From the Mount St Helens Wikipedia page:
Mount St. Helens is most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale caused an eruption that reduced the elevation of the mountain’s summit from 9,677 ft (2,950 m) to 8,363 ft (2,549 m), leaving a 1 mile (1.6 km) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume.
Someone just found a set of 34 slides, from photos taken on a plane circling the volcano as it erupted. These never-before-published photos are worth a look.
To me, the most amazing thing about this event is how, in the space of a few days, an almost perfectly conical mountain was practically leveled.
I was in Seattle on my way back to Eugene from climbing up at Squamish. I had driven by the mountain pre-eruption a few weeks earlier, and now it was almost all gone. Coincidentally, while shopping at Pike Street Market in Seattle my Nikon FM was stolen so I had nothing to shoot the eruption with. The ash cloud was enormous, even from that distance. It took me a while to get back home as I-5 was closed and I had to get out on the coast.
Many years earlier I’d climbed it and hiked around the crater. Gad, lucky me I guess. Many years after the eruption I flew into the crater with a friend in his small plane.
We unwittingly visited the site on the 20th anniversary of the eruption, and found we were sharing the visitor’s area with a bunch of news trucks. The devastation was still apparent, both in the immediate area and in the waterways surrounding the mountain.
I was camping 30 miles from it when it blew and we were told to evacuate. I had just bought a Canon F1 and had it loaded with Kodacrhome. I got some fanstastic pictures of the eruption as we departed South. (Which was the lucky side. If I had been that close on the Northeast side, I wouldn’t be writing this.)
As you noted about the shape of the mountain, that particular fact struck me as I saw it intact the day before, and then saw ⅓ of the mountain gone the next day. Awesome doesn’t cover the half of it.