Volcanic hazard assessment and prediction of upcoming eruptions is based on the record of past eruptions on various time scales, integrated with results of volcano surveillance. Younger, historical eruptions are often documented by observation, through scientific instruments and/or eyewitnesses' reports. Activity further back in time are reconstructed based on deposits of past eruptions. Such information facilitates to understand the systematics of the processes acting at individual volcanoes or in volcanic areas. For an all-encompassing hazard assessment, three components are taken into consideration: (1) eruptions styles and magnitudes that represent the typical behaviour of a volcano; (2) the chronological pattern of the eruption sequence (the probabilistic approach); and (3) the current activity (the deterministic approach).
Eruption styles and magnitudes
A central aspect of volcanic hazard assessment is the question on what will happen when a volcano erupts. Many volcanoes develop one or several particular eruption styles that are "typical" for them. The eruption behaviour of a volcano is usually directly linked with its tectonic setting. While large-scale plate tectonics govern the geochemical composition of the magma, local to regional tectonic features such as faults direct the magma ascent through the crust.
For many historical volcanic eruptions there are direct observations. To investigate eruptions of the earlier past, the deposits are interpreted: what are they composed of, what kind of spatial distribution do they exhibit? This allows to constrain the behaviour of past eruptions: what quantities of magma and gas were expelled? How far did the rocks fly or flow during and immediately after the eruption? Was the eruption explosive or effusive? Were there any "secondary phenomena", such as slope failures or tsunamis?
Eruption probabilities
How likely is it that a volcano will erupt? When will it probably erupt again? Any risk assessment is only of value when it has taken the probability of an event into consideration.
Just as the expected type and strength of a future eruption is derived from past volcanic activity, so too is its likelihood. The tephrostratigraphic and historical record facilitates to identify not only the eruption styles and magnitudes but also the temporal sequence of events. Statistical time series analyses of the intervals between eruptions – the so-called repose times, i.e., the periods in which the system builds up towards the next eruption – provides a powerful tool to estimate the probability of a new eruption to occur within a given time in the future.
Since volcano monitoring is cost-intensive, requiring specific instrumentation and highly-trained personnel, it is usually impossible to monitor all volcanoes to a degree that would be desirable. A probabilistic evaluation assists the responsible national and regional Geologic Surveys and related authorities to decide which volcanoes are to be given priority for surveillance.
Volcano monitoring and early warning
It is impossible to tame the forces of a volcanic eruption. But awareness of the danger combined with reception and understanding of precursory signs of an impending eruption offers the possibility to take precautionary measures, and, where appropriate, evacuate the population. Volcano monitoring and the related early warning is implemented over short timescales. Months, weeks, days, and finally hours and minutes. For sure, a volcano can explode without any warning. But often there are signs of unrest. Changes in the volcano's activity level are recorded and evaluated: are the changes taking place gradually or abruptly; are they minor or do they have dramatic dimensions?
What are the first signs that an eruption is brewing?
Harbingers of an impending volcanic eruption include:
• Changes in the amount and composition of the gases being silently ejected
• Changing fumarole temperatures
• Deformation of the volcanic edifice
• Small earthquakes
A variety of instruments is employed for monitoring purposes: silently released gases can be sampled directly. Gas emission is also determined by remote sensing techniques, that is, by satellites, ground based stations and hand-held devices. Seismic networks record earthquakes. GPS networks and satellites are available for the detection of deformation of the volcanic edifices. Since the recent past, devices such as webcams are also used for this purpose.
Knowledge bundling and cooperative approach Assessing volcanic hazards is a complex task. The more knowledge accumulates on the various parameters, the more precise and reliable the eruption forecast will be. Through ongoing work on individual eruptions, volcanoes, and volcanic areas, increasingly more profound information is becoming available for a better understanding and professional mitigation strategies in managing volcanic crises. An important aspect in this context is the human awareness of the threats they are exposed to. Higher risk awareness contributes to increased cooperation of the population, smoothing decision-making processes regarding future-oriented land use and air traffic planning.
