Forecasting Volcanic Activity

A forecast for a volcanic eruption is a probabilistic statement concerning the time, place, and character of an eruption before it occurs. It is analogous to forecasting the weather and is not as precise a statement as a prediction. Forecasting volcanic eruptions is a major component of the goal to reduce volcanic hazards. It is unlikely that we will be able to forecast the majority of volcanic activity accurately in the near future, but valuable information is being gathered about phenomena that occur before eruptions. One problem is that most forecasting techniques require experience with actual eruptions before the mechanism is understood. Thus, we are better able to predict eruptions in the Hawaiian Islands then elsewhere because we have had so much experience there. 

The methods of forecasting volcanic eruptions include: 

• Monitoring of seismic activity.
• Monitoring of thermal, magnetic, and hydrologic conditions. 
• Topographic monitoring of tilting or swelling of the volcano. 
• Monitoring of volcanic gas emissions. 
• Studying the geologic history of a particular volcano or volcanic centre.

Seismic Activity 

A Volcano reawakens Increased seismic activity is a good indicator of a forthcoming volcanic eruption. As a dormant volcano reawakens, rising magma fractures rock above. At first, the fracturing slowly increases the rate of seismic activity; then, both the fracturing and seismic activity accelerate a few days prior to an eruption. 

Our experience with volcanoes, such as Mount St. Helens and those on the big island of Hawaii, suggests that earthquakes often provide the earliest warning of an impending volcanic eruption. In the case of Mount St. Helens, earthquake activity started in mid-March before the eruption in May. Activity began suddenly, with near-continuous shallow seismicity. Unfortunately, there was no increase in earthquakes immediately before the May 18 event. In Hawaii earthquakes have been used to monitor the movement of magma as it approaches the surface. Several months before the 1991 Mt. Pinatubo eruptions, small steam explosions and earthquakes began. Mt. Pinatubo (present elevation 1700 m, or 5578 ft) was an eroded ridge, and, as a result, did not have the classic shape of a volcano. Furthermore, it had not erupted in 500 years; most of the people living near it did not even know it was a volcano! Scientists began monitoring earthquake activity and studying past volcanic activity, which was determined to be explosive. Earthquakes increased in number and magnitude before the catastrophic eruption, migrating from deep beneath the volcano to shallow depths beneath the summit. 

Geophysicists have proposed a generalized model for seismic activity that may help in predicting eruptions. The model is for explosive composite volcanoes, such as those in the Cascade Mountains, which may awaken after an extended period of inactivity. As a dormant volcano reawakens, the magma must fracture and break previously solidified igneous rock above the magma chamber in order to work its way to the surface. Several weeks before an eruption, increasing pressure creates numerous fractures in the plugged volcano conduit above the chamber. At first, the increase in seismic events will be very gradual, and a seismologist may need 10 days or so to confidently recognize an accelerating trend toward an eruption. Once the trend has been recognized, there will still be several days before the eruption occurs. Unfortunately, this short warning time may be insufficient for a large-scale evacuation. Thus to forecast eruptions, it may be best to use seismic activity in concert with other eruption precursors discussed below. It is fortunate that, in contrast to earthquakes, volcanoes provide warning signs prior to eruption.

Thermal, Magnetic, and Hydrologic Monitoring 

Monitoring of volcanoes is based on the fact that, before an eruption, a large volume of magma moves up into some sort of holding reservoir beneath the volcano. The hot material changes the local magnetic, thermal, hydrologic, and geochemical conditions. As the surrounding rocks heat, the rise in temperature of the surficial rock may be detected by remote sensing or infra-red aerial photography. Increased heat may melt snowfields or glaciers; thus, periodic remote sensing of a volcanic chain may detect new hot points that could indicate potential volcanic activity. This method was used with some success at Mount St. Helens before the main eruption on May 18, 1980. When older volcanic rocks are heated by new magma, magnetic properties, originally imprinted when the rocks cooled and crystallized, may change. These changes can be detailed by ground or aerial monitoring of the magnetic properties of the rocks that the volcano is composed of.

Topographic Monitoring 

 Inflation and tilting before eruption (a) Idealized diagram of Kilauea, illustrating inflation and surface tilting, accompanied by earthquakes as magma moves up. (U.S. Geological Survey Circular 1073, 1992) (b) The actual data graph, showing the east west component and the north south component of ground tilt recorded from 1964 to 1966 on Kilauea Volcano, Hawaii. Notice the slow change in ground tilt before eruption and rapid subsidence during eruption. 

Monitoring topographic changes and seismic behaviour of volcanoes has been useful in forecasting some volcanic eruptions. The Hawaiian volcanoes, especially Kilauea, have supplied most of the data. The summit of Kilauea tilts and swells before an eruption and subsides during the actual outbreak. Kilauea also undergoes earthquake swarms that reflect moving subsurface magma and an imminent eruption. The tilting of the summit in conjunction with the earthquake swarms was used to predict a volcanic eruption in the vicinity of the farming community of Kapoho on the flank of the volcano, 45 km (28 mi) from the summit. As a result, the inhabitants were evacuated before the event, in which lava overran and eventually destroyed most of the village. Because of the characteristic swelling and earthquake activity before eruptions, scientists expect the Hawaiian volcanoes to continue to be more predictable than others. Monitoring of ground movements such as tilting, swelling, opening of cracks, or changes in the water level of lakes on or near a volcano has become a useful tool for recognizing change that might indicate a coming eruption. Today, satellite based radar and a network of Global Positioning System (GPS) receivers can be used to monitor change in volcanoes, including surface deformation, without sending people into a hazardous area.

