Wednesday, April 7, 2010

Old Aerial Photos

Aerial photos taken April 7, 1980, compared to photos taken in the previous summer, allowed USGS geologists to map the changes in elevation and shape that had occurred in prior weeks. Keeping in mind that I had not had my first college-level class in geology yet, and that I was getting all of my information from mainstream media, I thought the bulge was interesting but it wasn't really the focus of my attention.
Map view of elevation changes (in feet) on the north flank of Mount St. Helens between August 1979 and April 7, 1980. Dashed line outlines the May 1980 crater. Red contours are negative values, blue contours are positive values (figure modified from USGS Professional Paper 1250, p.126).
(From the chronology at the Mount St. Helens Volcanic Monument web page) No, what really grabbed my attention was that behind the clouds of the last few days, the formerly two discrete summit craters had merged into one enormous, gaping, hole.
(USGS Photograph taken on April 7, 1980, by Peter Lipman, from the USGS pre-eruption photo archive.)
(USGS Photograph taken on April 6, 1980, by James G. Moore, from the USGS pre-eruption photo archive.) To me, at least, this was really starting to look like a real volcanic eruption, not just indigestion. Again referring to the pre-eruption chronology, it is stated that
When the new (1980) map was compared to the old map, it showed that parts of the north flank had moved outwards by over 300 feet! Although the aerial photos were taken on April 7, the new data on the bulge was not available to scientists until April 23.
And today's entry:

April 7 - The crater's dimensions measured approximately 1700 feet long by 1200 feet wide and 500 feet deep. Scientists obtained their first clear view of the vent at the bottom of the deepest part of the crater. A single circular "throat" about 20 feet in diameter was visible.

They also reported the first observations of substantial ponding of water. Two pools of muddy water large enough to float chunks of ice disappeared with each explosion then returned as more water accumulated after the explosion subsided.

The overall seismicity remained about the same and harmonic tremor was recorded for the first time in two days. Earthquakes were centered beneath the north slope at depths ranging from one half to three miles. USGS scientists installed a new tiltmeter at the Timberline Viewpoint.

While I know that the region's newspapers were talking about the bulge, and even a generally geo-ignorant person such as myself could see the changes in shape in the before and after pictures, I don't think most of us in the general public really perceived the magnitude of the mountain's expansion. And I know we (the public, that is) didn't perceive the danger inherent in oversteepening a mountainside over a fresh, hot magma chamber.

I just tracked down a tidbit of information I've been watching for; contrary to a point I made in a previous post,

Reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 March 1980, just two weeks after the first seismic signals of reawakening, and that the juvenile content of the ash had probably increased by early May. The magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material—a signature of extensive degassing-induced crystallization during magma ascent.
K.V. Cashman and R.P. Hoblitt, Geology; February 2004; v. 32; no. 2; p. 141-144; DOI: 10.1130/G20078.1

This is part 6 in a multi-part series on the events leading up to the catastrophic eruption of Mt. St Helens on May 18th, 1980, 30 years ago this spring. Part 1, Part 2, Part 3, Part 4, Part 5.

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