Volcano Monitoring and Research  

History of volcanology

It has been said that the science of "volcanology" originated with the accurate descriptions of the eruption of Vesuvius in A.D. 79 contained in two letters from Pliny the Younger to the Roman historian Tacitus. Pliny's letters also described the death of his uncle, Pliny the Elder, who was killed in the eruption. Actually, however, it was not until the 19th century that serious scientific inquiry into volcanic phenomena flourished as part of the general revolution in the physical and life sciences, including the new science of "geology." In 1847, an observatory was established on the flanks of Vesuvius, upslope from the site of Herculaneum, for the more or less continuous recording of the activity of the volcano that destroyed the city in A.D. 79. Still, through the first decade of the 20th century, the study of volcanoes by and large continued to be of an expeditionary nature, generally undertaken after the eruption had begun or the activity had ceased.

Perhaps "modern" volcanology began in 1912, when Thomas A. Jaggar, Head of the Geology Department of the Massachusetts Institute of Technology, founded the Hawaiian Volcano Observatory (HVO), located on the rim of Kilauea's caldera. Initially supported by an association of Honolulu businessmen, HVO began to conduct systematic and continuous monitoring of seismic activity preceding, accompanying, and following eruptions, as well as a wide variety of other geological, geophysical, and geochemical observations and investigations. Between 1919 and 1948, HVO was administered by various Federal agencies (National Weather Service, U.S. Geological Survey, and National Park Service), and since 1948 it has been operated continuously by the Geological Survey as part of its Volcano Hazards Program. The more than 75 years of comprehensive investigations by HVO and other scientists in Hawaii have added substantially to our understanding of the eruptive mechanisms of Kilauea and Mauna Loa, two of the world's most active volcanoes. Moreover, the Hawaiian Volcano Observatory pioneered and refined most of the commonly used volcano-monitoring techniques presently employed by other observatories monitoring active volcanoes elsewhere, principally in Indonesia, Italy, Japan, Latin America, New Zealand, Lesser Antilles (Caribbean), Philippines, and Kamchatka (Russia).

 The U.S. Geological Survey's Hawaiian Volcano Observatory, on the crater rim of Kilauea Volcano.

The World Organization of Volcano Observatories was established as the result of a meeting of representatives from world-wide volcano observatories, held in Guadeloupe in 1981. WOVO became International Association of Volcanology and Chemistry of the Earth's Interior Commission in the following year. The principal aims of the World Organization of Volcano Observatories are:

1. To stimulate cooperation between scientists working in observatories and to create or improve ties between observatories and institutions directly involved in volcano monitoring.
2. To facilitate an exchange of views and experience in volcano monitoring by convening periodic meetings including field-based ones, by periodic newsletters, and by promoting a specific e-mail observatories network service.
3. To maintain an up-to-date inventory (Directory of Volcano Observatories) of networks and of instrumentation and manpower that could be made available to any of the member institutions if a situation arises that requires scientific support.
4. To supply technical support at observatories in developing countries and to create a WOVO fund to be used by these observatories (activities of WOVO generally speaking are focused on dangerous volcanoes in developing countries).
5. To organise an international task force and to promote funding from international organisations that could help defray travel and related expenses of scientific support teams.

A strikingly successful example of volcano research and volcanic hazard assessment was the 1978 publication (Bulletin 1383-C) by two Geological Survey scientists, Dwight Crandell and Donal Mullineaux, who concluded that Mount St. Helens was the Cascade volcano most frequently active in the past 4,500 years and the one most likely to reawaken to erupt, "...perhaps before the end of this century." Their prediction came true when Mount St. Helens rumbled back to life in March of 1980. Intermittent explosions of ash and steam and periodic formation of short-lived lava domes continued throughout the decade. Analysis of the volcano's past behavior indicates that this kind of eruptive activity may continue for years or decades, but another catastrophic eruption like that of May 18, 1980, is unlikely to occur soon.

What does "volcano monitoring" actually involve? 

Basically, it is the keeping of a detailed "diary" of the visible and invisible changes in a volcano and its surroundings. Between eruptions, visible changes of importance to the scientists would include marked increase or decrease of steaming from known vents; emergence of new steaming areas; development of new ground cracks or widening of old ones; unusual or inexplicable withering of plant life; changes in the color of mineral deposits encrusting fumaroles; and any other directly observable, and often measurable, feature that might reflect a change in the state of the volcano. Of course, the "diary" keeping during eruptive activity presents additional tasks. Wherever and whenever they can do so safely, scientists document, in words and on film, the course of the eruption in detail; make temperature measurements of lava and gas; collect the eruptive products and gases for subsequent laboratory analysis; measure the heights of lava fountains or ash plumes; gage the flow rate of ash ejection or lava flows; and carry out other necessary observations and measurements to fully document and characterize the eruption. For each eruption, such documentation and data collection and analysis provide another building block in constructing a model of the characteristic behavior of a given volcano or type of eruption. Volcano monitoring also involves the recording and analysis of volcanic phenomena not visible to the human eye, but measurable by precise and sophisticated instruments. These phenomena include ground movements, earthquakes (particularly those too small to be felt by people), variations in gas compositions, and deviations in local electrical and magnetic fields that respond to pressure and stresses caused by the subterranean magma movements.