Monitoring Volcanic Gas Emissions 

The primary objective of monitoring volcanic gas emissions is to recognize changes in the chemical composition of the gases. Changes in both gas composition that is, the relative amounts of gases such as steam, carbon dioxide, and sulphur dioxide and gas emission rates are thought to be correlated with changes in subsurface volcanic processes. These factors may indicate movement of magma toward the surface. This technique was useful in studying eruptions at Mount St. Helens and Mt. Pinatubo. Two weeks before the explosive eruptions at Mt. Pinatubo, the emissions of sulphur dioxide increased by a factor of about 10.

Geologic History 

Understanding the geologic history of a volcano or volcanic system is useful in predicting the types of eruptions likely to occur in the future. The primary tool used to establish the geologic history of a volcano is geologic mapping of volcanic rocks and deposits. Attempts are made to date lava flows and pyroclastic activity to determine when they occurred. These are the primary data necessary to produce maps depicting volcanic hazards at a particular site. Geologic mapping, in conjunction with the dating of volcanic deposits at Kilauea, Hawaii, led to the discovery that more than 90 percent of the land surface of the volcano has been covered by lava in only the past 1500 years. The town of Kalapana, destroyed by lava flows in 1990, might never have been built if this information had been known before development, because the risk might have been thought too great. The real value of geologic mapping and dating of volcanic events is that they allow development of hazard maps to assist in land-use planning and preparation for future eruptions. Such maps are now available for a number of volcanoes around the world. 

Volcanic Alert or Warning 

Geologic behavior, color-coded condition, and response: Volcanic Hazards Response Plan; Long Valley Caldera, California.

At what point should the public be alerted or warned that a volcanic eruption may occur? This is an important question being addressed by volcanologists. At present, there is no standard code, but one being used with various modifications has been developed by the U.S. Geological Survey. The system is colour coded by condition; each colour green, yellow, orange, and red denotes increasing concern. This table was created specifically for the Long Valley caldera in California. Similar systems have been or are being developed for other volcanic areas, including Alaska and the Cascade Mountains of the Pacific Northwest. The colour-coded system is a good start; however, the hard questions remain: When should evacuation begin? When is it safe for people to return? Evacuation is definitely necessary before condition red, but when, during conditions yellow or orange, should it begin?

Volcanic Features

Geologic features that are often associated with volcanoes or volcanic areas include craters, calderas, volcanic vents, geysers, and hot springs.

Craters, Calderas, and Vents 

Crater

Vent

Depressions commonly found at the top of volcanoes are craters. Craters form by explosion or collapse of the upper portion of the volcanic cone and may be flat floored or funnel shaped. They are usually a few kilometres in diameter. 

Calderas are gigantic, often circular, depressions resulting from explosive ejection of magma and subsequent collapse of the upper portion of the volcanic cone. They may be 20 or more kilometres in diameter and contain volcanic vents, as well as other volcanic features, such as gas vents and hot springs. Volcanic vents are openings through which lava and pyroclastic debris are erupted at the surface of Earth. 

Vents may be roughly circular conduits, and eruptions construct domes and cones. Other vents may be elongated fissures or rock fractures, often normal faults, which produce lava flows. Some extensive fissure eruptions have produced huge accumulations of nearly horizontal basaltic lava flows called flood basalts. The best-known flood basalt deposit in the United States is the Columbia Plateau region in parts of Washington, Oregon, and Idaho, where basalt covers a vast area.

Hot Springs and Geysers 

Hot springs and geysers are hydrologic features found in some volcanic areas. Groundwater that comes into contact with hot rock becomes heated, and, in some cases, the heated water discharges at the surface as a hot spring, or thermal spring. In rare cases, the subsurface groundwater system involves circulation and heating patterns that produce periodic release of steam and hot water at the surface, a phenomenon called a geyser. World-famous geyser basins or fields are found in Iceland, New Zealand, and Yellowstone National Park in Wyoming.

Caldera Eruptions 

Calderas are produced by very rare, but extremely violent, eruptions. Although none have occurred anywhere on Earth in the last few hundred thousand years, at least 10 caldera eruptions have occurred in the last million years, three of them in North America. A large caldera-forming eruption may explosively extrude up to 1000 cubic km (240 ) of pyroclastic debris, consisting mostly of ash. This is approximately 1000 times the quantity ejected by the 1980 eruption of Mount St. Helens! Such an eruption could produce a caldera more than 10 km (6.2 mi) in diameter and blanket an area of several tens of thousands of square kilometres with ash. These ash deposits can be 100 m (328 ft) thick near the craters rim and a meter or so thick 100 km (62 mi) away from the source.6 The most recent caldera-forming eruptions in North America occurred about 600,000 years ago at Yellowstone National Park in Wyoming and 700,000 years ago in Long Valley, California. The area covered by ash in the eruption event, which produced the Long Valley caldera near the famous Mammoth Mountain ski resort. The most recent volcanic eruptions at Long Valley were about 400 years ago. Measurable uplift of the land, accompanied by swarms of earthquakes up to M 6 in the early 1980s, suggested magma was moving upward, prompting the U.S. Geological Survey to issue a potential volcanic hazard warning that was subsequently lifted. However, the future of Long Valley remains uncertain. The main events in a caldera-producing eruption can occur quickly in a few days to a few weeks but intermittent, lesser-magnitude volcanic activity can linger on for a million years. Thus, the Yellowstone event has left us hot springs and geysers, including Old Faithful, while the Long Valley event has left us a potential volcanic hazard. In fact, both sites are still capable of producing volcanic activity because magma is still present at variable depths beneath the caldera floors. Both are considered resurgent calderas because their floors have slowly domed upward since the explosive eruptions that formed them. The most likely future eruptions for Long Valley or Yellowstone would be much smaller than the giant caldera eruptions that occurred hundreds of thousands of years ago.