Gravity-Driven Flow
Lava flows are gravity-driven currents of molten rock that advance downslope from a volcanic vent or fissure. Their behavior is governed by the Jeffreys equation — essentially Poiseuille flow on an inclined plane. The interplay between gravitational driving force and viscous resistance determines flow velocity, while the balance between heat supply (effusion rate) and heat loss (radiation and conduction) controls how far the flow can travel before solidifying.
Viscosity: The Master Control
Viscosity varies by orders of magnitude between lava types and is the single most important control on flow morphology. Low-viscosity basaltic lavas produce thin, fast-moving pahoehoe and 'a'ā flows that can travel tens of kilometers. High-viscosity dacitic and rhyolitic lavas form thick, stubby domes that barely advance beyond the vent. The transition from smooth pahoehoe to rough 'a'ā occurs when strain rate exceeds a critical threshold that depends on viscosity and yield strength.
Cooling and Crust Formation
From the moment lava is exposed to air, its surface radiates heat at a rate proportional to T⁴ (Stefan–Boltzmann law). A solid crust forms within minutes, insulating the molten interior. This self-insulation is key to long-distance flow: lava tubes and crusted channels can transport melt tens of kilometers with minimal heat loss. The 1859 Mauna Loa flow traveled 50 km to the sea through an efficient tube system.
Hazard and Risk
Lava flows rarely kill because they usually advance slowly enough for evacuation, but they destroy everything in their path. Predicting flow paths requires accurate topographic data, effusion rate estimates, and viscosity modeling. Modern probabilistic flow models like MOLASSES and DOWNFLOW simulate thousands of possible paths over digital elevation models to produce hazard maps used by civil protection agencies worldwide.