Some common methods used to study invisible, volcano-related phenomena are based on:

1. Measurement of changes in the shape of the volcano-- volcanoes gradually swell or "inflate" in building up to an eruption because of the influx of magma into the volcano's reservoir or "plumbing system"; with the onset of eruption, pressure is immediately relieved and the volcano rapidly shrinks or "deflates." A wide variety of instruments, including precise spirit-levels, electronic "tiltmeters, and electronic-laser beam instruments, can measure changes in the slope or "tilt" of the volcano or in vertical and horizontal distances with a precision of only a few parts in a million.

2. Precise determination of the location and magnitude of earthquakes by a well-designed seismic network--as the volcano inflates by the rise of magma, the enclosing rocks are deformed to the breaking point to accommodate magma movement. When the rock ultimately fails to permit continued magma ascent, earthquakes result. By carefully mapping out the variations with time in the locations and depths of earthquake foci, scientists in effect can track the subsurface movement of magma, horizontally and vertically.

3. Measurement of changes in volcanic-gas composition and in magnetic field--the rise of magma high into the volcanic edifice may allow some of the associated gases to escape along fractures, thereby causing the composition of the gases (measured at the surface) to differ from that usually measured when the volcano is quiescent and the magma is too deep to allow gas to escape. Changes in the Earth's magnetic field have been noted preceding and accompanying some eruptions, and such changes are believed to reflect temperature effects and/or the content of magnetic minerals in the magma.

Recording historic eruptions and modern volcano-monitoring in themselves are insufficient to fully determine the characteristic behavior of a volcano, because a time record of such information, though perhaps long in human terms, is much too short in geologic terms to permit reliable predictions of possible future behavior. A comprehensive investigation of any volcano must also include the careful, systematic mapping of the nature, volume, and distribution of the products of prehistoric eruptions, as well as the determination of their ages by modern isotopic and other dating methods. Research on the volcano's geologic past extends the data base for refined estimates of the recurrence intervals of active versus dormant periods in the history of the volcano. With such information in hand, scientists can construct so-called "volcanic hazards" maps that delineate the zones of greatest risk around the volcano and that designate which zones are particularly susceptible to certain types of volcanic hazards (lava flows, ash fall, toxic gases, mudflows and associated flooding, etc.).

Scientist, wearing asbestos gloves and gas mask, samples volcanic gases from active vent.

Volcano-Monitoring Techniques

Illustration by B. Myers

Monitoring Strategy

Volcano monitoring methods are designed to detect and measure changes in the state of a volcano caused by magma movement beneath the volcano. Rising magma typically will trigger swarms of earthquakes and other types of seismic events, cause swelling or subsidence of a volcano's summit or flanks, and lead to the release of volcanic gases from the ground and vents. By monitoring these phenomena, scientists are sometimes able to anticipate an eruption days to weeks ahead of time and to detect remotely the occurrence of certain volcanic events like explosive eruptions and lahars. Scientists work as close as possible to the active vent(s) of a volcano so that they can observe and measure changes that often occur when magma rises toward the surface. When a volcano shows signs of unrest or is erupting, scientists often make several visits a week to conduct various surveys and to install and maintain instruments that enable us to track its activity 24 hours a day. If an eruption causes significant changes to nearby watersheds, for example by killing vegetation and depositing fresh volcanic debris over broad areas, scientists work extensively in river valleys to keep track of erosion and sedimentation downstream from the volcano. Scientists also collaborate with scientists specializing in satellite remote-sensing techniques to provide real-time warning of hazardous events (for example, eruption clouds).

Working directly on the rugged slopes of a volcano to measure and observe changes in its activity and to install and maintain a network of volcano-monitoring instruments are crucial for determining when a volcano might erupt. When a volcano begins to show new or unusual signs of activity, our monitoring data help scientists answer four critical questions for reducing the risk from volcanoes:

• Does the current unrest involve the movement of magma?
• If yes, when is an eruption most likely to occur, if at all?
• During an eruption, what real-time warnings are needed to prevent loss of life and property damage?
• When is the eruption really over?

The monitoring data scientists collect also helps address a variety of other important questions, including:

• What is the nature of a volcano's magma-reservoir system?
• What is the cause of specific volcano-seismic events?
• How do volcanic ash clouds disperse downwind of an erupting volcano?
• How susceptible to massive slope failures (landslides) are volcanoes?

Hydrologic Monitoring of Volcanoes

When water combines with loose rocks and sediment in river valleys to form a flood or lahar, large areas downstream from a volcano can be buried with water and sediment several meters thick. Scientists monitoring an active volcano face the critical challenge of detecting a potentially dangerous lahar in real time so that a warning can be issued by public officials to people downstream.

