The scientific study of volcanic phenomena, especially the processes, products, and hazards associated with active or potentially active volcanoes. It focuses on eruptive activity that has occurred within the past 10,000 years of the Earth's history, particularly eruptions during recorded history. Strictly speaking, it emphasizes the surface eruption of magmas and related gases, and the structures, deposits, and other effects produced thereby. Broadly speaking, however, volcanology includes all studies germane to the generation, storage, and transport of magma, because the surface eruption of magma represents the culmination of diverse physicochemical processes at depth. This article considers the activity of erupting volcanoes and the nature of erupting lavas. For a discussion of the distribution of volcanoes and the surface structures and deposits produced by them See also: Plate tectonics; Volcano
Volcanoes and humans
From the dawn of civilization, volcanic eruptions have intruded into human affairs, producing death and destruction, bewilderment, fear, superstition, and, ultimately, scientific curiosity. Deities or supernatural events, directly or indirectly linked to volcanoes and eruptions, figure prominently in the legends and myths of civilizations that developed in or near regions of active volcanism. During the last 500 years, at least 200,000 people have lost their lives as a result of volcanic eruptions (Table 1).
Three eruptions in the 1980s
appreciably increased public awareness of volcanic activity and of volcanology.
The eruption of Mount St. Helens (
Fig. 1 Climactic eruption of Mount St. Helens on May 18, 1980, about 5 h after the beginning of activity. The plume of ash and gases reached an altitude of about 15 mi (24 km). (Photograph by R. M. Krimmel, USGS)
On November 13, 1985, a very
small-volume eruption (0.007 mi3 or 0.029 km3) occurred at the summit crater of
17,680-ft-high (5389-m), glacier-capped Volcán Nevado del Ruiz,
In 1991, two eruptions captured
worldwide attention. On June 3, pyroclastic flows (nuées ardentes) triggered by
the collapse of a new lava dome at the summit of Unzen Volcano,
On average, about 50 to 60 volcanoes worldwide are active each year. About half of these constitute continuing activity that began the previous year, and the remainder are new eruptions. Analysis of historic records indicates that eruptions comparable in size to that of Mount St. Helens or El Chichón tend to occur about once or twice per decade, and larger eruptions such as Pinatubo about once per one or two centuries. On a global basis, eruptions the size of that at Nevado del Ruiz in November 1985 are orders of magnitude more frequent.
It was not until the nineteenth
century that serious scientific inquiry into volcanic phenomena became part of
the rapidly developing science of geology. Even though the building of a small
observatory was completed in 1847 on the flank of Mount Vesuvius
In March 1988, the USGS, in a
cooperative program with the State of
In 1999 the USGS formally designated
a long-term program of volcano-monitoring studies at Long Valley Caldera
Nature of magmas
The eruptive characteristics, products, and resulting landforms of a volcano are determined predominantly by the composition and physical properties of the magmas involved in the volcanic processes (Table 2). Formed by partial melting of existing solid rock in the Earth's lower crust or upper mantle, the discrete blebs of magma consist of liquid rock (silicate melt) and dissolved gases. Driven by buoyancy, the magma blebs, which are lighter than the surrounding rock, coalesce as they rise toward the surface to form larger masses. See also: Igneous rocks; Lithosphere
Concentration of volatiles
During its ascent, the magma enters zones of lower temperature and pressure and begins to crystallize, producing crystals suspended in the liquid—physically analogous to the formation of ice crystals when water begins to freeze. Other solid fragments may also be incorporated from the walls and roof of the conduit through which the magma is rising. As crystallization progresses, volatiles and the more soluble silicate components are concentrated in the remaining liquid. See also: Phenocryst; Xenolith
At some point during magma ascent, decreasing confining pressure and increasing concentration of volatiles in the residual liquid initiate the separation of gas from the liquid. From that point on to its eruption, the magma consists of three phases: liquid, solid, and gas. Volcanic gases generally are predominantly water; other gases include various compounds of carbon, sulfur, hydrogen, chlorine, and fluorine. All volcanic gases also contain minor amounts of nitrogen, argon, and other inert gases, largely the result of atmospheric contamination at or near the surface.
In laboratory experiments, at a temperature of 2000°F (1100°C) and pressure of 5 kilobars (500 megapascals), a melt of rhyolitic composition can contain in solution about 10% (by weight) of water, a basaltic melt about 8%. At lower pressure, the solubility of water in any magma decreases correspondingly. With continued ascent, water and other volatiles in excess of their solubilities in the magma will exsolve, vesiculate, and ultimately increase “gas pressure” of the magma to provide the driving force for eruptions. This process may be compared with the uncorking of a bottle of champagne, especially if it has been shaken; the gas separates from the wine and forms bubbles, which in turn expand violently (explode) when the cork is removed suddenly.
