Many of the great volcanoes that formed over subductive margins are composite volcanoes which disgorge mainly pyroclastic materials including ash as well as gases. These edifices are the classic ‘volcano’ shape, and in a cross-section such volcanoes look layered. Composite volcanoes, with their acidic, gas-filled lavas, are also notoriously dangerous. They often erupt explosively with little or no warning. Pyroclastics can be hurled far from the crater and volcanic ash can go into the stratosphere and affect the climate of a part of the while of the world. Ash can create health hazards that choke human and animal life away from the volcano.
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