Volcanoes form at plate boundaries or over hot spots in the Earth's crust. The type of eruption depends on the composition of the magma and amount of trapped gases. Shield volcanoes produce quiet eruptions of basaltic lava forming wide slopes while cinder cone volcanoes have explosive eruptions of thicker magma forming steep slopes. Composite volcanoes have characteristics of both. Major examples discussed are Kilauea, Krakatau, Mount St. Helens, and Mount Pinatubo. Scientists monitor volcanoes for warning signs of an impending eruption like earthquakes or gas changes.