The actual proportion of gas to lava
liberated during eruptions cannot be directly determined; the amount of gas can
be lower or higher than the values from laboratory experiments depending on the
actual crystallization and degassing histories of the magmas. For many
eruptions, volatiles measured in the lava constitute less than 1% (by weight) of
the lava erupted during the same interval. In initially unsaturated magmas, high
gas pressures may be developed by supersaturation of volatiles as their residual
liquid phase becomes concentrated during crystallization. In recent decades, a
combination of refined laboratory methods (to analyze melt inclusions and
phenocrysts) and modern remote-sensing techniques (to measure volcanic gases in
atmosphere) have been used to obtain data for well-studied eruptions. For
example, data obtained at Mount St. Helens (1980), Redoubt Volcano,
Part of the gas liberated at volcanoes probably comes from the same deep-seated source as the silicate portion of the magma, but some may be of shallower origin. Part of the steam may be the result of near-surface oxidation of deep-seated hydrogen or interaction of hot magma or rock with ground water or geothermal fluids in the proximity of the reservoir-conduit system. Some of the oxidation of the sulfur gases must have taken place close to the surface. At some volcanoes, such as Vesuvius, the carbon gases may derive in part from reaction of the magma with limestone at shallow depth. Ammonia and hydrocarbon components present in some gases probably are derived from the organic constituents of sedimentary rocks near the surface.
In some eruptions, such as the 1924
Temperatures of erupting magmas have
been measured in lava flows and lakes, pyroclastic deposits, and volcanic vents
by means of infrared sensors, optical pyrometers, and thermocouples. Reasonably
good and consistent measurements have been obtained for basaltic magmas erupted
from Kilauea and Mauna Loa volcanoes,
Temperature measurements on more silicic lavas are few and much less accurate because of the greater violence of the eruptions and the necessity of working at considerable distances as a safety precaution. In general, however, they suggest lower temperatures of eruption than those for mafic lavas. For example, for andesitic and more silicic lavas, available temperature estimates have ranged from about 1800 to 1330°F (980 to 720°C). Thermocouple measurements, made only 20 h after eruption of dacitic pyroclastic flows of Mount St. Helens (August 1980), yielded a temperature range of 1337–1540°F (725–838°C).
A few field measurements of the
viscosity of flowing basic lavas have been made by means of penetrometers
(instruments that measure the rate of penetration into liquid of a slender rod
under a given strength of thrust) and by the shearing resistance to the turning
of a vane immersed in the liquid. Viscosities also have been calculated from
observed rates of flow in channels of known dimensions and slope or have been
estimated from the chemical composition of the lava (by extrapolation of
laboratory data on viscosities of simple molten silicate compounds). The best
direct determinations of viscosity are vane-shear measurements in a lava lake at
Types of volcanic eruptions
The character of a volcanic eruption is determined largely by the viscosity of the liquid phase of the erupting magma and the abundance and condition of the gas it contains. Viscosity is in turn affected by such factors as the chemical composition and temperature of the liquid, the load of suspended solid crystals and xenoliths, the abundance of gas, and the degree of vesiculation. In very fluid lavas, small gas bubbles form gradually, and generally are able to rise through the liquid, coalescing to some extent to form larger bubbles, and escape freely at the surface with only minor disturbance. In more viscous lavas, the escape of gas is less free and produces minor explosions as the bubbles burst their way out of the liquid. In still more viscous lavas, at times there appears to be a tendency for the essentially simultaneous formation of large numbers of small bubbles throughout a large volume of liquid. The subsequent violent expansion of these bubbles during eruption shreds the frothy liquid into tiny fragments, generating explosive showers of volcanic ash and dust, accompanied by some larger blocks (volcanic “bombs”); or it may produce an outpouring of a fluidized slurry of gas, semisolid bits of magma froth, and entrained blocks to form high-velocity pyroclastic flows, surges, and glowing avalanches (nuées ardentes). Also, rising gases may accumulate beneath a solid or highly viscous plug, clogging the vent until it acquires enough pressure to cause rupture and attendant explosion that hurls out fragments of the disrupted plug.
