In laboratory experiments designed to model lava flow processes, liquid polyethylene glycol
wax is forced through either a small hole or a long narrow slit onto the base of a tank of cold
water, where it spreads laterally while cooling and solidifying at its surface. We observe the
surface structure of the flow, and its dependence on the flow rate, thermal conditions and
basal roughness. In each case, solidification of a crust during spreading gives rise to a
number of different surface morphologies, each of which forms under a restricted range of …