Distinctive annual-ring patterns in trees in a forest stand 10 km east of Mount St. Helens indicate that the volcano's voluminous layer We tephra was erupted after the A.D. 1481 growth season, late in 1481, or early during 1482. Combined with earlier research that established A.D. 1480 as the date of eruption of the underlying, similar layer Wn tephra, the new tree-ring data provide evidence that these two major explosive tephra eruptions about 500 yr ago were separated by about 2 yr. During that eruptive sequence, a voluminous explosive eruption was therefore followed by more than 1 yr of minor explosive activity before another voluminous eruption occurred. This record suggests that Mount St. Helens might produce a second large explosive eruption during its current period of activity.
Examinations of the tephra deposits that correlate with narrowring patterns in trees at Mount St. Helens also demonstrate that distinctive ring patterns can form in trees across a large range of tephra-layer thicknesses. The examinations also suggest that tephra coarseness interacts with tephra-layer thickness in determining the effects that tephra fallout will have upon forest trees.