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