Katmai, Alaska (#4)
Katmai National Park, Alaska. The largest volcanic eruption of the 20th century occurred in 1912. It covered a huge area with volcanic ash, and filled the valley in the foreground with thick pyroclastic-flow deposits. Once known as the "Valley of Ten Thousand Smokes", the thick pyroclastic deposits have now cooled and only a few fumaroles remain. This 1962 air photo shows Trident Volcano erupting in the background.
Photo by Dave Bogert, National Park Service.