La Soufriѐre Eruption 2020-2021 Glossary

Banded Tremor – Bursts of tremor (seismic signal) that can last for hours separated by periods of quiet that can last equal lengths of time.

Depressurization – a decrease in pressure within a volcano caused by the release of magma, gases or fluids during an explosive event.

Eruption column – This is a mixture of volcanic ash, rock particles and gases projected into the air by volcanoes during explosive eruptions.

Eruption column collapse – occurs when the weight of the volcanic particles in the column exceeds the upward buoyancy of the column. The volcanic particles fall back down to the ground under the influence of gravity and can form pyroclastic density currents and surges.

Explosive eruption – In an explosive eruption, pressurised gases trapped in rock or magma expand rapidly, breaking the rocks apart violently. Explosive eruptions usually send ash (fine rock particles) high into the atmosphere in the form of an eruption column.

Hybrid Earthquakes – these seismic signals combine features of both high-frequency VTs and low-frequency long period earthquakes. They can occur due to the nature of the source zone e.g. a fluid filled space or due to nature of the rock the seismic signal travels through on its path to the seismometer.

Lahars – a rapidly flowing dense mixture of rock debris, ash and water originating from a volcano. Also known as ‘mudflows’ or ‘debris flows’, lahars resemble the behaviour of wet concrete as they flow. It can occur during and after eruptions, and also for several years after an eruption when volcanic deposits are remobilised during heavy rainfall events.

Lateral Blast – A volcanic explosion that ejects material outwards at a high velocity from the side of the volcano.

Lava Dome – An often rounded or hemispherical mass that is formed on a volcano when thick (viscous), slow-moving lava is squeezed from the vent during an effusive (quiet) eruption. Lava domes are often found at volcanoes in the Eastern Caribbean as the lava is typically thick and sticky.

Long-Period Earthquakes – caused by changes in pressure due to movement of magma within the volcano. These earthquakes often indicate that a volcano is about to erupt.

Plinian eruptions – A violent explosive eruption that generates sustained and steady eruptive columns of heights that can range from 40km to 50km, before spreading laterally.

Pumice – A lightweight volcanic rock formed during explosive eruptions. Pumice usually has many small spaces left behind by gases released during formation.

Pyroclastic flows – hot and very fast moving mixtures of ash, rock fragments and gas that are usually hot and can travel down the sides of the volcano for several kilometers and can even travel across water. Such flows form when an eruption column or a lava dome collapses.

Pyroclastic Surges – Mixtures of ash, rock fragments and gas that are usually hot and can travel faster and further than pyroclastic flows. Pyroclastic surges are more dilute than pyroclastic flows and hence less dense which means that they can also move uphill (over hills and ridges), thus their effects can be very dangerous and unpredictable.

Rockfall – The fast movement of rock pieces down the side of a mountain or slope.

Seismic Amplitude – refers to the size of a seismic wave.

SO2 Flux – The measurement of the amount (mass) of SO2 that is present in the volcanic plume.

Sub-Plinian eruptions – An explosive eruption that generates an unsteady and low volume eruption column that can reach heights of 20km.

Sulphur Dioxide (SO2) – Sulphur dioxide is one of the gases that is released from an erupting volcano and carries a pungent odour. The presence of SO2 can indicate that fresh magma from a deeper source is being degassed and signals that an eruption is ongoing.

Swarm – A group of many earthquakes of similar size occurring closely together over a certain area and time none of which may be identified as a main event.

Tephra – Materials of all types and sizes that are erupted from a volcano and deposited from the air onto surrounding areas.

Tremor – A type of seismic signal observed at active volcanoes. This low-frequency* signal is usually associated with magma movement.

*Frequency – the amount of vibrations per second.

Volcanic Ash – Fine material carried upwards in an eruption column before it settles. Ash falls can blanket the entire island and may be thick enough to collapse buildings and destroy vegetation. Volcanic ash can be corrosive to machinery and motors, it can affect boat and plane engines and any equipment that relies on an air intake. Volcanic ash can cause irritation to eyes, skin and breathing (particularly in vulnerable persons). Certain kinds of ash (depending on the size and content) can have other longer term health impacts.

Volcanic Bombs – A fragment of magma that is blown out during an explosive eruption and can travel several kilometers from the vent.

Volcanic Hazard Zones – Defined areas around a volcano that are likely to be affected by the products of an eruption e.g. ash fall, pyroclastic flows, rockfall etc.

Volcanic Plume – A violent mixture of hot volcanic particles and gases ejected explosively from a volcano that can reach high into the atmosphere.

Volcano–tectonic earthquakes – high-frequency* vibrations caused by rock fracture (breaks) or minor fault movement associated with deformation (“swelling” of the flanks of the volcano). These vibrations are the result of fluids moving through cracks within the earth’s crust.

*Frequency – the amount of vibrations per second.

Volcanic Vent – An opening on the Earth’s surface where lava, rock fragments, hot gases and other volcanic material can escape when an eruption occurs.

Vulcanian eruption – A violent explosive eruption that has less than 1km3 of material but has a high rising eruption column (10-20 km).