Eruptions of Mount St. Helens, a highly explosive and frequently active volcano in the Cascade Range have, within the past 40 000 years, produced more than 100 tephra deposits now recognizable as distinct strata. The volcano has also erupted abundant pyroclastic flows, surges, and ash clouds, as well as lava flows and domes. Tephra deposits and those other products record a complex eruptive history of Mount St. Helens and provide information about the hazard it poses to people and property. The tephra strata also serve as time-stratigraphic marker beds that are widespread in Pacific Northwest States.
Mullineaux, D. R. (1996). Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington. US Geological Survey Professional Paper, 1563. https://doi.org/10.3133/pp1563