Magma that reaches the surface builds a volcano. The term refers both to the vent from which lava, ash, and gas erupt and to the mountain constructed by accumulated eruptive products. Volcanoes occur mainly at plate boundaries and over mantle hotspots.

Eruptive style ranges from gentle lava flows to catastrophic explosions. Magma composition, gas content, and the plumbing system control the behavior. Shield volcanoes, stratovolcanoes, and cinder cones are common morphological types. Calderas form when large magma chambers collapse.

Volcanoes create fertile soils, geothermal resources, and new land, yet they also pose hazards of lava flows, ash falls, pyroclastic flows, and tsunamis. Monitoring networks track seismicity, deformation, and gas emissions to forecast eruptions. Millions of people live on the flanks of active volcanoes, balancing risk against the benefits of the land.

• Vent and mountain formed by erupted material
• Occurs at plate boundaries and hotspots
• Ranges from effusive to highly explosive
• Both resource and hazard for nearby populations