Tephra Fallout: Grain-Size Sorting

When a volcanic eruption column collapses or spreads laterally, it releases ash and lapilli of every size into the wind. This simulator tracks individual clasts leaving an eruption column of adjustable height, computing each one's real terminal settling velocity from a gravity–drag force balance that depends on grain diameter and air density, then advecting it downwind before it lands. Landing points accumulate into a growing ground deposit whose height and colour show the two signatures every field volcanologist looks for after a real eruption: the deposit thins with distance from the vent, and the grains that build it get finer the farther downwind you go, because slow-falling fine ash rides the wind long after fast-falling coarse lapilli have already dropped out near the source.