Volcanoes are not random phenomena, but owe their existence, location, morphology, and eruptive styles to tectonic plate motions. Volcanic landforms vary widely in size and morphology from the stereotypical towering glacier-clad stratovolcano. They range from small spatter cones to massive shield volcanoes several thousand cubic kilometers in volume or broad volcanic depressions, cinder cones being the most common volcanic construct. Their eruptions likewise span many orders of magnitude in volume. Evaluation of eruption magnitudes and frequencies contrasts the vast majority of mild-to-moderate eruptions (∼80% with volcanic explosivity index, or VEI ≤2) with the more infrequent larger, higher-impact events (5% with VEI ≥4). Increased risk from population growth in proximity to volcanoes has been mitigated by enhanced monitoring and hazard-reduction efforts.