An even more difficult and less obvious challenge for scientists, however, comes in the weeks and years after an eruption that significantly alters a volcano's watersheds--monitoring the long-term threat of sediment transport and increased flooding.  For example, annual sediment yields following the explosive 1980 eruption of Mount St. Helens were as much as 500 times greater than typical background level. After 20 years, the average annual suspended-sediment yield in the Toutle River downstream from the 1980 landslide deposit was still 100 times above typical background level. 

Why is this a potential problem? Such high sediment yields often cause river channels leading away from an active volcano to gradually fill with new, loose sediment. As such channels partially fill with sediment, their capacity to convey water within their banks is reduced, which commonly results in more frequent flooding during periods of intense rainfall. The experience at Mount St. Helens, and more recently with 1991 eruption of Mount Pinatubo in the Philippines, shows that effective mitigation measures must remain functional for decades following a major volcanic disturbance in order to reduce the likelihood of flooding.

Methods of hydrologic monitoring

Detecting lahars in real time-- Detection of lahars and other debris flows close to their sources provides an opportunity for timely warnings to people in downstream areas if adequate communication systems exist. USGS scientists have developed an inexpensive, durable, portable, and easily installed system to detect and continuously monitor the arrival and passage of debris flows and floods in river valleys draining active volcanoes. This system has the potential to save many lives from one of the most dangerous hazards posed to people who choose to live along rivers leading away from an active volcano.

Measuring sediment on the move-- Keeping track of how much sediment is carried downstream from volcanoes and deposited in river channels near farmland and communities is a major goal of a hydrologic monitoring effort. Most sediment is transported from volcanically-disturbed watersheds during periods of heavy rain. Scientists use stream-gaging instruments to measure the volumes of both water and sediment carried by rivers.

Surveying river channels-- New volcanic deposits consisting of loose, fragmented rocks are no match for the erosive power of running water, which can quickly carve wide and deep channels. In order to keep track of the erosion and corresponding sedimentation downstream, scientists make regular surveys in the river channels.

Monitoring Volcano Ground Deformation

The surface of a volcano often changes shape when magma moves beneath it or rises into its cone. Hundreds of shallow cracks or deep faults tens to hundreds of meters long may develop in hours or days. The ground can change shape by rising up, subsiding, tilting, or forming bulges that are clearly visible to people familiar with the volcano.

Scientists use a variety of methods to monitor a volcano's changing shape or deformation. Some methods are as simple as using a steel tape to measure a widening ground crack. Most volcano deformation, however, can only be detected and measured with precise surveying techniques, sensitive instruments placed on the ground or in deep holes, and satellite-based technology. Whatever the method, our goal is the same: determine the changes occurring at a volcano that help us provide eruption warnings and to understand how volcanoes work.

Methods

Electronic Distance Measurements-- Measuring the distance between benchmarks placed on a volcano tens to thousands of meters apart with electronic distance meters.

EDM at Kilauea Volcano  

Tiltmeters-- Measuring tiny changes in the slope angle or "tilt" of the ground with tiltmeters is one of the oldest methods for monitoring volcano deformation.

Installing tiltmeter  

Global Positioning System-- The Global Positioning System (GPS) can pinpoint horizontal and vertical movement of the ground in real time and during surveys.

Augustine Volcano    

Satellite Radar Inferometry-- Comparison of radar-generated images from satellites recorded months to years apart can reveal deformation patterns over a broad area with remarkable clarity.

Interferogram showing uplift  

Monitoring Volcano Seismicity

Earthquake activity beneath a volcano almost always increases before an eruption because magma and volcanic gas must first force their way up through shallow underground fractures and passageways. When magma and volcanic gases or fluids move, they will either cause rocks to break or cracks to vibrate. When rocks break high-frequency earthquakes are triggered. However, when cracks vibrate either low-frequency earthquakes or a continuous shaking called volcanic tremor is triggered.

Most volcanic-related earthquakes are less than a magnitude 2 or 3 and occur less than 10 km beneath a volcano. The earthquakes tend to occur in swarms consisting of dozens to hundreds of events. During such periods of heightened earthquake activity, scientists work around the clock to detect subtle and significant variations in the type and intensity of seismic activity and to determine when an eruption is occurring, especially when a volcano cannot be directly observed.

Methods

A seismometer is an instrument that measures ground vibrations caused by a variety of processes, primarily earthquakes. To keep track of a volcano's changing earthquake activity, scientists typically must install between 4 and 8 seismometers within about 20 km of its vent, with several located on the volcano itself. This is especially important for detecting earthquakes smaller than magnitude 1 or 2; sometimes, these tiny earthquakes represent the only indication that a volcano is becoming restless. If a seismometer is located more than 50 km away, these tiny earthquakes could go undetected. Dramatic improvements in computer technology and increased scientific experience with volcano seismicity from around the world have improved our ability to provide eruption warnings and to characterize eruptions in progress. Computers have enabled us to locate earthquakes beneath a volcano faster and with greater accuracy than was possible just 5 years ago, and now scientists can determine in real time the changing character of a volcano's earthquake activity. They've also helped us to "map" subsurface structures like fault zones and magma reservoirs. Even so, the traditional paper or "analog" records generated by seismographs are still crucial to us for interpreting the seismic activity beneath a volcano.  

