Volcano Terms

Oct. 1, 2004 — -- 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.

Ash
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.

Basalt
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.

Caldera
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.

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

Lahar
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.