The Guardian has an interesting explanation on the physics of pyroclastic flow - that mixture of rocks, ash, and superheated gas that subductive volcanoes emit. Pyroclastic flow can reach 400 kph and can travel uphill as rapidly as it does downhill. Scientists have found that increased pressure at the rear of the flow raises the front, creating a sort of air hockey table effect in which surface friction is greatly reduced.

So remember 400 kph when volcanic warnings are issued. - William