Copy of `GeoNet - Volcano glossary`

The wordlist doesn't exist anymore, or, the website doesn't exist anymore. On this page you can find a copy of the original information. The information may have been taken offline because it is outdated.

GeoNet - Volcano glossary Category: Sciences > Volcano glossary Date & country: 19/11/2010, UK Words: 15

Viscosity
The ability of a liquid to flow. Basalt magma has a relatively low viscosity making it runny, whereas rhyolite magma has a high viscosity making the magma thick and sticky.

Silica
Another name for silicon dioxide, the basic building block of volcanic rocks. Silica is a major constituent of most magmas and the amount of silica controls the viscosity of the magma

Scoria
A frothy basaltic rock, full of small gas bubbles. Often a black or red colour.

Pyroclastic flow
A surface-hugging eruption cloud of very hot gas and volcanic particles that moves rapidly across the ground surface, away from the vent.

Rhyolite
A type of highly viscous magma with high silica content; it is found as pumice (in airfall deposits or ignimbrites), lava or obsidian. Rhyolite is also the name given to the volcanic rock formed from rhyolitic magma.

Lava
Magma that reaches the surface; molten material that is thrown out or has flowed from a volcano.

Magma
Rock material in a molten state within a volcano or underneath the earth's surface. Once magma reaches the surface it becomes lava.

Lahar
Mud flow of water and volcanic material commonly caused by the bursting of a crater lake, eruption from a snow-capped volcano or from prolonged torrential rain.

Debris avalanche
A sudden collapse of volcanic material from an unstable side of a volcano.

Eruption plume
A cloud of volcanic ash emitted from a volcanic vent or volcano.

Fissure
A large crack in the ground allowing magma to travel up and erupt onto the surface

Dacite
A type of volcanic rock intermediate between andesite and rhyolite, Mt Edgecumbe is an example of this.

Caldera
A large volcanic depression up to 50 km across, formed by a collapse during and after an eruption. The northern part of Lake Taupo is a caldera.

Basalt
A type of fluid magma with low silica content. Forms dark coloured rock (often red or black), such as the scoria cones of Auckland.

Andesite
A type of magma with intermediate viscosity and silica content. Forms large composite volcanoes, sometimes called stratovolcanoes, made up of alternating ash and lava layers, such as Mt. Ruapehu. Andesite is also the name given to the volcanic rock formed from andesite magma.