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Stop 9

Now that you’ve seen and touched a piece of Antarctica, go a few steps farther toward Building 20 to view another unusual rock. This piece of Mount St. Helens lava, is dedicated to the memory of David Johnston, the USGS scientist who died in the catastrophic explosion of the mountain on May 18, 1980.  This rock is a fairly large piece of “breadcrust” lava from the deposit of an andesitic pyroclastic flow (rock and ash flow) that descended the southwest flank of Mount St. Helens approximately 9 km from the pre-1980 summit. Tree-ring dating techniques bracket the age of the deposit between AD 1489 and 1556, about 450 to 500 years ago.

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U.S. Department of the Interior, U.S. Geological Survey
Maintainer: Susan Garcia
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URL: http://quake.usgs.gov/50years/self-guided/stop9.html
Last modification: July 22, 2004