Geology glossary

Volcanic Ash Advisory Centers (VAAC) are research centres who monitor volcanic ash clouds in real time. Each time, a volcano erupts a significant ash cloud, short reports are issued and transmitted directly to air control centres. The 9 VAAC are located in London, Toulouse, Tokyo, Darwin, Anchorage, Washington, Montreal and Buenos Aireas and collectively cover most of the globe.

The Volcanic Ash Advisory Centers (VAAC) are groups of experts and government-funded bodies to montor volcanic ash clouds all over the planet in real time. As Europe experienced in April 2010, volcanic ash clouds are hazardous for aircraft and must be avoided to fly through, even if that means cancellation of flights.

As of 2010, there are 9 VAAC located in different areas and each focussing m... Read all

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.

Pathways of rising magma inside vertical fissures.

Dikes are imaginable as the veins of a volcano, the pathways of rising magma. A dike is called a -usually more or less vertical- flat, sheet-like magma body that cuts unconformingly through older rocks or sediments.

Most dikes can be described as fractures into which magma intrudes or from which they might erupt. The fracture can be caused by the intrusion of pressurized magma, or vice versa, ... Read all

Volcanic tremor is a continuous seismic signal with regular or irregular sine wave appearance and low frequencies (0.5-5 Hz).

Volcanic tremor is usually in the form of harmonic tremor which has a very uniform appearance, whereas spasmodic tremor is pulsating and consists of higher frequencies with a more irregular appearance.
Tremor can be caused by different processes inside the volcano, including resonance triggered by magma flowing through cracks and vents, or continuous low-frequency earthquakes so closely spaced th... Read all

A volcaniclastic rock or loose deposit made of (older) volcanic fragments.

The science of studying volcanoes.

Volcanology comprises the study of volcanoes and volcanic phenomena and is mostly regarded as a sub-part of geology, but is interwoven with other science disciplines as well: chemistry, physics, but also sociology, history, archaeology. Read all

A vulcanian (note the different term from "volcanic") eruption is an intermediately violent type of explosive eruption, stronger than strombolian explosions but much weaker than Plinian eruptions.
During vulcanian eruptions, a relatively large solid plug is ejected when magmatic gas pressure that had build up beneath it overcomes the strength of the plug.
In the same way as Stromboli island for strombolian eruptions, the term vulcanian was coined after the neighboring island of Vulcano in the Eolian Islands, where such eruptions had been observed during the eruption in 1888.

Vulcanian eruptions can follow longer or shorter intervals of repose during which gasses in the magma beneath the plug slowly build up. The smallest vulcanian explosions typically produce eruption columns of approx 1-2 km height, while larger ones approaching sub-plinian size can erupt columns reaching several to more than 10 kilometers height.
Often, the eruptions are accompanied by violent gun... Read all

An inclusion of foreign rock in an igneous rock.

Xenoliths in volcanic rocks are often pieces of the volcano's (non-volcanic) substratum picked up by, engulfed within and erupted with the rising magma. Read all

Zeolites are alumino-silicate minerals where usually water molecules are built into the crystal cructure. Zeolites are commonly forming as secondary minerals in volcanic rocks.

Zeolites (Greek, zein, "to boil"; lithos, "a stone") are hydrated aluminosilicate minerals and have a micro-porous structure.

The term was originally coined in the 18th century by a Swedish mineralogist named Axel Fredrik Cronstedt who observed, upon rapidly heating a natural mineral, that the stones began to dance about as the water evaporated. Using the Greek words which mean "stone that boil... Read all