Volcano, vent in the crust of Earth or another planet or satellite, from which issue eruptions of molten rock, hot rock fragments, and hot gases. A volcanic eruption is an awesome display of Earth’s power. Yet, while eruptions are spectacular to watch, they can cause disastrous loss of life and property, especially in densely populated regions of the world.
Sometimes beginning with an accumulation of gas-rich magma (molten underground rock) in reservoirs near Earth’s surface, they can be preceded by emissions of steam and gas from small vents in the ground. Swarms of 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. 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 cored 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.

One feared phenomenon accompanying some explosive eruptions is the nuée ardente, or pyroclastic flow, a fluidized mixture of hot gas and incandescent particles that sweeps down a volcano’s flanks, incinerating everything in its path. Great destruction also can result when ash collects on a high snowfield or glacier, melting large quantities of ice into a flood that can rush down a volcano’s slopes as an unstoppable mudflow. (See the table of the world’s major volcanoes by region.)
Strictly speaking, the term volcano means the vent from which magma and other substances erupt to the surface, but it can also refer to the landform created by the accumulation of solidified lava and volcanic debris near the vent. One can say, for example, that large lava flows erupt from Mauna Loa volcano in Hawaii, referring here to the vent; but one can also say that Mauna Loa is a gently sloping volcano of great size, the reference in this case being to the landform. Volcanic landforms have evolved over time as a result of repeated volcanic activity.

Mauna Loa typifies a shield volcano, which is a huge, gently sloping landform built up of many eruptions of fluid lava. Mount Fuji in Japan is an entirely different formation. With its striking steep slopes built up of layers of ash and lava, Mount Fuji is a classic stratovolcano. Iceland provides fine examples of volcanic plateaus, while the seafloor around Iceland provides excellent examples of submarine volcanic structures.
Volcanoes figure prominently in the mythology of many peoples who have learned to live with eruptions, but science was late in recognizing the important role of volcanism in the evolution of Earth. As late as 1768, the first edition of the Encyclopædia Britannica gave voice to a common misconception by defining volcanoes as “burning mountains, which probably are made up of sulphur and some other matter proper to ferment with it, and take fire.”

Today geologists agree that volcanism is a profound process resulting from the thermal evolution of planetary bodies. Heat does not easily escape from large bodies such as Earth by the processes of conduction or radiation. Instead, heat is transferred from Earth’s interior largely by convection—that is, the partial melting of Earth’s crust and mantle and the buoyant rise of magma to the surface. Volcanoes are the surface sign of this thermal process. Their roots reach deep inside Earth, and their fruits are hurled high into the atmosphere.
Lava, gas, and other hazards
The list of hazards associated with volcanic eruptions is long and varied: lava flows, explosions, toxic gas clouds, ash falls, pyroclastic flows, avalanches, tsunamis, and mudflows. In addition to these immediate dangers, volcanic activity produces secondary effects such as property damage, crop loss, and perhaps changes to weather and climate. These hazards and long-term effects are described in this section.

The root zone of volcanoes is found some 70 to 200 km (40 to 120 miles) below the surface of Earth. There, in Earth’s upper mantle, temperatures are high enough to melt rock and form magma. At these depths, magma is generally less dense than the solid rocks surrounding and overlying it, and so it rises toward the surface by the buoyant force of gravity. In some cases, as in the undersea zones where the tectonic plates of Earth’s crust are separating, magma may move directly up to the surface through fissures that reach as deep as the mantle. In other cases, it collects in large underground reservoirs known as magma chambers before erupting to the surface. Molten rock that reaches the surface is called lava.

Massive volcanic explosions are caused by the rapid expansion of gases, which in turn can be triggered by the sudden depressurization of a shallow hydrothermal system or gas-charged magma body or by the rapid mixing of magma with groundwater. The ash, cinders, hot fragments, and bombs thrown out in these explosions are the major products observed in volcanic eruptions around the world. These solid products are classified by size. Volcanic dust is the finest, usually about the consistency of flour. Volcanic ash is also fine but more gritty, with particles up to the size of grains of rice. Cinders, sometimes called scoriae, are the next in size; these coarse fragments can range from 2 mm (0.08 inch) up to about 64 mm (2.5 inches).
Fragments larger than 64 mm are called either blocks or bombs. Volcanic blocks are usually older rock broken by the explosive opening of a new vent. Large blocks ejected in such explosions have been hurled as far as 20 km (12 miles) from the vent. Volcanic bombs, in contrast, are generally incandescent and soft during their flight. Some bombs take on strange, twisted shapes as they spin through the air. Others have a cracked and separated crust that has cooled and hardened in flight; they are called “breadcrust bombs.”

A directed blast in which one side of a volcanic cone fails, as happened at Mount St. Helens in the United States in 1980, can cause destruction over several hundred square kilometres on the failed flank of the volcano. This is especially true if the blast cloud is heavily laden with fragmental debris and becomes dense and fluidized. It then takes on characteristics similar to a pyroclastic flow.
Pyroclastic flows are the most dangerous and destructive aspect of explosive volcanism. Variously called nuées ardentes (“glowing clouds”), glowing avalanches, or ash flows, they occur in many sizes and types, but their common characteristic is a fluidized emulsion of volcanic particles, eruption gases, and entrapped air, resulting in a flow of sufficiently low viscosity to be very mobile and of sufficiently high density to hug the ground surface. A pyroclastic flow can pour over the lip of an erupting vent, or it may form when an ash column becomes too dense to continue rising and falls back to the ground. In major caldera collapses associated with explosive volcanoes (see below Calderas), huge pyroclastic flows may issue from the ring fractures as the caldera block subsides.