Frequently Asked Questions
Answers to commonly asked questions about Holocene volcanoes and their eruptions based on data from Volcanoes of the World.
One of the most difficult problems of standardization has been the varying usage of the word "volcano." Definitions of "volcano" range from individual vents, measured in meters, through volcanic edifices measured in kilometers or tens of kilometers, to volcanic fields measured in hundreds of kilometers. In a database compilation, the disadvantage of the narrowest definition is not so much the multiplicity of names introduced, as the dismembering of a single volcanic plumbing system's history into apparently unrelated separate records. The interiors of ancient volcanoes, now eroded and exposed for geologic study, show us that most subsurface magma chambers--the suppliers of lavas to overlying volcanoes--are at least several kilometers in diameter. We also know that many contemporary volcanoes grow by additions from countless flank vents as well as activity at a central crater. Consequently, we have tended to group closely spaced "volcanoes" such as the historical vents of the Canary Islands (many listed as separate volcanoes in the Catalog of Active Volcanoes of the World) by the major volcanic edifice on which they are found. Volcanoes listed here are rarely closer than 10 km to their nearest neighbor, and are commonly separated by at least 20 km.
Another problem is simply the identification of volcanoes. Prominent, steaming cones are easy to recognize, but water, ice, erosion, collapse processes, or dense vegetation can mask very dangerous volcanoes. For example, Lake Taupo, in the center of New Zealand's North Island, is beautifully tranquil, with no obvious features alerting non-geologists to its particularly violent history. In the Alaskan summer of 1975, two volcanologists traced an ever-thickening ash layer to a vent now covered by the Hayes Glacier, and a "new" volcano was added to the NE end of the Aleutian arc. Also in Alaska, five decades passed before the true source of this century's largest eruption was recognized: subsurface magma connections led to prominent collapse of Mount Katmai in 1912, and this was assumed to be the eruption's source until careful fieldwork showed it to be Katmai's inconspicuous neighbor, Novarupta. These examples illustrate why the listings generated from this database must be recognized as incomplete. Inclusion in this compilation may depend on thoroughness of mapping--quite variable through the world's volcanic regions--and the most dangerous volcanoes may be those not yet recognized.
The arrival of volcanic products at the surface of the Earth or other planetary bodies is termed an eruption. At first glance it might appear surprising that the ambiguity regarding what constitutes a volcano extends to eruptions as well. Some definitions of the word include purely gaseous expulsions, but we confine the term to events that involve the explosive ejection of fragmental material, the effusion of liquid lava, or both. Other definitions restrict eruptions to magmatic events, but the fragmental material ejected may be old as well as new. The explosive interaction of volcanically generated heat and near-surface water can cause dramatic eruptions without any fresh volcanic material reaching the surface and from a volcanic hazards perspective can be as important to document as magmatic events.
The answer to this common question depends upon use of the word "active." At least 20 volcanoes will probably be erupting as you read these words (Italy's Stromboli, for example, has been erupting for more than a thousand years); roughly 60 erupted each year through the 1990s; 154 in the full decade 1990-1999; about 550 have had historically documented eruptions; about 1300 (and perhaps more than 1500) have erupted in the Holocene (past 10,000 years); and some estimates of young seafloor volcanoes exceed a million. Because dormant intervals between major eruptions at a single volcano may last hundreds to thousands of years, dwarfing the relatively short historical record in many regions, it is misleading to restrict usage of "active volcano" to recorded human memories: we prefer to add another identifying word (e.g. "historically active" or "Holocene volcano").
The definition of "volcano" is as important in answering the number question as the definition of "active." Usage has varied widely, with "volcano" applied to individual vents, measured in meters, through volcanic edifices measured in tens of kilometers, to volcanic fields measured in hundreds of kilometers. We have tended toward the broader definition in our compilations, allowing the record of a single large plumbing system to be viewed as a whole, but this approach often requires careful work in field and laboratory to establish the integrity of a group's common magmatic link. The problem is particularly difficult in Iceland, where eruptions separated by many tens of kilometers along a single rift may share the same magmatic system. A "volcanic field," such as Mexico's Michoacán-Guanajuato field (comprising nearly 1,400 cinder cones, maars, and shield volcanoes derived from a single magmatic system, dotting a 200 x 250 km area) may be counted the same as a single volcanic edifice. Perhaps the most honest answer to the number question is that we do not really have an accurate count of the world's volcanoes, but that there are at least a thousand identified magma systems--on land alone--likely to erupt in the future.
Erupting now: perhaps 20
Each year: 50-70
Each decade: about 160
Historical eruptions: about 550
Known Holocene eruptions (last 10,000 years): about 1300
Known (and possible) Holocene eruptions: about 1500
Note that these figures do not include the large number of eruptions (and undescribed volcanoes) on the deep sea floor. Estimates of global magma budgets suggest that roughly 3/4 of the lava reaching Earth's surface does so unnoticed at submarine midocean ridges (see below).
