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Volcanic unrest
With contributions from Robert Tilling, Gillian Norton, John Ridgway
(revised 2006)

NAME: Volcanic unrest

BRIEF DESCRIPTION: Eruptions almost always are preceded and accompanied by volcanic unrest, indicated by variations in the geophysical and geochemical state of the volcanic system. Such geoindicators commonly include changes in seismicity, ground deformation, nature and emission rate of volcanic gases, fumarole and/or ground temperature, and gravity and magnetic fields. Volcanic unrest can also be expressed by changes in temperature, composition, and level of crater lakes, and by anomalous melting or volume changes of glaciers and snowfields on volcanoes. When combined with geological mapping and dating studies to reconstruct comprehensive eruptive histories of high-risk volcanoes, these geoindicators can help to reduce eruption-related hazards to life and property. However, not all volcanic unrest culminates in eruptions: in many cases the rising magma fails to breach the surface to erupt. Nevertheless, indicators that signal volcanic unrest may sometimes themselves cause harm to people without the actual onset of an eruption, such as increased seismicity (leading to landslides or structural damage) and degassing as at Lake Nyos, Cameroon in 1986.

SIGNIFICANCE: Natural hazards associated with eruptions of the world's 550 or so historically active volcanoes pose a significant threat to about 10% of the world's population, especially in densely-populated circum-Pacific regions. It is estimated that more than half a billion people are now at risk. Two indirect hazards - volcanogenic tsunami and post-eruption famine and disease . have accounted for most of the eruption-associated human fatalities. However, direct hazards related to explosive eruptions (e.g. pyroclastic flows and surges, debris flows, mudflows) are commonly the most deadly. Lava flows can cause great decreases in agricultural productivity and damage to property, but they rarely cause significant numbers of deaths, except in the case of fast moving flows, which killed about 100 people in the 1977 eruption of Nyiragongo, in central Africa.

HUMAN OR NATURAL CAUSE: Volcanism is a natural process, which has operated since the Earth was formed. Although a few attempts have been made to divert lava flows, people have had no influence whatsoever on the underlying causes of volcanism.

ENVIRONMENT WHERE APPLICABLE: Most active volcanic systems are located along or near divergent and convergent boundaries between the Earth's tectonic plates. However, some volcanoes (e.g. Hawaii) occur thousands of kilometres from the nearest plate boundary and result from melting and eruptive processes associated with the passage of a tectonic plate over a fixed thermal anomaly (or hotspot) in the mantle.

TYPES OF MONITORING SITES: Regions containing active or potentially active volcanoes, subaerial or submarine (including the deep ocean floor). Diagnostic monitoring sites commonly include active vents and fumaroles, crater lakes, and areas of ground cracking. Ideally, the sites should be distributed over the entire volcanic system, to monitor all areas where activity might be initiated.

SPATIAL SCALE: patch to regional/continental to global (the latter for monitoring possible climatic effects of those explosive eruptions that inject copious amounts of volcanic gases into the stratosphere).

METHOD OF MEASUREMENT: Optimum monitoring of volcanic unrest must be based on a combination of geophysical, geodetic and geochemical methods, rather than reliance on any single technique. These involve a network of monitoring sites at key locations around a volcanic centre at which repeated measurements are made of horizontal and vertical ground displacements (strainmeter, laser distance measurements, gravimeter, tiltmeter, GPS observations), seismicity (automatic event recording, 3-component and broad band seismometry, and special array techniques), and a wide range of geochemical parameters. Instruments located at depth (several hundred meters) in boreholes are very useful, because here signal to noise ratios are higher, and lower magnitude variations in parameters can be detected. Borehole instruments include seismometers, strainmeters, tiltmeters and dilatometers. Visual observations are also important in determining changes in the volcanic edifice, including volcanic lake levels and colours, tree and other vegetation damage around fumaroles, and outputs from hot springs and fumaroles.
Ground-based seismic and deformation monitoring approaches have proven to be the most reliable and diagnostic in early detection and tracking of volcanic unrest. These two approaches are commonly augmented by volcanic-gas, microgravity, geomagnetic, geo-electrical (e.g. resistivity), and remote-sensing studies. Satellite-based methods are increasingly used for measuring ground displacements and variations in thermal and volatile output at volcanic centres. Experience worldwide shows that volcano surveillance is best accomplished by on-site volcanic observatories or nearby centralized facilities, at which monitoring data are collected, processed and interpreted by experienced multi-disciplinary scientific teams. For remote locations, satellite monitoring is extremely important in determining onsets of eruptions and making critical measurements such as ash cloud height and direction of ash movement.

