The fusion pore was pyroclastic.

In September, 1996, I rented a car in Portland and drove out to see Mount Saint Helens, which erupted in 1980. It is an awesome sight. I have spent a lot of time in mountains in Colorado, but I wasn't prepared for the enormity of Saint Helens.


I downloaded the full size images taken before and after the eruption from a slide set provided by the USGS.