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Volcanology

 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 (Washington) on May 18, 1980 (Fig. 1), caused the worst volcanic disaster in the history of the United States, resulting in the loss of 57 lives. Yet, this eruption was much less destructive than other historic eruptions (Table 1). In March-April 1982, El Chichón, an obscure and largely forgotten volcano in southeastern Mexico, produced three major explosive bursts, which obliterated all settlements within a radius of about 5 mi (8 km) and perhaps caused more than 2000 deaths. Not only was this eruption the most destructive in Mexico's history, but some atmospheric scientists have claimed that the massive injection of sulfate aerosols into the stratosphere by El Chichón may have affected global climate, perhaps lowering the average temperature in the Northern Hemisphere by 0.18 or 0.36°F (0.1 or 0.2°C).

 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, Colombia, the northernmost active volcano in the Andes. Despite the small amount of material erupted, the hot ejecta mixed with melted snow and ice to generate highly mobile mudflows that swept down the steep drainages flanking the volcano. These mudflows killed more than 25,000 people downvalley, resulting in the second-worst volcanic disaster in the twentieth century (the worst is the 1902 Mont Pelée eruption at Martinique).

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, Kyushu, Japan, killed 41 people, including three volcanologists filming the volcanic activity. Intermittent weaker activity persisted at Unzen, and periodic collapses of the still-growing lava dome continued to pose a volcanic hazard to the city of Shimabara downslope from the volcano. During the course of the Unzen eruption, at least 12,000 people evacuated their homes temporarily. A much larger explosive eruption occurred at Mount Pinatubo, Luzon, Philippines, on June 15–16, 1991, after a dormancy of about 600 years. This eruption caused more than 300 fatalities and widespread destruction of structures, civil works, and cropland, and forced the evacuation of nearly 80,000 people, including 17,000 U.S. military personnel and their dependents stationed at Clark Air Base. Most of the fatalities attributed to the June 15 climactic eruption were caused by the collapse of roofs, laden with ash wetted by heavy rains of typhoon Yunya, which struck the island of Luzon at the same time. Since the 1991 eruption, destructive mudflows triggered by heavy rainfall during the monsoon seasons have caused additional fatalities and considerable property damage; such posteruption mudflows will pose a continuing volcano hazards into the twenty-first century, when the debris-choked valleys draining Mount Pinatubo are expected to reestablish the preeruption stream gradients. The Pinatubo eruption ranks as the second largest eruption in the world in the twentieth century, after that of Novarupta (Katmai), Alaska, in 1912. Moreover, Pinatubo injected into the stratosphere at least twice the volume of aerosols as did El Chichón in 1982, and the resultant stratospheric volcanic cloud affected the global climate until the mid-1990s.  See also: Climate history

In the Caribbean region, Mont Pelée had been responsible for the world's worst volcanic disaster, in 1902 (Table 1). In mid-1995, Soufriere Hills, a volcano on the island of Montserrat (British West Indies), which had been dormant for more than three centuries, began to erupt. The most intense activity occurred in 1996–1997, mostly involving nuées ardentes triggered by a series of collapses of actively growing, unstable lava domes; sporadic weaker activity has continued. This eruption is not large and has produced few fatalities, but it has caused tremendous socioeconomic and political impact. All of the island's means of livelihood and infrastructure have been lost, and the present population (3500–4000) is only about a third of that before the eruption. As of 2000, the British and Montserrat governments were still undecided about the long-range plans for the island's rehabilitation and possible return of the evacuated population.

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.