Types of eruptions customarily are designated by the name of a volcano or volcanic area that is characterized by that sort of activity (Table 3), even though all volcanoes show different modes of eruptive activity on occasion and even at different times during a single eruption.
Eruptions of the most fluid lava, in
which relatively small amounts of gas escape freely with little explosion, are
designated Hawaiian eruptions. Most of the lava is extruded as successive, thin
flows that travel many miles from their vents. Lava clots or spatter thrown into
the air in fountains (Fig. 2) may remain fluid enough to flatten out on striking
the ground, and commonly to weld themselves to form cones of spatter and cinder.
An occasional feature of Hawaiian activity is the lava lake, a pool of liquid
lava with convectional circulation that occupies a preexisting shallow
depression or pit crater. Recent data, however, indicate that eruptions of
Hawaiian volcanoes—the namesake for the “Hawaiian” (dominantly effusive) type of
eruptive activity—may not be as nonexplosive as suggested by observations of the
historical eruptions at the Kilauea and Mauna Loa volcanoes. Geological and
dating studies of prehistoric volcanic ash deposits demonstrate that the
frequency of explosive eruptions from Kilauea is comparable to those for many of
the composite volcanoes of the Cascade Range of the
Fig. 2 Typical of “Hawaiian eruptions,” an
80-ft-high (25-m) fountain of fluid basaltic lava plays during an eruption of
Strombolian eruptions are somewhat more explosive eruptions of lava, with greater viscosity, and produce a larger proportion of pyroclastic material. Many of the volcanic bombs and lapilli assume rounded or drawn-out forms during flight, but commonly are sufficiently solid to retain these shapes on impact.
Generally still more explosive are the vulcanian type of eruptions. Angular blocks of viscous or solid lava are hurled out, commonly accompanied by voluminous clouds of ash but with little or no lava flow.
Peléean eruptions are characterized by the heaping up of viscous lava over and around the vent to form a steep-sided hill or volcanic dome. Explosions, or collapses of portions of the dome, may result in glowing avalanches (nuées ardentes).
Plinian eruptions are paroxysmal eruptions of great violence—named after Pliny the Elder, who was killed in A.D. 79 while observing the eruption of Vesuvius—and are characterized by voluminous explosive ejections of pumice and by ash flows. The copious expulsion of viscous siliceous magma commonly is accompanied by collapse of the summit of the volcano, forming a caldera, or by collapse of the broader region, forming a volcano-tectonic depression. The term ultraplinian has been used occasionally by some volcanologists to describe especially vigorous plinian activity.
In contrast to the foregoing magmatic eruptions, some low-temperature but vigorous ultravulcanian explosions throw out fragments of preexisting volcanic or nonvolcanic rocks, accompanied by little or no new magmatic material. Certain explosion pipes and pits, known as diatremes and maars, have been produced by ultravulcanian explosions.
Volatilization of meteoric or ground water when it comes in contact with hot solid rocks in the vicinity of the volcano's magma reservoir causes the usually mild, but occasionally violent, disturbances known as phreatic explosions. No fresh magmatic material is erupted during phreatic (steam-blast) activity, which may precede major magmatic eruptions. The term “phreatomagmatic” describes a highly variable type of volcanic activity that results from the complex interaction between fresh magma/lava and subsurface or surface water (ground water, hydrothermal water, meteoric water, seawater, lake water).
The differences between the
different types of eruptions are gradational but ultimately are dependent on the
variation in explosivity and “size” of the eruption. Some volcanologists have
proposed the volcanic explosivity index (VEI) to attempt to standardize the
assignment of the size of an explosive eruption, using volume of eruptive
products, duration, height of ash plume, and other criteria. Table 3 shows the
general relationships and the necessarily arbitrary distinctions between
eruption type, explosivity, and eruptive volume for nearly 6300 eruptions during
the Holocene (the past 10,000 years of the Earth's history), for which such
information is known or can be reasonably estimated. Of these eruptions, about
11% are nonexplosive Hawaiian-type eruptions (always assigned a VEI of 0
regardless of eruptive volume). Explosive eruptions considered to be small to
moderate in size (VEI 1 to 2) constitute about 69% of all eruptions and are
gradationally classified as Hawaiian, strombolian, or vulcanian. Only 127
plinian or ultraplinian eruptions rate VEIs of 5 or greater (very large). The
May 18, 1980, eruption of
Monitoring active volcanoes
A major component of the science of volcanology is the systematic and, preferably, continuous monitoring of active and potentially active volcanoes. Scientific observations and measurements—of the visible and invisible changes in a volcano and its surroundings—between eruptions are as important, perhaps even more crucial, than during eruptions. Measurable phenomena important in volcano monitoring include earthquakes; ground movements; variations in gas compositions; and deviations in local gravity, electrical, and magnetic fields. These phenomena reflect pressure and stresses induced by subsurface magma movements and or pressurization of the hydrothermal envelope surrounding the magma reservoir.
