Flow front behaviour of komatiite lavas.

If komatiite lavas were as turbulent and fast flowing as generally considered, they should have been capable of substantial physical erosion, as well as thermal erosion. Physical erosional processes have not been adequately addressed previously. However, the majority of the surface area of the bases of komatiite lavas are conformable, not erosional, and tops are coherent to quench fragmented, not autobrecciated. These features are more consistent with passive, laminar flow, not turbulent, fast flowing behaviour as generally thought. Recent findings on the flow behaviour and flow propagation processes for subaerial basalt lavas indicate that flow front velocities in medial to distal settings are significantly less than the flow velocities in near vent tubes and channels. Flows propagate by thin lobate and sheet breakouts, rapidly forming a skin or crust and then inflating by at least an order of magnitude over hours to days. Final preserved flow thickness is thus significantly greater than the dynamic flow thickness of the propagating flow front. Exaggerated Reynolds Numbers result for the distal margins of flows if final flow thickness and near vent velocities are used instead of dynamic flow front thickness and velocity. The Jeffreys equation was used to determine likely flow front thicknesses for komatiite lavas in medial to distal settings on low slopes. Using the results in the Reynolds Number equation indicates that at expected low flow front velocities many komatiites would have propagated in a laminar flow state.