Nevados Ojos del Salado volcano on the Chile/Argentina border is the world's highest volcano above sea level, but it rises only about 2,000 m above its base. The broad summit of Mauna Loa shield volcano is 2,700 m lower than Nevados Ojos del Salado, but it's height above base is almost 10 times that of the Andean volcano.
|Volcano||Country||Elevation above sea level
|Ojos del Salado, Nevados||Chile/Argentina||6887||22,595|
The summits of the world's ten highest Holocene volcanoes (above) are all constructed above the structural highs of the Andes mountains. The highest volcano with documented historical eruptions is Llullaillaco, which had three in the 2nd half of the 19th century. Active fumaroles, however, mark the summit crater of Nevados Ojos del Salado, 267 km to the south and 148 m higher than Llullaillaco. The youthful nature of Nevados Ojos del Salado suggests that its lack of historical eruptions stems only from its remote location, and it is rightfully the world's highest volcano. The only higher mountain in the Americas, Argentina's Aconcagua at 6962 m, was listed as active by Darwin during the voyage of the Beagle, but Chilean colleagues tell us that the mountain is not a volcano and its height results from imbricate thrust faulting.
A very different picture emerges when considering the height of volcanic edifices themselves, as measured from their constructional bases rather than sea level. Massive oceanic shield volcanoes of Hawaii, such as Mauna Loa, rise as much as 9,000 m above the sea floor. These volcanoes are by far the world's largest by volume, dwarfing the continental-margin stratovolcanoes of the Andes. Furthermore, the weight of the countless overlapping lava flows forming these shield volcanoes substantially depresses the oceanic crust beneath them. Geophysical evidence indicates that the full height of Mauna Loa above its base is an astounding 19 kilometers, more than twice the height of Mount Everest above sea level.
Unfortunately, determining the true base of a volcano is often difficult, and we have accurate height-above-base data for only about half of the world's volcanoes.
Clearly some eruptions last for a very long time, like Stromboli's 2400+ year continuing pyrotechnic show (see the "How many active volcanoes are there in the world?" question). At the turn of the century the following 15 volcanoes have been erupting more or less continuously through the last three decades (the reporting span of SEAN/GVN) and are likely to remain active for some time: Stromboli and Etna (Italy); Erta Ale (Ethiopia); Manam, Langila, and Bagana (Papua New Guinea); Yasur (Vanuatu); Semeru and Dukono (Indonesia); Sakura-jima (Japan); Santa Maria and Pacaya (Guatemala); Arenal (Costa Rica); Sangay (Ecuador); and Erebus (Antarctica). However, other eruptions end swiftly: 10% of those for which we have accurate durations lasted no longer than a single day, most end in less than 3 months, and few last longer than 3 years. The median duration is about 7 weeks.
We don't think so.
A look at the number of volcanoes active per year, over the last few centuries, shows a dramatic increase, but one that is closely related to increases in the world's human population and communication. We believe that this represents an increased reporting of eruptions, rather than increased frequency of global volcanism: more observers, in wider geographic distribution, with better communication, and broader publication. The past 200 years (see plot below) show this generally increasing trend along with some major "peaks and valleys" which suggest global pulsations. A closer look at the two largest valleys, however, shows that they coincide with the two World Wars, when people (including editors) were preoccupied with other things. Many more eruptions were probably witnessed during those times, but reports do not survive in the scientific literature.
If these apparent drops in global volcanism are caused by decreased human attention to volcanoes, then it is reasonable to expect that increased attention after major, newsworthy eruptions should result in higher-than-average numbers of volcanoes being reported in the historical literature. The 1902 disasters at Mont Pelee, St. Vincent, and Santa Maria (see 1902 arrow) were highly newsworthy events. They represent a genuine pulse in Caribbean volcanism, but we believe that the higher numbers in following years (and following Krakatau in 1883) result from increased human interest in volcanism. People reported events that they might not otherwise have reported and editors were more likely to print those reports.
Additional strong evidence that the historical increase in global volcanism is more apparent than real comes from the lower plot below. Here only the larger eruptions (generating at least 0.1 km3 of tephra, the fragmental products of explosive eruptions) are plotted. The effects of these larger events are often regional, and therefore less likely to escape documentation even in remote areas. The frequency of these events has remained impressively constant for more than a century, and contrasts strongly with the apparent increase of smaller eruptions with time.
Finally, we plot below the record since reasonably comprehensive reporting of global volcanism began in the 1960s. Note that the number of confirmed erupting volcanoes has leveled off between 50 and 70 per year through the past four decades, and a linear regression line through the data indicates that volcanism has been virtually constant.
The assignment of official hazard alert levels is the responsibility of national or regional volcano observatories under the umbrella of the World Organization of Volcano Observatories (WOVO). The following discussion of volcanic hazard alert levels is adapted from the WOVO website, with permission.