Seismographs record seismic signatures, Mount Pinatubo, Philippines  

Monitoring Volcanic Gases

Scientists have long recognized that gases dissolved in magma provide the driving force of volcanic eruptions, but only recently have new techniques permitted routine measurement of different types of volcanic gases released into the atmosphere. Sulfurous volcanic gas and visible steam are usually the first things people notice when they visit an active volcano. A number of other gases also escape sight unseen into the atmosphere through hot fumaroles, active vents, and porous ground surfaces. The gases escape as magma rises toward the surface, when it erupts, and even as it cools and crystallizes below ground. A primary objective in gas monitoring is to determine changes in the release of certain gases from a volcano, chiefly carbon dioxide and sulfur dioxide. Such changes can be used with other monitoring information to provide eruption warnings and to improve our understanding of how volcanoes work. In recent years, scientists have directed increased attention toward volcanic gas emissions because of the newly appreciated hazards they sometimes pose and their effects on the Earth's atmosphere and climate.

Gases released by most volcanoes are difficult to sample and measure on a regular basis, especially when a volcano becomes restless. Direct sampling of gas requires that scientists visit a hot fumarole or an active vent, usually high on a volcano's flank or within its summit crater. At some volcanoes, gases discharge directly into crater lakes. The remote location of these sampling sites, intense and often hazardous fumes, frequent bad weather, and the potential for sudden eruptions can make regular gas sampling sometimes impossible and dangerous. Measuring gases remotely is possible but requires ideal weather and the availability of suitable aircraft or a network of roads around a volcano. Consistent and favorable wind conditions are needed to carry gases from vents and fissures to where they can be measured. In some cases, automated on-site gas monitoring is feasible. Under corrosive conditions, only a few sensors are available, however, for continuously recording the concentrations of specific gases. Scientists face yet another challenge--acid gases, like SO₂, easily dissolve in water. Thus, volcanoes with abundant surface or subsurface water can prevent scientists from measuring the emission of acid gases as magma rises toward the surface and even after explosive eruptions. Because CO₂ is is less likely to be masked by the presence of water, measuring it when a volcano first becomes restless and between eruptions may be important for determining whether significant magma degassing is occurring.

Methods of studying volcanic gases

Measuring gas-emission rates in volcanic plumes-- The rate at which a volcano releases gases into the atmosphere (usually reported in metric tonnes per day) is related to the volume of magma within its magma-reservoir system and its hydrothermal system. By measuring changes in the emission rate of certain key gases, especially sulfur dioxide and carbon dioxide, scientists can infer changes that may be occurring in a volcano's magma reservoir and hydrothermal system. The emission rates of sulfur dioxide and carbon dioxide are measured using airborne or ground-based techniques. During large explosive eruptions, sulfur dioxide gas injected high into the atmosphere is measured by an instrument aboard a satellite.

Mount St. Helens

Direct gas sampling with laboratory analysis-- Direct sampling of gases escaping from fumaroles is currently the only way to fully characterize the composition of gases discharging from volcanoes and to collect data needed to determine the origin of specific gases. Unfortunately, direct gas sampling does not provide information about the emission rates of different gases. The most common method for sampling volcanic gases is to collect them directly from fumaroles in solution-filled bottles, and then to analyze the mixtures in the laboratory. In this photograph, gases are drawn through a metal tube inserted into a fumarole at Mageik Volcano in Alaska; the sample was later analyzed at a USGS laboratory in Menlo Park, California.

Mageik Volcano, Alaska

Continuous on-site gas monitoring-- ontinuous automated gas measurements can be made on a volcano directly in fumaroles, in the air near active fumaroles, and in the soil. At each gas measurement site, one or more chemical sensors measure the concentration of a specific volcanic gas, such as sulfur dioxide or carbon dioxide, and these data are transmitted by radio to a volcano observatory. These sensors can provide a real-time record of changes in gas concentration that may occur on a time scale as short as a few minutes. This site is located on the rim of Halemaumau crater in the summit caldera of Kilauea Volcano, Hawaii.

Soil-efflux measurements-- Soil-efflux measurements can be made in areas where volcanic gases, typically carbon dioxide, rise from depth and discharge into the upper soil layers near the surface. Dozens of measurements are needed to map areas of high gas concentration. In this photograph, scientists are measuring the concentration of carbon dioxide gas in the soil at a site near Horseshoe Lake at the base of Mammoth Mountain volcano in California.

