Volcano Terms

Volcanic eruptions are among Earth's most dramatic geological events. They can destroy an area, but also eventually lead to new formations and fertile ground.

The word "volcano" is derived from the name of an island, Vulcano, in the Mediterranean Sea off Sicily, where people once believed Vulcano was the chimney of the forge of Vulcan — the blacksmith of the Roman gods. Scientists have since come to understand the mechanisms that create volcanoes and the stirrings that cause them to erupt.

Volcanoes are usually conical mountains that form around a vent connecting with pools of molten rock below the surface of the Earth. Pressure forces molten rock upward into the volcano. The vent can become clogged as the magma solidifies at the surface, but increasing pressure may break through weak zones in the Earth's crusts. This is a volcanic eruption.

Below is a list of terms used to describe the features of volcanoes.


Fine particles of pulverized rock that are blown from an explosion vent. Measuring less than one-tenth of an inch in diameter, ash may be either solid or molten when first erupted. Ash is extremely abrasive, similar to finely crushed window glass, mildly corrosive, and electrically conductive, especially when wet.


The most common type of rock formed from the cooling of lava. Basalt, which comes in a range of dark colors, contains a high percentage of iron and magnesium.


The large, basin-shaped crater at the top of a volcano. A caldera is formed when the original peak collapses into an empty chamber below.

Dormant volcano

The term used to describe a volcano that is "sleeping," or presently inactive, but may erupt again.

Extinct volcano

A volcano that is not presently erupting and is not likely to do so for a very long time in the future.


A crack from which volcanic gases, mostly water vapor, escape into the atmosphere.


A mudslide caused by the mixing of volcanic ash and debris with water. A lahar, usually caused by heavy rainfall after an eruption, looks like a mass of wet concrete carrying rocks that range in size from gravel to boulders 30 yards in diameter. A lahar can also be triggered during an eruption by the quick melting of snow or the ejection of water from a crater lake.


What magma, or molten rock, is called once it reaches the surface. Lava, which can reach 2,000 degrees Fahrenheit or more, usually moves at speeds between one-third and two-thirds of a mile per hour, which you can easily outwalk. (A normal walking speed is 2 mph to 4 mph.) However, lava flowing down a channel can speed along considerably faster, up to about 23 mph, which is about as fast as a sprinter running a 100-yard dash.


Molten rock from Earth’s interior. When magma reaches the surface, it’s called lava.


The zone of the earth below the crust and above the core.

Plate tectonics

Scientific theory explaining the movement of the continents of Earth’s surface. The surface is divided into several large, rigid plates about 50 miles deep that move on top of Earth’s hot, malleable interior.

Pyroclastic flow

An avalanche of hot lava fragments, which can cascade down at speeds of 50 mph or faster. The rock fragments range in size from ash to boulders. The hot temperatures of rocks and gas inside pyroclastic flows, generally between 400 and 1,300 degrees Fahrenheit, can cause plants and other combustible materials to catch fire. Most pyroclastic flows consist of two parts: a flow of coarse fragments that moves along the ground and a turbulent cloud of ash on top.


A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the usual conical structure that is produced by the ejected material.

Source: U.S. Geological Survey