 Scientific inquiry

 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 (Italy), modern volcanology perhaps began with the founding of well-instrumented observations at Asama Volcano (Japan) in 1911 and at Kilauea Volcano (Hawaii) in 1912. The Hawaiian Volcano Observatory, located on Kilauea's caldera rim, began to conduct systematic and continuous monitoring of seismic activity preceding, accompanying, and following eruptions, as well as other geological, geophysical, and geochemical observations and investigations. Operated by the U.S. Geological Survey (USGS), the Hawaiian Volcano Observatory pioneered and refined most of the commonly used volcano-monitoring techniques that are employed by other observatories studying active volcanoes elsewhere, principally in Iceland, Indonesia, Italy, Japan, New Zealand, Lesser Antilles (Caribbean), Philippines, and Kamchatka (Russia). In response to the Mount St. Helens eruption in 1980, the David A. Johnston Cascades Volcano Observatory was established by the U.S. Geological Survey. A sister observatory to the Hawaiian observatory, it monitors the eruptions of Mount St. Helens and serves as the center for the study of the other potentially active volcanoes of the Cascade Range (in California, Oregon, and Washington).

In March 1988, the USGS, in a cooperative program with the State of Alaska and the university of Alaska, established the Alaska Volcano Observatory, with facilities and staff in both Anchorage and Fairbanks.

In 1999 the USGS formally designated a long-term program of volcano-monitoring studies at Long Valley Caldera (east-central California) as the Long Valley Observatory. The last volcanic activity in the Long Valley region was about 200 years ago. Since May 1980 the caldera has exhibited measurable volcanic unrest, as seen by greatly increased seismicity activity and an accumulated ground uplift of nearly 2 ft (0.7 m).  See also: 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, Alaska (1989), and Mount Pinatubo (1991) indicate that many magma systems were gas-saturated at the time of eruption. Some magmas, however, undergo considerable preeruption degassing during shallow subsurface storage and transport.  See also: Crystallization; Magma

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 eruption of Kilauea, Hawaii, temperatures are low and the gas is wholly or very largely steam. Fresh magmatic material may be entirely absent. In these phreatic (steam-blast) eruptions, the steam is simply heated ground water from the rocks adjacent to the magma reservoir and volcanic conduit. In other eruptions, such as that of Parícutin, Mexico, in 1943, the large volume of steam given off simultaneously with lava and smaller amounts of magmatic gas far exceeds the theoretical saturation limit of the magma, indicating that volatilized ground water was involved.

 Physical properties

 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, Hawaii, and a few other volcanoes. Measured temperatures typically range between 2100 and 2200°F (1150 and 1200°C), and many measurements in cooling Hawaiian lava lakes indicate that the basalt becomes completely solid at about 1800°F (980°C). Perhaps the most reliable temperature determinations of Hawaiian lavas are obtained from experimentally calibrated geothermometers, involving the precise chemical analysis of the abundance of calcium or magnesium in the glass matrix (the quenched liquid phase). At Nyamlagira volcano, central Africa, in vents and in flows close to vents, temperatures ranged from about 1900 to 2000°F (1040 to 1095°C). Temperatures during the 1950–1951 eruption of Oshima, Japan, were in the same range. Locally, temperatures as high as 2550°F (1400°C) have been reported but not well documented; these anomalously high temperatures have been ascribed to the burning of volcanic gases in vents.  See also: Geologic thermometry

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 Kilauea, which yielded a best estimate of about 1900 poises (190 Pa · s) at a temperature of 2100°F (1150°C). Calculations based on rate of flow at both Kilauea and Mauna Loa, Hawaii, gave viscosities of 3000–4000 poises (300–400 Pa · s) for lava close to the vents, increasing at greater distances from the vents as the lava cools and stiffens to immobility. At Hekla, Iceland, a somewhat more silicic lava in the vent had a viscosity of about 10,000 poises (1000 Pa · s); and at Oshima, Japan, the lowest viscosities in two streams near the vent during the 1951 eruption were 5600 and 18,000 poises (560 and 1800 Pa · s), respectively. In general, empirical observations of flow behavior of more silicic lavas indicate that they are more viscous than basaltic lavas, but direct field measurements of their viscosities have not yet been made.  See also: Lava; Magma; Pyroclastic rocks; Viscosity

  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 Pacific Northwest.