Remote Sensing for Monitoring Volcanoes

This composite satellite image shows the movement of an eruption cloud from Mount Spurr Volcano in Alaska (upper left). The cloud of volcanic ash and gas was erupted at about midnight local Alaska time on September 16, 1992, and was carried by strong winds eastward across Canada and the United States. This eruption cloud is noteworthy because it traveled as a coherent mass for 5 days after the eruption and disrupted commercial air traffic in Canada and the United States. Increasingly, satellites are being used by scientists around the world to track eruption clouds in near real time, especially from Alaskan volcanoes. Alaska daylight time equals GMT (Greenwich Mean Time) minus 8 hours.

The launch of new satellites each year and new developments in remote-sensing techniques have expanded the capability of scientists worldwide to monitor volcanoes using satellites. For the purpose of studying volcanoes, remote sensing is the detection by a satellite's sensors of electromagnetic energy that is absorbed, reflected, radiated, or scattered from the surface of a volcano or from its erupted material in an eruption cloud. A variety of sensors are used to measure wavelengths of energy that are beyond the range of human vision; for example ultra- violet, infrared, and microwave.

The application of remote-sensing techniques for volcano monitoring is far from routine at volcano observatories and the techniques are not likely to replace conventional ground-based monitoring methods. For well-monitored volcanoes, satellite observations are complementary in nature and they can be extremely important for tracking eruption clouds. For many of the world's volcanoes that are either extremely remote or not monitored well, satellite observations of volcanic activity may be all that is available because of the extensive coverage they provide.

Applications of Satellite Monitoring

• Tracking Eruption Clouds
• Measuring Sulfur Dioxide Gas in Eruption Clouds
• Detecting Hot Features on Volcanoes

Studying Past Eruptive History

Scientists look at volcanic deposits around the volcano to determine their age, type (lava flows, mudflows, ash flows), size, and distance from the volcano. This data will help to determine if the volcano is active. Scientists look for patterns of activity. Does the volcano erupt or produce mudflows at regular intervals? Does it have a consistent sequence of events, such as a number of effusive eruptions followed by an explosive one or several small eruptions followed by a large one? Combining the sizes and ages of different types of deposits will help us evaluate the risk of a particular volcanic hazard at a particular volcano.

Past history can only help us in a limited way, however, because experience with many volcanoes has shown that no two eruptions of a single volcano are exactly alike and no two volcanoes produce exactly the same sequence of eruptions. Since each volcano is unique, predictions concerning the next eruption of any given volcano always have an element of uncertainty. This is why any volcano showing current signs of activity needs to be monitored in "real time."

Preparing for Volcanic Emergencies

Communication is key to saving lives

Recent advances in volcano monitoring, new and refined volcano-hazard assessments, and better warning schemes have significantly improved our capability to warn of volcano hazards and impending eruptions. Our volcano information and warnings, however, no matter how timely or precise, will reduce volcanic risk only if they are communicated effectively to a wide audience, especially to people who live and work in potentially hazardous areas and to emergency-management specialists.

Increasing public awareness of volcano hazards

In addition to carrying out specialized studies on volcanoes and hazards posed by them, scientists participate in a wide variety of projects and activities intended to increase awareness of volcano hazards and minimize future consequences of volcano activity in the United States:

• participate in volcano-emergency planning workshops and emergency-response exercises
• convene international, regional, and local workshops focused on volcano-hazard issues
• prepare educational materials with partners, including exhibits, fact sheets, booklets, video programs, and maps
• collaborate with emergency-management specialists to develop effective warning schemes
• meet with community leaders and residents wanting information about potentially dangerous volcanoes in their area
• work with the news media and media producers
• lead educational field trips to active and potentially dangerous volcanoes for the public, officials, local residents, educators, and students
• help educators and students with classroom presentations, teacher workshops, field trips, and activities

In developing and delivering easily understandably hazard information that others may act on, scientists do not dictate or advocate specific mitigation measures, because such measures must be decided in view of social, political, and economic considerations that are beyond USGS responsibility and expertise. Rather, scientists try to provide the best possible scientific information about volcanoes that will help people to choose and manage the risks associated with active and potentially active volcanoes.

Eruption Warning and Real-Time Notifications

The best warning of a volcanic eruption is one that specifies when and where an eruption is most likely to occur and what type and size eruption should be expected. Such accurate predictions are sometimes possible but still rare in volcanology. The most accurate warnings are those in which scientists indicate an eruption is probably only hours to days away based on significant changes in a volcano's earthquake activity, ground deformation, and gas emissions. Experience from around the world has shown that most eruptions are preceded by such changes over a period of days to weeks. A volcano may begin to show signs of unrest several months to a few years before an eruption. In these cases, however, a warning that specifies when it might erupt months to years ahead of time are extremely rare.

