So some eruptions are effusive and some are explosive. The volcanoes are natural hazards, because they are relatively close to people. Helen's erupted catastrophically in 1980. The U.S. Geological Survey has produced hazards maps. Particularly dangerous are areas down valleys, where lahars could channel.
Calderas develop when magma chamber drains, and the top of the volcano collapses.
Wizard island is a new volcanic cone growing out of the lake. These volcanic chains exist because subduction is occurring along most of the rim of the Pacific. Hawaii Volcanoes - Big island of Hawaii is the active volcano of a hot spot. The other islands, to northwest, are extinct volcanoes that have moved off the hot spot. Formed by thousands of successive layers of basalt flows.