In a volcanic crisis, there is often worldwide interest in the volcano's hazard alert levels. With the exception of color codes for aviation, though, there is currently no standardized international volcano alert levels system. This is due to: (a) wide variation in the behavior of individual volcanoes and in monitoring capabilities, and (b) the different needs of populations, including different languages and symbolism of colors used. National volcano observatories have developed alert level protocols that are regionally variable and differ significantly in detail. The WOVO site contains links to the regional volcano observatories and the alert systems they utilize.
Organizations with interest in natural hazards are strongly cautioned against posting global volcano hazard alerts or eruption "forecasts" that do not originate from volcano observatories or regional agencies with both responsibility for and familiarity with those volcanoes. Posting of hazard alert levels can have major public safety and economic implications, and should not be done lightly. The data needed to provide alert levels come from onsite and remote monitoring instrumentation and are best evaluated by staff of regional volcano observatories who are the most familiar with activity at their volcanoes. The responsible observatories and organizations are listed on the WOVO website, and readers are directed to these organizations for information on current volcano alert levels.
Currently, there is no WOVO-endorsed source of worldwide Volcanic Alert Levels, with the exception of aviation color codes. For those seeking a near real-time overview of current reported activity that incorporates direct observatory sources WOVO recommends the Weekly Activity Reports compiled by the Smithsonian Institution's Global Volcanism Program (GVP) and the U.S. Geological Survey.
Many instances of aircraft flying into volcanic ash clouds have demonstrated the life-threatening and costly damages that can be sustained. Consequently, a universal volcanic alert level system for aviation has been developed (as part of the International Airways Volcano Watch, a universal warning system coordinated by the International Civil Aviation Organization, a UN specialist agency). This system uses four color codes, designed to help pilots, dispatchers, and air-traffic controllers quickly find the status of numerous volcanoes that might endanger aircraft.
The color codes reflect conditions at or near a volcano and are not intended to indicate hazards posed downwind by drifting ash - all discernible ash clouds are assumed to be highly hazardous and to be avoided. Furthermore, the aviation color code should not be extrapolated to represent hazards posed on the ground, which might be quite different.
Not all observatories currently provide information in this format, but where they do, the aviation color code is currently defined as below.
GREEN - Volcano is in normal, non-eruptive state or, after a change from a higher level:
Volcanic activity considered to have ceased, and volcano reverted to its normal, non-eruptive state.
YELLOW - Volcano is experiencing signs of elevated unrest above known background levels or, after a change from higher level: Volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.
ORANGE - Volcano is exhibiting heightened unrest with increased likelihood of eruption or, volcanic eruption is underway with no or minor ash emission. [specify ash-plume height if possible]
RED - Eruption is forecast to be imminent with significant emission of ash into the atmosphere likely. or eruption is underway with significant emission of ash into the atmosphere. [specify ash-plume height if possible]
Scientists use a wide variety of techniques to monitor volcanoes, including seismographic detection of the earthquakes and tremor that almost always precede eruptions, precise measurements of ground deformation that often accompanies the rise of magma, changes in volcanic gas emissions, and changes in gravity and magnetic fields. Although not diagnostic individually, these techniques, when used in combination at well-monitored volcanoes, have resulted in successful predictions. At Pinatubo volcano (Philippines) in 1991, a successful forecast saved thousands of lives. The USGS website discusses these monitoring techniques in more detail.
Monitoring-based forecasts are becoming much more reliable, but they remain imperfect. If scientists are fortunate, precursors to an eruption follow the same course as they followed before previous eruptions. Patterns often change, though, and wholly new behavior is observed. The best forecasts will be based on an integration of geologic history, realtime monitoring, and a deep understanding of the internal plumbing processes of the specific volcano. Even with the best of monitoring and interpretations, reliable forecasts are rarely possible more than a few days in advance of an eruption.
Some forecasts of volcanic eruptions are based on eruption recurrence intervals, but these are notoriously unreliable for two reasons: (a) few volcanoes are sufficiently well studied to provide an accurate eruptive history over the many hundreds of years necessary to establish a reliable recurrence interval; and (b) few volcanoes maintain the same behavior for long (more often than not, as soon as a repetitive pattern becomes apparent, the volcano changes behavior).
Volcano observatories make forecasts with great caution as they can have huge impacts on the affected populations, in some cases forcing people to leave behind homes, farms, and livestock. Inaccurate forecasts can lead to unnecessary obligation of scarce resources and/or undermine residents' confidence in future forecasts.
Reliable forecasts, however, can be made by volcano observatory staff, who have the experience to interpret their monitoring that detects eruption precursors. Most nations with volcanoes have tasked an established observatory, run by the government or by a university, to provide eruption forecasts to the public. All of these observatories are members of the World Organization of Volcano Observatories (WOVO).