Strategy of Volcano Warnings

The strategy that scientists use to provide volcano warnings in the United States involves a series of alert levels that correspond generally to increasing levels of volcanic activity. As a volcano becomes increasingly active or as our monitoring data suggest that a given level of unrest is likely to lead to a significant eruption, scientists declare a corresponding higher alert level. This alert level ranking thus offers the public and civil authorities a framework they can use to gauge and coordinate their response to a developing volcano emergency. Scientists currently use different alert levels (also referred to as status levels, condition levels, or color code) for providing volcano warnings and emergency information regarding volcanic unrest and eruptions. These levels are different for Long Valley caldera in California and for volcanoes in Alaska, the Cascade Range in the Pacific Northwest, and Hawaii for several reasons:

1. Volcanoes exhibit different patterns of unrest in the weeks to hours before they erupt, which means that uniform and strict criteria cannot be applied to all episodes of unrest
2. Communities, people, and economic activity are threatened by US volcanoes with different types of volcano hazards so that a warning scheme must address specific hazards from a volcano
3. US volcanoes are not monitored with the same degree of intensity, depending on degree of historical unrest and eruptions and potential future risk

Uplift or Inflation As a mass of new lava rises to the surface, it pushes the old rock aside and upward making a bulge or uplift on the surface. The process is often called inflation, because the expansion of a volcano due to the lava pushing up inside is similar to inflating a balloon by blowing new air into it. The inflation of a volcano is measured in several ways: by tilt meters that measure the angle of the ground surface, by laser ranging using mirrors placed on the mountain, and by precision surveys using aerial photographs.

Measurement of the volume of a bulge is very important because it provides an indication of how large the later eruption will be, since the volume of the bulge on the surface is roughly equal to the volume of the new magma underground. For example, about one square mile of the north side of Mount St. Helens bulged outward about 450 feet (approximately one-tenth of a mile) before the May 18 eruption. Thus the volume of the bulge was about one square mile times about a tenth of a mile, or about a tenth of a cubic mile. By comparison, the volume of new lava expelled during the eruption was later estimated to be a few tenths of a cubic mile.

Case study: Pilot Project Mount Rainier Volcano Lahar Warning System (1998)

A two-year cooperative pilot project was under way to develop, deploy, and begin operation of an automated system to detect the occurrence of a lahar in the Puyallup River valley. The U.S. Geological Survey Volcano Hazards Program and the Pierce County, Washington, Department of Emergency Management are full partners in the pilot project. The Puyallup River drains the west flank of Mount Rainier, and the densely populated Puyallup valley extends about 70 km diagonally across Pierce County from Mount Rainier to the Port of Tacoma on Puget Sound. Upon detection of a lahar in the valley, the system is intended to issue an automatic notice to County emergency-management officials that would trigger immediate, preplanned emergency-response actions.

What is the lahar hazard at Mount Rainier?

Careful study of the deposits in the large valleys that drain Mount Rainier shows that, over the past 10,000 years, Mount Rainier has been the source of numerous lahars (volcanic debris flows) that buried now densely populated areas as far as 100 km from the volcano. Lahars are flowing mixtures of water and sediment that contain such a high concentration of rock debris that they look and behave like flowing wet concrete. They are capable of destroying buildings, bridges, and other man-made structures by battering, dislodgement, and burial. 

Prehistoric lahars originated on the steep flanks of the volcano and were channeled into the big valleys that carry water and sediment westward to Puget Sound or the Columbia River. Evidence from their deposits combined with observations of modern debris flows suggest that they traveled at speeds as fast as 70-80 km/hr at depths of 30 m or more in the confined parts of the valleys but slowed and thinned in the more distant, wider parts. During the past few thousand years, lahars that spanned valley floors well into the now densely populated Puget lowland have recurred, on average, at least every 500 to 1,000 years. There is every reason to expect that future lahars from Mount Rainier will be similar in behavior and frequency of occurrence to past lahars.

Aerial view of Puyallup River valley and the growing community of Orting; Carbon River on left and Puyallup River on right. The most recent large lahar to rush down this valley occurred about 500 years ago when part of Mount Rainier's west flank collapsed. The resulting lahar swept through the Puyallup valley, which contained an old-growth forest, and eventually reached Puget Sound. The lahar knocked down trees as large as 2-3 m in diameter and encased the logs as well as the lower parts of still-standing trees in muddy rock debris about 5 m thick. Some trees and stumps in the lahar deposit were unearthed during recent construction of new homes on the valley floor. This photograph is by S.R. Brantley on September 29, 1992  

Why is an automated lahar-detection system needed?

Geologic evidence indicates that many of the large prehistoric lahars from Mount Rainier originated as surges of meltwater initiated by rapid melting of snow and ice during eruptions. The meltwater torrents transformed to lahars by incorporation of loose sediment from the volcano flanks. Such a lahar, initiated by a small eruption in 1985 at a Colombian volcano, Nevado del Ruiz, took more than 20,000 lives in Armero, a valley-floor community located about 75 km from the volcano's summit. The lahar took about 2.5 hours to reach Armero. People who perished in Armero and other towns around the volcano could easily have been spared if only they had known that the lahar was coming and that safety was within an easy walk only a few hundred meters away. Buildings in the middle of Armero were completely swept away by the lahar.

