe lavas with maximum flow distances of ≥1,000 km. The characteristics of the flows are thereafter controlled by the nature of the environment into which they are extruded, or the environment into which they then flow, or both. Modern high-precision sonar bathymetric surveys now allow large areas of the seafloor to be mapped to the detail of defining individual lava flow fields, lava flows, and lobes. There are, however, major differences in the effects of an ambient aqueous medium, especially in deep-water environments, on subaqueous basaltic eruption processes, and the deposit types produced.