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.