volcanic eruption, an eruption of molten rock, hot rock fragments, and hot gases through a volcano, which is a vent in a planet’s or satellite’s crust. Volcanic eruptions can cause disastrous loss of life and property. They range from relatively gentle eruptions, as typically seen in Hawaiian volcanoes, to massively destructive ones, such as the eruption of Vesuvius that destroyed Pompeii in 79 CE. Volcanic eruptions have captured the imagination of people over millennia, and they feature in several mythologies as well as works of fiction. These eruptions also play a role in climate change, with expelled gases such as carbon dioxide contributing to global warming, while ash, dust, and gases such as sulfur dioxide can drive global temperatures down. 
Volcanic eruptions occur as a result of heat moving under Earth’s surface. They often begin with an accumulation of gas-rich magma (molten underground rock) in reservoirs near Earth’s surface, though they may be preceded by emissions of steam and gas from small vents in the ground. Small earthquakes, which may be caused by a rising plug of dense, viscous magma oscillating against a sheath of more permeable magma, may also signal volcanic eruptions, especially explosive ones. In some cases, magma rises in conduits to the surface as a thin and fluid lava, either flowing out continuously or shooting straight up in glowing fountains or curtains. The eruptions of Hawaii’s volcanoes fall into this category. In other cases, entrapped gases tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is hollowed out by an explosive blast, and solid fragments are ejected in a great cloud of ash-laden gas that rises tens of thousands of metres into the air. An example of this phenomenon is the 1980 eruption of Mount Saint Helens. Many explosive eruptions are accompanied by a pyroclastic flow, a fluidized mixture of hot gas and incandescent particles that sweeps down a volcano’s flanks, incinerating everything in its path. If the expelled ash or gases collect on a high snowfield or glacier, they may melt large quantities of ice, and the result can be a disastrous flood or landslide that rushes down a volcano’s slopes.

Volcanic eruptions can also result in secondary damage, beyond the direct loss to life and property from the eruption itself. Volcanic ash can cause respiratory illnesses such as silicosis and can be particularly harmful to infants and people with chronic lung diseases. Gases such as hydrogen chloride, carbon monoxide, and hydrogen fluoride can cause both short- and long-term problems. Eruptions can cause economic harm that affects workers’ livelihoods and can force mass migrations of people in affected regions. The 2010 eruption of Iceland’s Eyjafjallajökull also demonstrated the threat posed to jet aircraft by high clouds of volcanic ash; this eruption led aviation authorities to ground flights across northern and central Europe for several days. Volcanoes can be classified by the manner in which they erupt. These six types of volcanic eruptions, starting with the least explosive, make up one classification system: Icelandic, Hawaiian, Strombolian, Vulcanian, Pelean, and Plinian. Each name corresponds to a region or to a specific volcano or historical eruption that exemplifies the type. Volcanic eruptions are not limited to Earth. Jupiter’s moon Io is subject to strong gravitational forces due to Jupiter’s mass as well as interaction with Jupiter’s other moons Europa and Callisto. These forces cause distortions in Io’s shape and make it the most volcanically active body in the solar system. Mars is also known for several volcanoes, with Olympus Mons being the largest known volcano in the solar system. The volcanoes of Mars are shield volcanoes, which have a relatively flat profile, as Mars’s low gravity allows for longer and more widespread lava flows. (According to some estimates, Olympus Mons has been built up by eruptions for more than a billion years, which has resulted in its 700-km [435-mile] diameter.)

There are different types of volcanic eruptive events, including: pyroclastic explosions, with is fast-moving hot gas and volcanic matter hot ash releases lava flows gas emissions glowing avalanches, when gas and ashes release. Volcanic eruptions can also cause secondary events, such as floods, landslides and mudslides, if there are accompanying rain, snow or melting ice. Hot ashes can also start wildfires. Volcanic eruptions can impact climate change through emitting volcanic gases like sulfur dioxide, which causes global cooling, and volcanic carbon dioxide, which has the potential to promote global warming.