Figure 2: Lava flows advancing in tractor-tread style over thick snowpack with little obvious effect.

(a) Front of ′a′a lava 2 m thick slowly advancing towards observation pit in the snowpack. Person for scale. (b) Front of a sheet flow 0.5–1 m thick quickly advancing over an area of snowpack 1–4 m deep. Person for scale. (c) Schematic sketch showing a cross-sectional view along the front of an ′a′a flow advancing over thick snowpack.