May 18, 1980 - 8:32 A.M. PDT

Animated initial eruption sequence.

USGS Photograph taken on May 18, 1980, by Robert Krimmel.

At 8:32 AM, May 18, 1980, the largest earthquake recorded at Mt. St. Helens crumbled the weak north face of the mountain. Suddenly, an enormous mass of superheated rock and gas was released, and the cataclysm began.

Within ten minutes, the ash column from the mountain reached 12 miles high (63,000 feet); by noon, ash was falling as far away as Idaho.

Closer to the mountain, a low-hanging cloud of superheated gas and pulverized rock (known as a pyroclastic surge) had been launched by the initial landslide across the surrounding ridges, killing everything its path. Mudflows swelled by melted glaciers roared down the surrounding river valleys, eventually reaching the Columbia River and dropping enough sediment to close the river to navigation for more than a month.

Mt. St. Helens had surprised; no one had expected in 1979 what Mt. St. Helens would do before June 1980. Never had a volcano erupted so catastrophically in the United States in historic time. A major reassessment of the volcanic hazards of the United States would be launched, for the risks were dramatically more apparent, and there are other slumbering mountains that rest far closer to major population centers.

And the mountain continues to surprise. Even now, the mountain is regrowing. Mt. St. Helens' story is far from over.

Slab erupted from the 2004-2006 dome-building eruptions.

For more information:

Fire Mountains of the West by Stephen L. Harris

USGS Cascades Volcano Observatory

USGS Cascades Volcano Observatory - Mt. St. Helens

Wikipedia - Mount St. Helens
Portland's KGW - May 18, 1980 newscast 

UPDATE:
Boston.com has a great collection of photographs.

May 17, 1980

Mt. St. Helens, May 17th, 1980. Ash-shrouded and quiet.

From the USGS Mt. St. Helens 30th Anniversary site:

The mountain remained quiet. Seismic activity reached the lowest level for May, with only 18 earthquakes larger than 3.0 recorded, including 6 larger than magnitude 4.0. In response to pressure from property owners and with the Governor's consent, law enforcement officials escorted about 50 carloads of property owners into the Red Zone to retrieve possessions. Those who entered were required to sign liability waivers at the roadblocks and to leave by nightfall. Authorities agreed to allow another caravan of property owners in at 10:00 a.m. the following morning, May 18.

March 19, 1980

Mt. St Helens, viewed from Spirit Lake. Photo from USGS.

Mt. St Helens, viewed from the southwest. Photo from USGS.

Mount St. Helens, a quiet, regal peak located about sixty miles north-northwest of Portland, Oregon.

One of a chain of Cascade volcanoes, formed by the submergence of a small tectonic plate (the Juan de Fuca Plate) underneath the far larger North American plate, the mountain's volcanism was well known. St. Helens had even erupted within the short recorded history of the Pacific Northwest, throwing up embers and ash in the 1830s, 1840s, and 1850s. Since then, the mountain had been silent, becoming a recreation destination by 1980.

On March 20, 1980, that changed, as the first of thousands of earthquakes began to rumble beneath the mountain. For only the second time in the twentieth century, a Lower 48 volcano awakened from its slumber. The second would prove very different from the first.

For more information:

Fire Mountains of the West by Stephen L. Harris

USGS Cascades Volcano Observatory

USGS Cascades Volcano Observatory - Mt. St. Helens

Wikipedia - Mount St. Helens

Mt. Redoubt Continues to Erupt

Mt. Redoubt has erupted in a series of eruptions, including one this morning that produced an ash cloud rising to 65,000 feet.

The Alaska Volcano Observatory is the best source for the latest information.

Mr. Redoubt Erupts

Mt. Redoubt, southwest of Anchorage, Alaska, has begun erupting.

Go to the Alaska Volcano Observatory for the latest.

Mt. Redoubt - Watching and Waiting

Mount Redoubt, as seen by USGS observers on January 31st, 2009.

So, is no news good news?

Mt. Redoubt Is Stirring

After turning away from matters seismological* after the Yellowstone Lake earthquake swarm, the Ring of Fire has decided to put on a show of its own.

Mount Redoubt, about 100 miles southwest of Anchorage, is preparing to erupt. Starting the weekend of January 24th-25th, 2009, seismic activity suddenly increased, activity that suggested magma was on the move inside the mountain.

Redoubt is known as an active volcano, most recently erupting in 1989-90. Generally, such eruptions are only really hazardous if you are on the mountain, downstream from the mountain on a river or stream fed by flows from the mountain, or downwind from the volcano. Resoubt is relatively remote in that there are no settlements on it; however there are some oil facilities and camps on the Drift River which could be affected by an eruption-generated mudflow. (These mudflows are called lahars, and result from snow and glaciers rapidly melting due to hot ash and pyroclastic flows. This sudden release of water scours the mountainside and collects soil in ash into a thick muddy mass that moves down a river, wreaking havoc. The mudflows on the Toutle River after the Mt. St. Helens eruption in 1980 are a textbook example of the hazard a lahar can pose.)

The real concern is volcanic ash. Volcanic ash in essentially pulverized rock, which means it is abrasive and can be heavy when it is allowed to collect. So, in a state that receives 70% of its supplies through the Anchorage airport will be watching very closely to see which way the wind blows when Redoubt erupts.

More information:
Alaska Volcano Observatory - Mt. Redoubt
Anchorage Daily News



*Sorry about the arch prose. I watched a Sherlock Holmes MYSTERY! last night ("The Second Stain"), and the show had its usual effect. For a period of at least 24 hours after watching, I have to repress urges to walk with a proper cane, smoke a pipe, and talk with a proper English accent. I blame you, Jeremy Brett.