I had the great fortune (or misfortune) to be living in E Central Washington when Mt. St. Helens erupted. I was actually on the coast that weekend, so I missed the show that came from the ash fall and super heated clouds of ash. We got approximately 6" (15cm) over the whole farm. It was foul smelling and nasty to work with, but it made a moisture cap and we had a good wheat crop that year. Beans had not been planted yet (fortunately) and final tillage and planting worked it in the soil adequately.

But was it ever abrasive. We wore stuff out at an astounding rate. I tore a pickup motor down that had been toasted in the first week and the wear was phenomenal. But it was also incredibly smooth and polished.

I know that there is Mt. St. Helens ash being used as lapping compound to this day. It worked really good.

Thanks for a wonderful explanatory article. Always a pleasure.