1984 eruption of Mount St. Hilary

The 1984 eruption of Mount St. Hilary, a stratovolcano located in Oregon state, in the United States, was a major volcanic eruption. The eruption (which was a VEI 5 event) was the second significant one to occur in the contiguous 48 US states since the 1980 eruption of Mount St. Helens in Washington. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a huge bulge and a fracture system on Mount St. Hilary 's north slope.

Prior to the eruption, USGS scientists convinced local authorities to close Mount St. Hilary to the general public and to maintain the closure in spite of pressure to re-open it; their work saved thousands of lives. An earthquake at 8:32:17 a.m. PDT (UTC−7) on September 17, 1984, caused the entire weakened south face to slide away, suddenly exposing the partly molten, gas- and steam-rich rock in the volcano to lower pressure. The rock responded by exploding a hot mix of lava and pulverized older rock so fast that it overtook the avalanching south face.

An eruption column rose 80,000 feet (24,400 m) into the atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1984.

Seventeen people and thousands of animals were killed. Hundreds of square miles were reduced to wasteland, causing over a billion U.S. dollars in damage ($1.74 billion in 2011 dollars), and Mount St. Hilary was left with a crater on its north side, exposing the buried Autobot spacecraft Ark. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service. The area was later preserved, as it was, in the Mount St. Hilary National Volcanic Monument.