Mount St. Helens: An overview of the eruptive history and petrology, tephra deposits, 1980 pyroclastic deposits and the crater

Mount St. Helens

Post-meeting field trip, August 19-22; max/min participants, 15/12
This field trip will provide an introduction to several fascinating features of Mount St. Helens. The trip begins with a rigorous ~15 km hike from the Johnson Ridge Observatory (9 km NNE of the crater-vent) across the 1980 pumice plain to Windy Ridge (3.6 km NE of the crater-vent). During the hike we examine features that document the dynamics and progressive emplacement of pyroclastic flows. The next day we drive to classic tephra outcrops on the east flank and observe changes in thickness and character of these fall deposits as we traverse their respective lobes. We examine clasts in the deposits and discuss how the petrology and geochemistry of Mount St. Helens deposits reveal the evolution of the magmatic system through time. We also investigate the stratigraphy of the 1980 blast deposit and review the chronology of this famous eruption as we drive through the remains of the blown-down forest. The third day also includes another rigorous hike of 12 km round-trip, this time climbing from the base of Windy Ridge (el. 1240 m) into the mouth of the crater (el. 1700 m). During the trip we examine stranded masses of 1980 debris avalanche deposit, beautifully preserved 1980 pyroclastic flow deposits (including a spectacular canyon-wall exposure) as well as seeing the “volcano in cross section” in the crater walls.

We will depart from the Oregon Convention Center in Portland at 2:30 PM on 19 August and stay overnight the first night in Castle Rock; all subsequent nights are at the tent camp.  We will return to Portland on the morning of the 23rd with a planned arrival at PDX by 12:00 PM or to the downtown area by 1:00 PM. Please note that this is a camping trip that includes two strenuous all-day hikes in hot weather over rough terrain with significant climbs and descents. Large multi-person tents with cots as well as food at the camp are provided. Participants need to bring sleeping bags (summer weight is recommended) and field gear.

Estimated Cost
$600/person.  Includes transportation, tents with cots, breakfast, lunch, and dinner, except dinner the first night in Castle Rock.

John Pallister, U.S. Geological Survey
Heather Wright, U.S. Geological Survey
Alexa Van Eaton, U.S. Geological Survey
Jim Vallance, U.S. Geological Survey
Peter Kokelaar, University of Liverpool