We define volcanic unrest 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. When unrest is preceded by periods of quiescence over centuries or millennia it is particularly difficult to foresee how a volcano might behave in the short-term. As a consequence, one of the most important problems is to assess whether un- rest will culminate in an eruption or not. Here, we review and evaluate global unrest reports of the Smithsonian Institution Global Volcanism Program (GVP) between January 2000 and July 2011. The aim of the evaluation is to establish the nature and length of unrest activity to test whether there are common temporal patterns in unrest indicators and whether there is a link between the length of inter-eruptive periods and unrest duration across different volcano types. A database is created from the reported information on unrest at 228 volcanoes. The data is categorised into pre-eruptive or non-eruptive unrest indicators at four different subaerial volcano types and submarine volcanoes as de␣ned by the GVP. Unrest timelines demonstrate how unrest evolved over time and highlight different classes of unrest including reawakening, pulsatory, prolonged, sporadic and intra- eruptive unrest. Statistical tests indicate that pre-eruptive unrest duration was different across different volcano types. 50% of stratovolcanoes erupted after about one month of reported unrest. At large calderas this median average duration of pre-eruptive unrest was about twice as long. At almost ␣ve months, shield volcanoes had a signi␣cantly longer unrest period before the onset of eruption, compared to both large calderas and stratovol- canoes. At complex volcanoes, eruptive unrest was short lived with only a median average duration of two days. We find that there is only a poor correlation between the length of the inter-eruptive period and unrest duration in the data; statistical signi␣cance was only detected for the pair-wise comparison of non-eruptive unrest at large calderas and stratovolcanoes. Results indicate that volcanoes with long periods of quiescence between eruptions will not necessarily undergo prolonged periods of unrest before their next eruption.
Our findings may have implications for hazard assessment, risk mitigation and scenario planning during future unrest crises.