FREQUENCY OF MEASUREMENT: For frequently active volcanoes, measurement should be continuous. For potentially active volcanoes currently in repose, geophysical and geochemical baseline monitoring data should be obtained and then followed by repeat measurements at least every few years. However, after the recognition of possible departure from baseline behaviour, the monitoring networks should be expanded and measurements should be made on a more frequent, preferably continuous, basis. Eruptive activity can develop very quickly over several weeks, so even on volcanoes in repose the aim should be to take measurements as frequently as possible.

LIMITATIONS OF DATA AND MONITORING: The main limitation in detecting and tracking volcanic unrest is simply that no more than a small percent of the world's volcanoes are now being monitored. The majority of high-risk volcanoes are in countries that lack sufficient economic and scientific resources to conduct the necessary monitoring. Even in the richer nations, efforts to reduce government expenditures are compromising the effectiveness of existing monitoring programs.

APPLICATIONS TO PAST AND FUTURE: The recent increase in knowledge about how volcanoes work has led not only to a better understanding of the eruptive process and products of previous volcanic events, but also to a sharper recognition of the need for multi-disciplinary, integrated field and laboratory studies. Much progress has been made in the forecasting of non-explosive eruptions at some well-monitored volcanoes, but the prediction of the onset and size of explosive eruptions remains elusive. Because no two volcanoes behave exactly alike, monitoring and related studies must be done at many more volcanoes before a predictive capability for explosive eruptions can be achieved.

POSSIBLE THRESHOLDS: With current knowledge and volcano-monitoring techniques, it is not yet possible to determine a fixed threshold value in the magnitude or duration of volcanic unrest, which, if exceeded, inexorably leads to eruptive activity. However, at a few well-monitored volcanoes, scientists are beginning to recognize patterns of build-up of precursory geoindicators that characterize magma movement and/or hydrothermal-pressurization effects at a given volcano.


Fisher, R.V., Heiken, G. & Hulen, J.B. 1997. Volcanoes - crucibles of change. Princeton University Press, 317p.

McGuire, B., Kilburn, C.R.J &.Murray, J (eds) 1995. Monitoring active volcanoes: strategies, procedures and techniques. London: University College London Press.

Scarpa, R. &.Tilling, R.I 1996. Monitoring and mitigation of volcano hazards. Berlin: Springer-Verlag.

Sigurdsson, H, Houghton, B, Rymer, H, Stix, J & McNutt, S 2000. Encyclopedia Of Volcanoes. Academic Press.

Tilling R.I. Two e-books are available: Volcanoes. (, and Monitoring Active Volcanoes (

OTHER SOURCES OF INFORMATION: Emergency preparedness/disaster relief agencies, geological surveys, Global Volcanism Network (Smithsonian Institution, National Museum of Natural History, MRC 129, Washington DC 20560, USA) International Association of Volcanology and Chemistry of the Earth's Interior, World Organization of Volcano Observatories (see, U.S. Geological Survey Office of Earthquakes, Volcanoes and Engineering, World Data Center-A for Natural Hazards; IDNDR; UNDRO. A particularly useful web site can be found at

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Injections of volcanic ash and gases high into the atmosphere during explosive eruptions can have significant global environmental effects. Large explosive eruptions that form stratospheric clouds of volcanic aerosols (e.g. Tambora, Indonesia, in 1815; El Chichon, Mexico, in 1982; and Mount Pinatubo, Philippines, in 1991) produce measurable effects on global climate, such as a hemispheric cooling of up to 0.5 degrees C, which can persist for several years. In-flight encounters between jet aircraft and volcanic ash present a growing hazard, as air traffic increases worldwide. Volcanism can also have significant effects on the landscape (such as the changes that followed the 1980 Mount St Helens eruption) and on ecosystems.

OVERALL ASSESSMENT: Early recognition and systematic monitoring of unrest in volcanic areas is essential. It can significantly mitigate eruption-related hazards by improving understanding of volcanic phenomena before, during and after eruptions, by refining long-term and short-term eruption-forecasting capability, and by providing the fundamental data for preparing hazard-zonation maps and for assessing volcano hazards.

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