The behaviour of the fronts of komatiite lavas in medial to distal settings

Field characteristics of komatiites and observations of flow behaviour and propagation processes for subaerial basalt lavas are used to reconsider the flow behaviour of Archean komatiite lavas. Planar, conformable bases at contacts with pelitic sedimentary strata, the existence of coherent to quench fragmented tops, but the absence of autobreccias, indicate that many komatiite lavas were emplaced passively under laminar flow conditions. To determine how widely this may apply to active komatiite lavas, we model flow front thicknesses. Noting that the preserved flow thickness is thus significantly greater than the thickness of the propagating flow front, we apply the concept of inflation of basalt lava flows to komatiites. Application of the Jeffreys equation allows lava thickness to be determined as a function of flow velocity, viscosity, density and slope of the terrain. Using the results together with estimates of flow front velocities and viscosity in the Reynolds number equation indicates that at expected low flow front velocities in medial to distal settings, most komatiites would have propagated in a laminar flow state. We therefore envisage that komatiites were turbulent in near vent settings and capable of physical erosion, and at times, as previously proposed, channel-forming thermal erosion. As flow area increased downstream, and magma supply rate to the flow front and the flow front velocity decreased significantly, the flow state would have transformed to laminar flow. Preserved flow thicknesses are often considerably greater than the calculated flow front thicknesses for such low viscosity lavas indicating that the final preserved thicknesses are either due to flow inflation, pending in topography, or that some komatiites were intrusions. (C) 1999 Elsevier Science B.V. All rights reserved.