We met a Frenchman who travels the world skiing volcanoes. He said that the cone shaped volcanoes like Mt. St. Helens before it erupted have perfectly graded slopes and few of the gullies and crevasses one finds on normal mountains. The slope of a volcano is dictated by the viscosity of the magma, and the viscosity is dictated by its acidity. He uses light mountaineering skis to climb the volcano, and then skis back down.