Fig. 2  Typical of “Hawaiian eruptions,” an 80-ft-high (25-m) fountain of fluid basaltic lava plays during an eruption of Kilauea volcano, Hawaii, in 1973. Compare with the plinian eruption of Mount St. Helens in Fig. 1, involving much more viscous dacitic lava. (Photograph by R. I. Tilling, USGS)

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 Mount St. Helens rated a VEI of 5, but just barely. During the past 10,000 years, only four eruptions rated VEIs of 7, including the 1815 eruption of Tambora (Indonesia), the largest known eruption in the world in recorded history. Several of the large pre-Holocene caldera-forming eruptions would qualify for VEI ratings of 8. The VEI scale, like the Richter magnitude scale for earthquakes, is open-ended; that is, there is no maximum rating. However, to date, no eruptions of VEI 9 size (eruptive volume of 104 km3) have been recognized, and magma reservoirs of such volume are virtually unknown in the geologic record.  See also: Holocene

 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 (Wyoming) and Long Valley calderas and some other volcanoes [such as, Mount Etna (Italy) and Okmok (Alaska)].  See also: Geodesy; Satellite navigation systems; Seismographic instrumentation

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 (Italy), Kilauea (Hawaii), and Merapi (Indonesia). At Kilauea, volcanic gases have been monitored at 25 sites by sampling twice a week and analyzing chromatographically more than 10 different gas species.  See also: Gas chromatography

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 Mount St. Helens. Another recently developed remote gas-monitoring technique uses Fourier transform infrared (FTIR) spectroscopy for measurement of sulfur dioxide (SO2), the ratio of sulfur dioxide to hydrochloric acid (SO2/HCl), and silicon tetrafluoride (SiF4) in volcanic plumes; this technique has been applied at the Kilauea (Hawaii), Etna and Vulcano (Italy), and Asama and Unzen (Japan) volcanoes. The continuous monitoring of emission at selected sites, together with periodic regional surveys, has demonstrated that the rate of carbon dioxide emission in the Long Valley Caldera region greatly increased in 1989. In areas of highest emission, the high CO2 concentrations in the soil are killing trees by denying their roots oxygen and by interfering with nutrient uptake. Large but gradually declining volumes of CO2 gas continue to seep from this volcanic system. The periodic determination of the absorption of certain gases (such as chlorine and sulfur) in alkaline solutions provides a time-integrated measure of their emission.

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 Long Valley caldera (California) in the United States, and at Vulcano and other sites in Italy. Nonetheless, gas-monitoring techniques must be considered largely experimental, in large measure because of the fugitive nature of gases, their complex pathways from magmatic source to measurement site, and interactions with the hydrology of the volcano and its surroundings.

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 Kilauea suggests that this technique may provide more advanced notice of impending eruptions than seismic and ground deformation monitoring. However, much more data and testing are required before geoelectrical monitoring techniques can be considered to be as routinely and universally applicable as seismic and ground deformation methods.  See also: Earth, gravity field of; Geoelectricity; Geophysical exploration

 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 Mount St. Helens experience. As early as 1975, scientists predicted that this volcano was the one in the conterminous United States most likely to reawaken and to erupt, possibly before the end of the century. This prophecy was followed in 1978 by a detailed analysis of the types, magnitudes, and areal extents of potential volcanic hazards that might be expected from a future eruption of Mount St. Helens. The volcano erupted catastrophically on May 18, 1980, and the volcanic events and associated hazards largely followed the scenario outlined earlier. Because of improved volcano monitoring with the establishment of the David A. Johnston Cascades Volcano Observatory, all of the eruptions of Mount St. Helens since June 1980 have been predicted successfully.

The response by United States and Philippine volcanologists to the reawakening of Mount Pinatubo in early April 1991 (Fig. 6) constitutes another successful case in volcanic hazards mitigation. Seismic and volcanic gas monitoring studies prompted scientists to recommend to the U.S. and Philippine governments evacuation of the surrounding region and so more than 200,000 people were moved. About 36 h later, the cataclysmic eruption of June 15–16 took place, obliterating the region within a 5–6–mi (8–10–km) radius. Given the huge volume of the eruption (preliminary estimate is about 0.5 mi3 or 2 km3) and the widespread devastation, the death toll almost certainly would have been tens of thousands, rather than hundreds.

