A sample of tephra erupted by Mount St. Helens on May 18, 1980. The tephra was collected between about 40 and 60 km downwind from the volcano. Most of the tephra consists of pumice. The largest fragment is about 5 mm in diameter. Tephra between 2 and 64 mm in diamter is called lapilli; tephra <2 mm in diamter is called ash.
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Last modified: October 14, 1998