Background

Most volcanic eruptions are preceded by a period of volcanic unrest that perhaps is best defined as the deviation from the background or baseline behaviour of a volcano towards a behaviour which is a cause for concern in the short-term because it might prelude an eruption (Phillipson et al., 2013).

Although it is important that early on in a developing unrest crisis scientists are able to decipher the nature, timescale and likely outcome of volcano reawakening following long periods of quiescence there are still major challenges when assessing whether unrest will lead to an eruption in the short-term or wane with time.

Analysis of volcanic unrest

An analysis of 228 cases of reported volcanic unrest between 2000 and 2011 (Phillipson et al. (2013); Figure 18.1) recognises five primary observational (predominantly geophysical and geochemical) indicators of volcanic unrest:

Ground deformation: Restless volcanoes often undergo periods of ground uplift or subsidence driven for example by pressure changes in their magma reservoir or overlying geothermal reservoir. In some cases pressure increase may break the ground surface. Ground deformation is generally recorded by ground or space-borne techniques [see also Chapter 17].

Degassing: Plumes of gas may be released from craters or other vents (fumaroles) on a volcanic edifice craters. Alternatively the amount of gas released may increase or the chemical composition of gases may change over time. Ground and space-borne techniques are usually applied to monitor degassing behaviour [see also Chapter 17].

Changes at a crater lake: These changes include variations in lake temperature, lake levels, level of water chemistry, lake colour and gas release and are generally recorded using ground-based or air-borne techniques.

Thermal anomaly: Anomalous temperature changes of the ground or of fumarolic gases can be recorded by ground-based, air or space-borne sensors [see also Chapter 17].

Seismicity: The movement of magma, fluids and gas can cause seismic signals at restless volcanoes as does the breaking of rock from stress increases at depth. Particular seismic wave forms are generated from such processes which may provide clues as to what is driving unrest at a particular volcano. Seismic observations are generally made on the ground.