REBUILDING MOUNT ST. HELENS

On May 18, 1980, Mount St. Helens, Washington, exploded in a spectacular and devastating eruption that shocked the world. The eruption removed 2.7 cubic kilometers of rock from the volcano's edifice, the bulk of which had been constructed by nearly 4,000 years of eruptions of lava domes and flows.

Following the 1980 eruption, Mount St. Helens remained active. A large lava dome began episodically extruding in the center of the volcano's newly-formed crater. This dome-building eruption lasted until 1986 and added about 80 million cubic meters of rock to the volcano. During the two decades following the May 18, 1980 eruption, horseshoe-shaped Crater Glacier grew around the east, south, and west sides of the lava dome in the deeply shaded niche between the dome and south crater wall.

The most active Cascade Range volcano over the past 4,000 years with a complex 300,000-year history, Mount St. Helens erupted again in the fall of 2004 as a new period of dome building began in the 1980 crater. Between October 2004 and October 2006, nearly 90 million cubic meters of dacite lava erupted immediately south of the 1980-86 lava dome. The erupting lava separated the glacier into two parts, first compressing the east arm against the east crater wall and doubling its thickness, and then causing equally spectacular deformation of the glacier's west arm. As a result, the termini of both arms of the glacier have advanced northward, the east by over 200 meters, and the west by about 50 meters. The eruption has continued into 2007.

Repeated vertical aerial photographs document dome growth and glacier deformation. These photographs enabled photogrammetric construction of a series of high-resolution digital elevation models (DEMs) showing changes within the volcano's crater throughout the eruption. From the DEMs, Geographic Information Systems (GIS) applications were used to estimate extruded volumes and growth rates of the new lava dome. The DEMs were also used to quantify dome height variations, size of the vent, and the mechanics of dome emplacement.

Previous lava-dome-building eruptions at the volcano have persisted intermittently for years to more than one century. Over time, such events constructed much of the cone-shaped mountain seen prior to the May 18, 1980 eruption. These processes may eventually rebuild Mount St. Helens to its pre-1980 form.