Dome Building Years

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Dome Building Years
Back to Mt St Helens
Towards the end of 1980, the crater of Mt St Helens had an obvious vent hole, through which steam, ash and pumice would be released. The floor of the crater was built of these releases, a huge pile of large rocks... most of them were ridden with sulfur.

	In October, an upwelling of magma began to form a lava dome. In a week, the dome had grown to a height of 112 feet, and was about 300 feet around. grew in periodic extrusions of stubby lava flows, called lobes. This pattern changed in February 1983, when growth became continuous and mostly endogenous (internal). Periodic lobe growth, along with endogenous growth, resumed in early 1984. An aerial view of the June 1981 lobe with its "spreading center" is shown here.

By August of 1981, the dome was about 500 feet high, or about the height of a 44 story building. It was about a 1/4th mile in width. By 1983, the dome was quite an impressive, rising almost 700 feet into the crater. (The image to the right is "VIEW A"... you'll 'get it' later) After a flurry of activity during that year, however, the rate of growth slowed considerably.

	In 1984, Geologists use a EDM (Electronic Distance Meter) to measure angles and slope-distances to the lava dome. Changes in these angles and distances are used to calculate "deformation rates". An increase in deformation rates is an indication that magma is slowly entering the dome. Deformation rates often reach 30 feet per hour (10 m/hr) as magma rises and the dome expands before extrusion starts.
By 1985, the dome had grown to 770 feet, and was more than 1/2 a mile wide. This picture is "VIEW B"... it was taken with the same camera at the same location as "VIEW A", earlier on this page.
	As large as it was, though, it still had a considerable ways to go before it would grow the mountain to what it was! Using the growth rates as a model, it was estimated that it would take about 200 years to return the volcano to the cone shape that gave it its nickname, "Mt Fuji of the West". Events in the next year, however, would change that assessment.