 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 (Papua New Guinea) in the mid-1980s offers an excellent example of how an effective hazards-mitigation program can save lives, even though the anticipated eruption came a decade later than the original forecast. Beginning in the early 1970s, Rabaul began to exhibit signs of volcanic unrest, as seen by periodic earthquake swarms beneath the caldera and by ground uplift. During 1983 and 1984, the volcano's activity increased dramatically, with the monthly counts of earthquakes approaching 10,000 late in 1983 (Fig. 7a). This sudden escalation in seismicity was accompanied by sharply increasing rates of ground deformation, as indicated by geodetic monitoring. In October 1983, after considering socioeconomic factors and the scientific information provided by the Rabaul Volcano Observatory on the status of the volcanic unrest, Rabaul government officials declared a stage-2 alert, which implied that an eruption would occur within a few months. In response to this declaration, the citizens of Rabaul Town were made aware of the need to prepare for possible evacuation, staging areas for evacuation were designated, certain roads were widened to serve as evacuation routes, and several evacuation drills were conducted.

  Fig. 7  Activity at Rabaul Caldera, Papua New Guinea. (a) Earthquakes per month from July 1971 through late 1985. The sharp increase in earthquakes in late 1983, combined with a cumulative uplift of about 2 m (6.6 ft) since 1973, prompted government officials to declare a stage-2 alert (in effect October 29, 1983, to November 22, 1984), in anticipation of a possible eruption within a few months. However, the eruption did not occur until mid-September 1994, about a decade after the alert was lifted. (After R. I. Tilling, The role of monitoring in forecasting volcanic events, in B. McGuire et al., eds., Monitoring Active Volcanoes: Strategies, Procedures and Techniques, pp. 369–401, UCL Press Limited, London, 1995) (b) Photograph, taken by the space shuttle astronauts, of the 18-km-high (11-mi) volcanic plume produced by the Rabaul eruption, about 24 hours after it began on September 19, 1994. (NASA)

 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 Rabaul Town suffered massive destruction (principally from ash accumulated and roof collapse) and over 50,000 people were displaced, fewer than 10 people were killed (several from automobile accidents)—quite remarkable considering the rapid onset of the eruption with virtually no public warning to the population. The main reason for the low fatalities of the 1994 eruption was that people quickly “self-evacuated” with the first light ash falls. The people apparently had learned their lessons well years earlier, from the heightened awareness of volcanic hazards generated by the 1983–1985 volcanic crisis as well as from memories or stories of the 1937 eruption (similarly involving eruptive outbreaks at Vulcan and Tavurvur) among the long-time residents and their families.

 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.

The 1980 Mount St. Helens eruption apparently had minimal climatic impact, producing at most only a decrease of 0.18°F (0.1°C) in average temperature for the Northern Hemisphere. In contrast, the 1982 EI Chichón and the 1991 Pinatubo eruptions lowered temperature by 0.36–0.9°F (0.2–0.5°C). Significantly, the Mount St. Helens magma contains much less sulfur [<0.01 wt % of sulfur trioxide (SO3)] than those of EI Chichón or Pinatubo, 1–4 and 0.3–0.5 wt % respectively. Perhaps the best-known example of eruption-induced climate change is that associated with the eruption of Tambora volcano (Indonesia) in 1815, the most powerful eruption in historical time. Average temperature in the Northern Hemisphere was lowered by as much as 0.9°F (0.5°C), and parts of the United States and Canada experienced unusual summer frosts and loss of crops; 1816 was known as the year without summer. On a local scale, the fallout from gas-rich eruption plumes and associated acid rains can cause damage to crops and exposed metal parts of dwellings.  See also: Acid rain; Aerosol; Air pollution

Bibliography

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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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• Ali Fazeli = egeology.blogfa.com
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Additional Readings

•  U.S. Geological Survey Volcano Hazards Program
• Ali Fazeli = egeology.blogfa.com
• Global Volcanism Program
• Ali Fazeli = egeology.blogfa.com