Seismicity and ground deformation
The monitoring of volcanic seismicity and ground deformations before, during, and following eruptions has provided the most useful and reliable information. From the many decades of study of the active Hawaiian volcanoes and volcanoes elsewhere, it has been determined that a volcano generally undergoes measurable ground deformation when magma is fed into its near-surface reservoir-conduit system.
Vertical and horizontal ground displacements and slope changes can easily be detected and measured precisely by existing geodetic techniques. Slope changes can be measured with a precision of a microradian or less by various electronic-mechanical “tilt-meters,” or with somewhat less precision by leveling of short-sided arrays of benchmarks. Vertical displacements of benchmarks on a volcano, relative to a reference benchmark (or tide gage) unaffected by the volcano, can be determined to a few parts per million by leveling surveys. Horizontal distance changes between benchmarks can be measured with similar precision by various electronic distance measurement instruments.
Advances in geodetic applications of
satellite positioning and other forms of space geodesy, especially the Global
Positioning System (GPS), suggest that conventional ground-deformation
monitoring techniques will be supplanted by satellite-based monitoring systems,
if the acquisition and maintenance costs can be decreased substantially. Toward
the end of the twentieth century, significant progress was made in the
development and testing of near-real-time GPS monitoring networks at selected
volcanic systems [for example, Augustine Volcano (Alaska), Long Valley Caldera
(California)]. Another promising satellite-based geodetic technique is the InSAR
(interferometric synthetic aperture radar) method, which is capable of detecting
ground movements over extensive areas (that is, over the entire volcano). This
technique involves the interferometric analyses of one or more pairs of SAR
images acquired at time intervals; coherent ground movement is revealed by
ringlike or other regular anomalies reflecting differences (interferences) in
topography between the time-separated pair of satellite images being compared.
The InSAR technique, while not yet a routine volcano-monitoring tool, has been
shown to be successful at Yellowstone (
The ground deformation is related to, and accompanied by, intense earthquake activity reflecting the subsurface ruptures of the confining rocks of the expanding volcanic reservoir in response to the increased pressure exerted by infilling magma. Modern volcano observations employ well-designed seismic networks to monitor volcanic seismicity continuously in order to track subsurface movement of magma between and during eruptions. For well-studied volcanoes, experience has shown that premonitory seismicity usually provides the earliest signals of impending activity. Great advances have been made in volcano seismology since the early 1980s; in particular, recent studies demonstrate that the occurrence of, and variations in, long-period (low-frequency) events and volcanic tremor provide valuable insights in inferring the movement of magma and/or hydrothermal fluids before and during eruptions.
The number of earthquakes and the magnitude of ground deformation gradually increase as the magma reservoir swells or inflates until some critical strength threshold is exceeded and major, rapid migration of magma ensues to feed a surface eruption or a subsurface intrusion (magma drains from the reservoir and is injected into another part of the volcanic edifice without breaching the surface). With the onset of eruptive or intrusive activity, pressure on the “volcanic plumbing system” is relieved and the reservoir abruptly shrinks or deflates, causing flattening of slope (tilt), reduction in vertical or horizontal distances between surface points, and decrease in earthquake frequency. One or more of these inflation-deflation cycles may take place during an eruption, depending on its duration (Figs. 3 and 4). See also: Earthquake; Seismology
Fig. 3 Schematic diagrams showing three commonly observed stages in the course of an inflation-deflation cycle during a typical Hawaiian eruption. At stage 1, inflation begins; it peaks at stage 2. Stage 3 is the eruption-deflation stage. (After R. I. Tilling, Monitoring Active Volcanoes, USGS, 1983)
Fig. 4 Idealized graphs of (a) earthquake frequency and (b) tilt or distance changes as a function of time during the three stages of Fig. 3. (After R. I. Tilling, Monitoring Active Volcanoes, USGS, 1983)
Gas emission and other indicators
As magma ascends, the emission rate
or composition of gases exsolving from it and those generated by interaction of
ground water and hot rock or magma may change with time. The systematic
measurement at volcanic vents and fumaroles of such variations, though still
largely experimental, shows much promise as another volcano monitoring tool.
