Such continuous ground vibrations, commonly associated with eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface movement of fluids, either gas or magma.
The drawing above the photograph illustrates, in a highy exaggerated fashion, the nearly horizontal movement--about 85 feet in 20 days--of one of the measured points on the "bulge."Through April 21, Mount St.
Helens intermittently ejected ash and steam in bursts lasting from a few seconds to several tens of minutes.
The effect of the prevailing easterly wind was striking during the March-April eruptive activity, transforming the snow-covered Mount St. The ash blown out between March 27 and May 18 was derived entirely from the 350-year-old summit dome, shattered and pulverized by phreatic (steam-blast) processes driven by the explosively expanding, high-temperature steam and other gases.
No magma (molten rock and contained gases) was tapped during the initial eruptions.
The first crater was joined on the west by a second, slightly larger crater, and as the activity continued, both craters enlarged and ultimately merged.
Several avalanches of snow and ice, darkened by ash, formed prominent streaks down the mountain's slopes.
He named it in honor of a fellow countryman, Alleyne Fitzherbert, who held the title Baron St.
Helens and who was at the time the British Ambassador to Spain.
In contrast, the gently sloping shield volcanoes, such as those in Hawaii, typically erupt nonexplosively, producing fluid lavas that can flow great distances from the active vents.
Although Hawaiian-type eruptions may destroy property, they rarely cause death or injury.
Helens "Louwala-Clough," or "smoking mountain." The modern name, Mount St.