Mount Rainier is carefully monitored for signs of volcanic reawakening, and an eruption that could produce a catastrophic lahar initiated by vigorous release of meltwater is expected to follow days, weeks, or even months of readily detected symptoms of volcanic unrest. Thus, it is likely that there will be opportunity for citizens and communities to prepare for an impending eruption.

However, deposits of some of the large prehistoric lahars from Mount Rainier are rich in clay, implying that they contain abundant hydrothermally altered debris from within the volcano. Therefore, they are inferred to have originated as huge avalanches of water-saturated, clay-rich debris from massive gravity-driven failures of the volcano's flanks. Absence of geologic evidence substantiating coincidence of some of these large, clay-rich, prehistoric lahars with eruptions raises concern that some may have occurred with no attendant eruptive activity. They may have been triggered by intrusion of magma into the edifice, which would show symptoms like those observed before eruptions. On the other hand, they may have been triggered by earthquakes or hydrothermal-system explosions, or a volcano flank may simply have collapsed when it became sufficiently destabilized by progressing hydrothermal alteration. Such events could generate a massive lahar with no recognized precursory warning. A reliable lahar-warning system designed to detect such sudden events can provide notification to people downstream that a lahar is underway.

Inasmuch as lahars seek valley bottoms, people can quickly climb or drive to safety in many cases by simply evacuating the floor of a well-defined valley before the lahar arrives; they need go no farther than high ground adjacent to the valley. A critical issue is to know when evacuation is necessary. Travel time for a large lahar from Mount Rainier may be an hour or less to Orting, the city closest to Mount Rainier in the Puyallup valley, and possibly as little as 30 minutes may be available from detection of a large lahar to its arrival. Successful evacuation there will depend on detection of an approaching lahar, clear warning, public understanding of the hazard, and practiced response by citizens. Decreased lahar velocity as the valleys broaden downstream gives more time--about an additional hour--for response in the larger urban areas of Puyallup and Sumner, which are closer to Puget Sound.

It is critical that the lahar-detection system be completely automatic. Except during volcanic unrest when intense around-the-clock monitoring by a team of volcanologists is underway, the time from initiation of a lahar to its arrival in a populated valley-floor area is insufficient for analysis of the data by scientists before notices are issued. Thus the system must be designed to unfailingly detect a lahar with minimum opportunity for false alarms.

How does the system work?

Lahars will be detected by networks of five acoustic flow monitor (AFM) stations that have been placed within tens to hundreds of meters from the active flood plain in the upper reaches of both the Puyallup and Carbon River valleys. The network in each valley is located about 25 km upstream from Orting, which is near the confluence of the two valleys. Each AFM station consists of a microprocessor-based data logger that measures the amplitude, frequency, and duration of ground vibrations detected by an exploration-class geophone. When measurements exceed programmed thresholds, the data are radioed to the base-station computer.

Illustration by L. Faust
Simplified schematic of an acoustic-flow monitoring station

Two AFM (acoustic-flow monitor) stations in each valley are located above flood level but within the expected inundation zone of a significant lahar. Those stations, then, will serve as "deadman" devices whose destruction by a major lahar would be noted by the system. The other three stations in each valley are located above the anticipated lahar-inundation limit with the expectation that they will monitor ground vibrations and transmit data throughout passage of a lahar. Data from all stations are transmitted by radio to duplicate base-station computers located at the Law Enforcement Support Agency, City of Tacoma and Pierce County Emergency 9-1-1 Center in Tacoma and at the Washington State Emergency Operations Center at Camp Murray. Software, currently under development, analyzes the incoming data and triggers an automatic unequivocal notice when a significant lahar is detected.

The USGS-Pierce County partnership

The USGS Volcano Hazards Program and the Pierce County Department of Emergency Management entered into their formal two-year cooperative agreement in early summer 1998. Working closely together through the first summer, they selected the station sites and installed the station housings (55-gallon drums), the station hardware (geophones, radios, antennas), and telemetry repeater stations sufficient to ensure a high level of redundancy. At that stage, Pierce County largely managed site preparation and installation of the station housings, while the USGS took responsibility for procuring and assembling the station hardware.

During the remainder of the two-year project, the USGS role would be primarily to test and evaluate the stations for sensitivity and durability; set sensitivity parameters so as to filter out ground vibrations from normal floods, wind, or passing log trucks; develop and test the software that analyzes data from the stations and governs lahar detection and automatic notification; and train Pierce County personnel to take over full operation of the system. The Pierce County Department of Emergency Management will be responsible for preparing to assume full control and operation of the system and for developing effective emergency-response actions once a lahar is detected. Upon mutual agreement at the end of the pilot project, it was expected that Pierce County will assume ownership and full control of the Puyallup valley lahar-detection network.s

Common questions on volcano monitoring

What are the signs that a volcano is about to erupt?