With modern methods (such as gas chromatography and mass spectroscopy), the
composition of gas can be determined routinely soon after its collection. A
field gas chromatograph has been developed and has been tested at several active
volcanoes, including Etna (
Some success has been obtained at
Etna, Mount St. Helens, Galunggung (Indonesia), Mount Pinatubo (Luzon,
Philippines) and other volcanoes in measuring the fluctuation in the emission
rate of sulfur dioxide by means of a correlation spectrometer (COSPEC) in both
ground-based and airborne modes. The emission rate of carbon dioxide (CO2) was
monitored remotely, using a modified infrared spectrophometer during some of the
1980–1981 eruptions of
A significant advance in the monitoring of volcanic gas emission was the accidental discovery in the early 1980s that, with modification in computer algorithm and data processing, the Total Ozone Mapping Spectrometer (TOMS) instrument aboard the Nimbus 7 satellite can be used to measure the output of sulfur dioxide during an eruption and to track the movement and, ultimately, the dissipation of the sulfur dioxide–containing stratospheric volcanic clouds. The combination of COSPEC and TOMS measurements of sulfur dioxide provides a powerful volcano-monitoring tool. See also: Spectroscopy
Periodic measurement of gas
composition and emission rate, though providing important information about the
volcanic system, does not, however, give the critical data on short-term or
continuous fluctuations—as can seismic and ground deformation monitoring—that
might augur an impending eruption. Some attempts have begun to develop
continuous real-time monitoring of certain of the relatively abundant and
nonreactive gases, such as hydrogen, helium, and carbon dioxide. Particularly
encouraging is the continuous monitoring of hydrogen emission, utilizing
electrochemical sensors and satellite telemetry, that has been tested at Mount
St. Helens, Kilauea, Mauna Loa, and
Even more experimental than gas monitoring are techniques involving measurements of changes in the thermal regime, in gravitational or geomagnetic field strength, and in various measurable “geoelectrical” parameters, such as self-potential, resistivity, and very low-frequency signals from distant sources. All of these methods are premised on the fundamental model that an inflating (or expanding) magma reservoir causes thermal anomalies and related magma-induced changes in bulk density, electrical and electromagnetic properties, piezoelectric response, and so forth, of the volcanic edifice.
Primarily because of the complicating effects of near-surface convective heat transfer, thermal monitoring of volcanoes generally has proved to be nondiagnostic, although monitoring of temperature variations in crater lakes at some volcanoes—Taal (Philippines) and Kelut (Indonesia)—has given useful information.
Although modern gravity meters can
detect changes as small as 5 microGal (0.05 micrometer/s2), interpretation of
the results of gravity surveys is highly dependent on the ability to
discriminate between elevation-induced and mass-difference changes. Preliminary
analysis of some electrical self-potential data at
Volcanic hazards mitigation
Volcanoes are in effect windows into the Earth's interior; thus research in volcanology, in contributing to an improved understanding of volcanic phenomena, provides special insights into the chemical and physical processes operative at depth. However, volcanology also serves an immediate, perhaps more important, role in the mitigation of volcanic and related hydrologic hazards (mudflows, floods, and so on). Progress toward hazards mitigation can best be advanced by a combined approach. One aspect is the preparation of comprehensive volcanic hazards assessments of all active and potentially active volcanoes, including a volcanic risk map for use by government officials in regional and local land-use planning to avoid high-density development in high-risk areas. The other component involves improvement of predictive capability by upgrading volcano-monitoring methods and facilities to adequately study more of the most dangerous volcanoes. An improved capability for eruption forecasts and predictions would permit timely warnings of impending activity, and give emergency-response officials more lead time for preparation of contingency plans and orderly evacuation, if necessary.
Both these approaches must be buttressed by long-term basic field and laboratory studies to obtain the most complete understanding of the volcano's prehistoric eruptive record (recurrence interval of eruptions, nature and extent of eruptive products, and so on). Progress in mitigation of volcanic hazards must be built on a strong foundation of basic and specialized studies of volcanoes (Fig. 5). The separation of the apex from the rest of the triangle reflects the fact that “decision makers” must consider administrative and socioeconomic factors in addition to scientific information from volcanology. Nonetheless, volcanologists and other physical scientists—individually and collectively—must step out of their traditional academic roles and work actively with social scientists, emergency-management officials, educators, the news media, and the general public to increase public awareness of volcanoes and their potential hazards. Recent volcanic crises and disasters have shown that scientific data—no matter how much or how precise—serve no purpose in volcanic-risk management unless they are communicated effectively to, and acted upon in a timely manner by, the civil authorities.
