Abstract

Because of their high eruption temperatures and ultrabasic composition, komatiite lavas had low viscosities, which typically ranged from 0-1 to 10 Pa s. A major consequence of this low viscosity is that most lavas erupted as turbulent flows. An analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms"1 and Reynolds numbers much greater than the critical value of 2000. The lavas would have remained turbulent for most or all of their subse-quent flow and emplacement. Typical horizontal flow rates are estimated to range from 0-5 to 100 m2 s~ ' per unit width of flow. Such turbulent lava flows would have lost their heat by convection to the surroundings, at rates which are orders of magnitude greater than the rates for laminar flows, which cool by conduction. A quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 °C hr" ' to a few °C hr"1, while the flows remained turbulent. These rates are in an appropriate range to cause phenomena such as high nucleation rates, strong supersaturation of the lava, delayed nucleation of olivine, and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lower melting temperatures. It is