Short answer
Several things happen when a volcano is about to erupt, some of the most obvious are listed here:

1. The number and size of earthquakes increase in and around the volcano.
2. The ground deforms or "bulges" at the eruption site.
3. A lot more gas comes out of the volcano.

Longer answer
There are lots of signs that are examined, depending on how closely monitored the particular volcano is. Probably the most common type of monitoring is by seismicity. Even one seismometer can tell if there is an increase of seismic activity on a usually seismically-quiet volcano. If you have at least 3 seismometers, and they are strategically placed, you can triangulate on earthquakes to see if they are occurring in a place that indicates perhaps magma movement. By examining the seismic data over a period of time you may be able to determine if the earthquakes are migrating towards the surface (suggesting that magma is also migrating towards the surface since the earthquakes are probably being generated as magma breaks rocks that are in its way).

Another type of data that is used is the study of ground deformation. When magma moves up into the shallow plumbing of a volcano, it takes up space and pushes the surrounding rock outward. This also causes the surface of the volcano to deform. Some points move upward and any two points will move farther apart. By using very accurate leveling and distance-measuring techniques, these surface changes can be measured. Usually the changes are a few mm over a distance of a few hundred meters, but sometimes they are dramatic. For example prior to many eruptions at Kilauea, the summit bulges 1-2 meters upward. In the last few days prior to the big Mt. St. Helens eruption the northern flank was bulging outward at a few meters per day!

Some people like to monitor volcanoes by constantly monitoring gases that come out of fumaroles. Most active volcanoes have fumaroles where volcanic gases escape to the surface. It is relatively easy to monitor the temperatures of these gases, and an anomalous increase in temperature might be a sign that magma has moved closer to the surface. Monitoring the composition of the gases is more difficult to do, and changes in the composition are way more difficult to interpret. Many times just visual changes to fumarole areas are indications of impending activity. If the area of active degassing gets larger, if the plants nearby die suddenly, if the color of any lakes or ponds nearby changes...Many volcanoes have summit lakes through which heat and gases rise to the surface and escape. Many of these lakes have strange colors due to all the dissolved minerals in them, and many of the colored ones change color, pH, temperature, etc. These too, are signs of change below but are often difficult to interpret.

A number of people are studying ways in which to use satellite data to monitor volcanoes. It is possible to obtain thermal images of volcanic areas, and by comparing images on a monthly or bi-weekly basis, increases or decreases in temperatures can be detected. Additionally, some new technologies have allowed for the determination of very accurate topography from satellite data. This technology may someday allow for the remote monitoring of surface deformation associated with sub-surface magma movement. This process is still being developed. It usually takes too long to get satellite data processed for this technique to be useful in a rapidly-escalating crisis so it would be used over the long term, in the years to months prior to an eruption rather than the hours prior.

How often do volcanoes erupt and is there a predictable pattern?

Well that depends on the volcano. Some volcanoes erupt very often (and some like Kilauea almost never stop). On the other hand, some volcanoes are inactive for very long periods of time between eruptions. For example Mt. St. Helens erupted in the late 1800's and then again in 1980. That is considered a relatively short rest for volcanoes in the Cascade range. Pinatubo, however, last erupted about 400 years ago prior to its 1991 eruption. Lots of people didn't even consider it to be capable of erupting, it had become so eroded during those 400 years.

The largest eruptions come from volcanoes called rhyolite calderas, and these huge eruptions (which we haven't really witnessed since 186 AD in New Zealand) may occur at intervals of 10,000 to 30,000 years. Yellowstone, the largest caldera in the U.S.A. seems to erupt on average every 600,000 years!

Why does volcanic activity often lead to high rates of erosion and sedimentation?

Explosive eruptions that destroy vegetation and deposit volcanic rocks and ash over wide areas create conditions that (1) promote increased rates of surface runoff during rainstorms; and (2) dramatically increase the availability of loose debris that can be eroded and transported into river valleys. The destruction of vegetation combined with deposition of tephra on hill slopes reduces the amount of water that normally soaks into the ground or is transpired by plants. The increased overland flow of water erodes rock debris from hill slopes and carries it into river valleys.  There, sediment can accumulate and can alter the hydraulic characteristics of the river channel.

The net effect of such changes to watersheds is that post-eruption stream velocities and peak discharges during rainstorms are temporarily much higher than during pre-eruption conditions. Streams typically respond more quickly to a given amount of rainfall and produce higher flows as rainfall is quickly flushed through a watersheds.

Close view of a gully eroded into new volcanic deposits within 3 months of the eruption. The underlying soil layer, topped with pre-eruption vegetation and roots, prevented running water from eroding into even thicker tephra deposits erupted by the volcano hundreds of years ago. In many locations, however, these older deposits were also carried away by surface runoff during intense rainstorms and transported tens of kilometers downstream.