I thought this visualization is interesting in that it provided the effect of the volcano over a period of time. Instead of just showing the total area affected or the area affected as time passes, it shows the area that are affected the most amount of time by the overlapping shades.
The visualization might not be as useful in telling how much impact the volcano had on the area as an area that is affected by large particles or more particles is not differentiated from an area that is just barley affected. Nonetheless, it might prove to be useful to some people. I do think a color change or a better contrast of the shades of gray would have worked better than the current model though.