Volcano

A volcano is an opening in the earth's surface through which hot rock, gases, and rock fragments from within the earth are expelled to the surface. The hot rock material still underground is called magma; when it reaches the surface it is called lava. Volcanoes often build cone-shaped mountains made up of layers of lava and ash stacked on top of one another.

Volcanoes are most commonly found at the boundaries of tectonic plates, particularly where one plate slides beneath another or where plates move apart. There are also volcanoes that originate from hot spots within a plate itself, such as those of the Hawaiian Islands.

How it works

Molten rock accumulates in a magma chamber beneath the surface. Because magma is less dense than the solid rock surrounding it, it rises upward. The pressure of gases dissolved in the magma increases as the magma rises; if the chamber ruptures, the gas escapes and causes an explosion. When the magma is low in gas or is very fluid, the lava flows out gradually and produces a small explosion.

Types

Volcanoes are classified by their shape and what they eject. A shield volcano is broad with gentle slopes, composed of fluid lava. A stratovolcano is tall with steep sides, made of alternating layers of lava and ash; these typically produce violent eruptions. There are also small ash cones and narrow fissures from which lava flows.

Ejecta and hazards

Materials expelled include lava, ash, and small rock fragments (tephra), as well as gases such as water vapor, carbon dioxide, and sulfur dioxide. The greatest hazards include pyroclastic flows of ash and hot gas, lahars (mudflows from melting snow and ice), and ash rising into the atmosphere that affects aircraft and respiratory health. Volcanic ash also enriches soil after settling, which is why many people live in volcanic areas.

Monitoring and prediction

Volcanic research centers use small seismographs, ground surface changes, and gas measurements to detect rising magma. Early warning allows people to be evacuated, but the exact timing of eruptions cannot be predicted with certainty.