What Is A Volcano

STRANGE BUT TRUE

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.

What Is An Eruption?

The arrival of volcanic products at the Earth's surface is termed an eruption. Some definitions of the word include purely gaseous expulsions, but we confine the term to events that involve explosive ejections of fragmental material, the effusion of liquid lava, or both. This fragmental material 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.

How Many Active Volcanoes Are There In The World?

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.

How Many Active Volcanoes Known?

• 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).

Volcanism distributed by tectonic setting. Pie diagram on left shows proportion of documented historical eruptions from subduction zones (black), midocean ridges (stipple), and hot spot settings (white). Pie diagram on right show proportion of annual magma budget in the same settings (with same symbols) to emphasize the dramatic contrast between the volcanism that we see and that we don't. Data for right diagram from Crisp (1984) and left from Smithsonian.

How Long Does An Eruption Last?

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.

Has Volcanic Activity Been Increasing?

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 4 decades, and a linear regression line through the data indicates that volcanism has been virtually constant. The efforts described under Volcanic Activity Reports have paid off, and it appears that we are now capturing a nearly complete record of Earth's subaerial volcanism.