A volcano is a geological structure or opening through which molten rock, volcanic gases, ash and other materials from Earth's interior reach or approach the surface. The term can refer to the physical landform, the vent or conduit system through which material moves, or the broader volcanic system responsible for eruptions. Volcanoes occur because Earth's internal heat drives geological processes capable of generating magma and transporting it through the crust.
Some volcanoes form as prominent mountains, while others appear as relatively low volcanic fields, fissures, submarine vents or broad plateaus. A volcano is therefore not necessarily a conical mountain. Its essential geological characteristic is a connection, temporary or persistent, between a source of magma and the surface environment. Volcanic activity can construct new crust, alter landscapes, release gases, produce fertile soils and create geothermal resources, while explosive eruptions can also cause severe hazards to people, infrastructure, ecosystems and aviation.
Volcanoes are closely associated with plate tectonics, although volcanism also occurs in settings that do not fit simple plate boundary categories. Many volcanoes occur at convergent boundaries where one tectonic plate descends beneath another in a process called subduction. Water and other volatile components released from the descending slab can contribute to melting in the overlying mantle, generating magma that rises through the crust.
Other volcanoes occur at divergent boundaries where tectonic plates move apart and mantle material rises and partially melts because pressure decreases, a process called decompression melting. Hotspots can produce volcanic activity away from plate boundaries, as illustrated by the Hawaiian volcanic chain, although the detailed origin and persistence of hotspots remains an active area of geological research. Mid ocean ridges, continental rifts, subduction zones and intraplate volcanic provinces therefore represent several distinct volcanic environments.
A volcano generally contains a complex plumbing system rather than a single simple pipe. Magma may originate at considerable depth, accumulate temporarily in one or more magma reservoirs, migrate through fractures and dykes, and eventually reach the surface through conduits or fissures. Some magma solidifies underground before reaching the surface, forming intrusive igneous rocks such as granite, gabbro or diorite depending on composition and cooling history.
Magma that reaches the surface becomes lava, while fragmented material produced during an eruption can become volcanic ash, lapilli, bombs and other pyroclastic deposits. Volcanic gases commonly include water vapour, carbon dioxide and sulfur dioxide, together with smaller quantities of other gases. The composition, temperature, gas content and viscosity of magma strongly influence how a volcano behaves.
Volcanoes are classified using several different criteria, including shape, eruptive behaviour, magma composition, geological setting and current activity. Major morphological types include shield volcanoes, stratovolcanoes, cinder cones and lava domes, while fissure eruptions may occur without producing a conventional central cone. Shield volcanoes tend to have broad, gently sloping profiles because they are built mainly from relatively fluid basaltic lava. Stratovolcanoes generally have steeper sides and are constructed from repeated layers of lava, ash, tephra and other volcanic deposits.
Cinder cones are relatively small, steep sided cones composed primarily of loose pyroclastic fragments accumulated around a vent. Lava domes form when highly viscous lava accumulates close to a vent instead of flowing far away. These categories overlap in some volcanic systems, and individual volcanoes can change behaviour during their geological histories.
Volcanic activity is often described using terms such as active, dormant and extinct, but these labels require careful interpretation. An active volcano is generally one that is erupting or has demonstrated relatively recent geological activity and remains capable of erupting. Dormant usually describes a volcano that is not currently erupting but may erupt again, whereas extinct traditionally implies that future eruption is considered highly unlikely.
There is no universal time interval that cleanly separates these categories because volcanic systems can remain quiet for centuries or thousands of years before becoming active again. Monitoring organisations therefore examine seismicity, ground deformation, volcanic gas emissions, temperature changes, hydrothermal activity and other indicators rather than relying solely on whether an eruption has occurred in recent human history. A volcano that appears quiet at the surface may still contain hot magma or hydrothermal systems beneath it.
Volcanoes have profound effects on Earth's surface and atmosphere. Eruptions can build islands and mountains, create new land, produce mineral deposits and supply nutrients to soils through weathering of volcanic rocks. Basaltic volcanic rocks form a substantial portion of oceanic crust, particularly at mid ocean ridges. Large explosive eruptions can inject sulfur dioxide and fine particles into the stratosphere, where chemical and radiative processes can temporarily alter climate by increasing the reflection of incoming sunlight.
Volcanic carbon dioxide also contributes naturally to the long term carbon cycle, although modern anthropogenic emissions are much larger than typical annual volcanic emissions. Volcanism also influences groundwater systems, geothermal energy and hydrothermal ecosystems. At the same time, lava flows, ashfall, pyroclastic density currents, lahars, volcanic gases, landslides and tsunamis generated by volcanic activity can produce serious hazards.
The word volcano ultimately derives from Vulcano, one of the Aeolian Islands of Italy, whose volcanic activity was historically associated with Vulcan, the Roman god of fire. Scientific understanding of volcanoes developed from early observational descriptions into modern volcanology, which combines geology, geophysics, geochemistry, remote sensing and atmospheric science. Historical eruptions provided important evidence about volcanic processes, while modern instruments can detect tiny earthquakes, centimetre scale ground deformation and changes in gas chemistry before or during eruptions.
Volcanologists use seismometers, global positioning systems, satellite radar, thermal imaging, gas analysers, gravimeters and other techniques to investigate volcanic systems. This multidisciplinary approach is important because magma remains inaccessible at depth in most cases, meaning that scientists must infer its movement and state from indirect physical and chemical signals.
A common misconception is that every volcano is a mountain with a crater at its summit and that every eruption is an explosive event. In reality, volcanic systems vary enormously in size, shape, composition, duration and eruptive style. Some eruptions produce slowly advancing lava flows with relatively little explosive activity, whereas others generate enormous columns of ash and gas, pyroclastic density currents and widespread tephra. Some eruptions occur underwater, and much of Earth's volcanic activity takes place beneath the oceans. Volcanoes can also have multiple vents, fissures, parasitic cones, calderas and extensive underground plumbing systems. Understanding a volcano therefore requires consideration of its magma supply, geological setting, structure, history and current physical state rather than simply identifying its surface shape.
