Volcano

A volcano is a rupture in the crust of a planetary body that allows molten rock, volcanic ash, and various gases to escape from a magma chamber below. On Earth, most volcanism occurs at tectonic plate boundaries or above geological hotspots. When magma reaches the surface, it can produce lava flows, explosive ash plumes, and a variety of volcanic materials.

Volcanoes vary in eruption style according to magma viscosity and gas content. Basaltic magma, which is relatively fluid, tends to produce effusive eruptions with lava flows that can build shield volcanoes. Higher-silica magmas such as andesite or rhyolite trap more gases and are prone to explosive eruptions, forming stratovolcanoes and lava domes. Eruptions eject lava, volcanic ash, pumice, and various pyroclastic materials; they can also release gases such as water vapor, carbon dioxide, and sulfur dioxide.

Common landforms include shield volcanoes with broad, gentle slopes; stratovolcanoes with steep profiles and alternating eruptive styles; and cinder cones built from tephra. Volcanoes can also occur underwater, creating seamounts and island arcs.

Hazards from volcanic activity include lava flows, pyroclastic flows, ashfall, gas emissions, lahars, landslides, and tsunamis triggered by eruptions or sector collapse. Many hazards can be forecast with monitoring of seismic activity, ground deformation, gas emissions, and thermal anomalies.

Notable examples include Mauna Loa and Kilauea in Hawaii, Mount Etna in Sicily, Vesuvius in Italy, and Mount St. Helens in the United States. Global volcanism is concentrated around the Pacific Ring of Fire and at continental rifts and hotspots. Volcanoes may awaken, become dormant, or become extinct, though some dormant systems erupt again after long periods of quiescence.