The Physics of Lava Flow

After being exploded, extruded, or oozed out of the ground, lava tends to eventually go somewhere. Most of us probably think of Hawaii and the rivers of molten rock flowing down and away from the volcano and lava fountains. However, intermediate and rhyolitic lavas flow as well, but they tend to be so viscous as to clump up into large piles called lava domes.

These piles of rock move at a glacial pace. Lava domes are viscous enough that they tend to pile up instead of flow outward, so once enough mass has built up behind the outer wall they can collapse; sometimes causing a pyroclastic flow to move down the volcano.

Basaltic flows are generally the rivers of lava and moving walls of crumbly rock. Pahoehoe flow is the smooth, oozing flow and 'A'a flows are the crumbly, slow walls of burning rock. Although they are generally safe as far as human life goes, they will (slowly) destroy much of what is in their path as they flow away from the volcano. Fortunately, Chris Kilburn et al have come up with a helpful, if incomplete and rough, model that lets us determine the expected total length of these flows.

The function describes the max length (Lm), with sigma being the crust extension failure rate, S being tensile strength (Pa), rho being density (kg m-3), g the gravity constant, k bulk thermal diffusivity (m2 s-1), and Q being mean volumetric flow (m3 s1). Most of these are actually highly variable between any two volcanoes, but we can use general numbers to give ourselves a rough idea of just how far we would normally have to worry about a lava flow.