On the formation of eruption columns following explosive mixing of magma and surface‐water

Abstract

When magma vents into the sea or a crater lake, the ensuing magma‐water interaction can affect the style of eruption dramatically. If the mass of surface water incorporated into the erupting material is small, (<15% of the total mass), then typically, this water vaporizes, and the density and temperature of the erupting mixture decreases. As a result, the minimum eruption velocity for which a Plinian‐style eruption column may develop decreases. If a larger mass of cold surface water is added to the mixture, then part of this water may not vaporize, the initial mixture has the saturation temperature, and the initial density increases again. For sufficiently large masses of surface water mixed into the erupting magma, the ascending mixture cannot become buoyant. Instead, relatively cold, wet and dense ash flows spread laterally from a collapsing fountain. With a small or moderate eruption rate, <108 kg/s, the height of rise of a buoyant column does not vary significantly with the surface water content. However, for very large eruption rates, >108 kg/s, the height progressively decreases with surface water content. This occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column, so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air. The transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake‐jima in 1983 and for the changes in the eruptive activity at Surtsey in 1963–1964 as the access of seawater to the vent became more restricted. We also present calculations which suggest that the accretionary lapilli, which are often found in wet flow deposits, may result from condensation of vapor in both the cold, wet collapsing fountains and in the flows themselves.