Renewed volcanic activity in the Cascade Arc , such as the 1980 eruption of Mount St. Helens , has offered a great deal of evidence about the structure of the Cascade Arc . One effect of the 1980 eruption was a greater knowledge of the influence of landslides and volcanic development in the evolution of volcanic terrain . A vast piece on the north side of Mount St. Helens dropped and formed a jumbled landslide environment several kilometers away from the volcano . Pyroclastic flows and lahars moved across the countryside . Parallel episodes have also happened at Mount Shasta and other Cascade volcanoes in prehistoric times
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Cascade Volcanoes