Fig. 5 Schematic diagram showing the general process of volcanic hazards assessment and development of mitigation strategies. (After R. I. Tilling, ed., Volcanic Hazards, American Geophysical Union, 1989)
One of the best examples of good
volcanic hazards assessment and eruption forecasts is provided by the
The response by
Fig. 6 Climatic eruption of Mount Pinatubo (Luzon, Philippines) on June 15, 1991, with U.S. Clark Air Base in the foreground. The base of the eruption column measures about 16 mi (25 km) across. (USAF photograph by Robert Lapointe)
In contrast, the other two major volcanic disasters in the 1980s—El Chichón (1982) and Nevado del Ruiz (1985) [Table 1]—are sobering examples of failures in mitigation of hazards. In the case of El Chichón, the eruption came as a surprise because there were no geologic data about its eruptive history and no preeruption volcanic hazards assessment or monitoring. The Ruiz catastrophe, however, is a more tragic case because considerable geologic data existed; warning signs were recognized a year earlier and limited volcano monitoring was initiated; and a preliminary hazards-zonation map, produced more than a month before the destructive eruption, correctly pinpointed the areas of greatest hazard. Sufficient warnings were given by the scientists on site at Ruiz, but for reasons still not clear, effective evacuation and other emergency measures were not implemented by the government authorities.
In contrast, the volcanic crisis at
Rabaul Caldera (
Fig. 7 Activity at
The rate of seismicity and ground
deformation continued to increase for another 6 months following the declaration
of the stage-2 alert, but then the level of unrest declined rapidly (Fig. 7a).
The expected eruption did not happen, and the officials ended the alert in
November 1984. For the next 10 years, caldera activity fluctuated with
relatively low levels, but slightly higher than pre-1983 rates. Then, on
September 19, 1994, following only 27 hours of precursory seismic and
ground-deformation activity, explosive eruptions began at Vulcan and Tavurvur,
the two vents on opposite sides of the caldera that have been the sites of
previous historical eruptions (Fig. 7b). While
Volcanic ash and aviation safety
During recent decades, with the advent of high-performance jet engines, an unrecognized volcano hazard emerged: in-flight encounters between jet aircraft and volcanic ash clouds produced by powerful explosive eruptions. This hazard stems from the following: (1) volcanic ash clouds are not detectable by the aircraft's onboard radar instrumentation; and (2) if the gritty, jagged ash particles are ingested into the aircraft's jet engines, the high operating temperatures can partially melt the ash. Severe abrasion and ash accumulation within the engine, along with adherence of melted ash to critical engine parts and openings, combine to degrade engine performance and, at worst, can cause engine flameout and power loss. Since the early 1970s, more than 60 volcanic ash–aircraft encounters have occurred, with several of the aircraft experiencing total power loss and requiring emergency landings; fortunately, to date, no fatal crashes have resulted from such encounters. However, many millions of dollars of damage to aircraft have been incurred; for example, an encounter between a Boeing-747 jetliner and the ash cloud from an eruption of Redoubt Volcano (Alaska) in December 1989 required more than $80 million dollars to replace all four engines and repair other damage. Volcanologists worldwide are now working closely with the air-traffic controllers, civil aviation organizations, and the air-carrier industry to mitigate the hazards of volcanic ash to civil aviation. A part of this effort involves the operation of nine regional Volcanic Ash Advisory Centers around the world to provide early warning of explosive eruptions and to issue advisories of potentially dangerous volcanic ash clouds produced by them.
Impact of explosive volcanism on global climate
Large explosive eruptions that eject copious amounts of volcanic aerosols into the stratosphere also can affect climate on a global basis. For example, long-lingering stratospheric volcanic clouds—for example, from the great 1883 eruption of Krakatau (between Java and Sumatra, Indonesia), the 1963 eruption of Agung volcano (Bali, Indonesia), and the 1982 eruption of EI Chichón (Chiapas, Mexico)—produced spectacular sunrises and sunsets all over the Earth for many months because of the interaction of suspended aerosols and the atmosphere. Studies indicate that the sulfate aerosols in volcanic clouds form a layer of sulfuric acid droplets. This layer tends to cool the troposphere by reflecting solar radiation, and to warm the stratosphere by absorbing radiated Earth heat; in general, the combined effect is to lower the Earth's surface temperature.
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