07 Volcanoes and Volcanism
Learn how magma forms and moves, how its properties influence eruption styles and volcanic landforms, and why volcanoes create hazards that vary by place and time.
From to the surface
is the movement of and volcanic gases from within Earth to or near its surface. An eruption can build new land, reshape a landscape, and create hazards near a volcano or far downwind and downstream.
is molten rock below ground. It is generally less dense than the surrounding solid rock, so it can rise through cracks and fractures. Once reaches the surface, it is called lava. Before an eruption, may collect in an underground reservoir, where it can cool, crystallize, mix with other , or change composition.
How forms
forms when some, but not all, of a rock melts. Three common processes can cause this:
: Hot mantle rises and pressure decreases, allowing melting without adding heat. This occurs at mid-ocean ridges and beneath some hotspots.
: Water and other volatiles released from a descending tectonic plate lower the melting temperature of nearby mantle. This is common above subduction zones.
: Hot rises into cooler crust and transfers heat to surrounding rock, which may then partially melt.
These processes explain how molten rock can form in different tectonic settings. The 's later movement and changing conditions help determine what happens at the surface.
Why eruptions differ
Eruption behavior depends on temperature, composition, crystal content, gas content, and the rate at which reaches the surface. One important property is , a fluid's resistance to flow. Hot, low-silica basaltic is usually relatively fluid, while higher-silica , such as rhyolite, is generally more viscous.
Gas can escape more easily from fluid . In viscous , gas may become trapped as pressure falls during ascent, encouraging explosive fragmentation. These are broad patterns rather than fixed rules: water interacting with , changes in supply, and other conditions can alter an eruption. A single volcano may produce different eruption styles over time.
mainly pour out lava or produce lava fountains. Fluid basaltic lava can spread across broad areas and build layers.
blast gas, ash, and larger rock fragments into the air. They may form an eruption column. If the column collapses, hot mixtures of gas and volcanic fragments can race down the slopes as .
Some eruptions alternate between explosive activity and lava extrusion. Viscous lava may build a dome near the vent, and a later collapse can generate dangerous flows.
eject , the general term for fragments of volcanic material. consists of particles smaller than 2 millimeters. Despite its name, it is made of rock and volcanic glass, not soft ash from combustion.
Landforms built by eruptions
The materials erupted and the way they accumulate shape volcanic landforms:
are broad and gently sloping, built mainly by repeated flows of fluid lava. Hawaiian volcanoes are familiar examples.
Cinder cones are usually small, steep-sided cones formed as fragments ejected from a vent fall and accumulate nearby. They commonly have a summit crater.
Stratovolcanoes, also called composite volcanoes, are commonly steep-sided and built from layers of lava and fragmental deposits. Many occur above subduction zones.
Lava domes form when viscous lava piles up near its vent instead of flowing far. Dome growth or collapse can be hazardous.
Calderas are large depressions formed when the ground above a partly emptied reservoir collapses. They are much larger than the summit crater of a typical small cone.
Fissure eruptions issue from long cracks rather than a single central vent. Repeated, voluminous flows can create extensive lava fields or plateaus.
Landforms can overlap. A volcano's shape records its eruption history, but does not guarantee that every future eruption will have the same style.
Hazards and preparedness
Volcanic hazards vary with the volcano, the eruption, and the local terrain. Some occur during eruptions, while landslides can also happen during quiet periods.
Lava flows can burn, bury, and destroy structures, roads, and farmland. Their paths are strongly influenced by slope and terrain.
and surges are fast-moving, hot mixtures of gas and volcanic fragments. They can overwhelm areas near a volcano; less-dense surges can spread over ridges.
Ashfall can travel long distances. It can impair breathing, reduce visibility, damage machinery and aircraft engines, harm crops, contaminate water, and overload roofs when deposits are heavy, especially if wet.
form when water mixes with loose volcanic material and can travel down valleys. Water may come from rain, melting snow or ice, or crater lakes.
Volcanic gases, including water vapor, carbon dioxide, and sulfur dioxide, can affect health and air quality. Some gases can collect in low areas.
Landslides and debris avalanches can occur when a volcano's flank becomes unstable, sometimes without an eruption. Displacement of water by volcanic activity or landslides can also generate tsunamis in some settings.
Hazard maps, monitoring, and timely warnings help communities prepare. The hazards to expect depend on a volcano's past activity and local terrain; no single hazard profile applies to all volcanoes.
Connecting the main ideas
can form through decompression, the addition of volatiles, or heat transfer. Its properties and changing conditions influence whether eruptions are mainly effusive, explosive, or a combination. Eruptions build a range of landforms, while also producing hazards such as lava flows, ashfall, , , volcanic gases, and landslides. Understanding both eruption history and local terrain helps communities prepare for the hazards most relevant to them.