Volcano Monitoring

signs that a volcano may be about to erupt may be observed only if volcanoes are carefully and routinely monitored. Volcanic monitoring is aimed at detecting the precursory phenomena described above and tracking the movement of magma beneath volcanoes. In the United States, the United States Geological survey is in charge of comprehensive volcano moni toring programs in the Pacific Northwest, Alaska, and Hawaii.

one of the most accurate methods of determining the position and movement of magma in volcanoes is using seismology, or the study of the passage of seismic waves through the volcano. These can be natural seismic waves generated by earthquakes beneath the volcano or seismic energy released by geologists who set off explosions and monitor how the energy propagates through the volcano. Certain types of seismic waves travel through fluids like magma (compressional- or P-waves), whereas other types of seismic waves do not (shear- or S-waves). The position of the magma beneath a volcano can be determined by detonating an explosion on one side of the volcano, and having seismic receivers placed around the volcano to determine the position of a "shadow zone" where P-waves are received, but S-waves are not. The body of magma that creates the shadow zone can be mapped out in three dimensions by using data from the numerous seismic receiver stations. Repeated experiments over time can track the movement of the magma.

other precursory phenomena are also monitored to track their changes with time, which can further refine estimates of impending eruptions. Changes in

Arching lava fountain on Kilauea, Hawaii, February 25, 1983 (J. D. Griggs/USGS)

the temperature of the surface can be monitored by thermal infrared satellite imagery, and other changes, such as shifts in the composition of emitted gases, are monitored. Other promising precursors may be found in changes to physical properties, such as the electrical and magnetic field around volcanoes prior to eruptions.

Changes in the geochemical nature of gases and fluids coming out of volcanic vents and fumaroles can be used as indicators of activity beneath volcanoes. These changes depend largely on the changing rates of magma degassing beneath the volcano, and interactions of the magma with the groundwater system. Monitoring of gases is largely designed to look for rapid changes or nonequilibrium conditions in hydrochloric and sulfurous acids, carbonic acids, oxygen, nitrogen, and hydrogen sulfide. Convergent margin andesitic types of explosive volcanoes show the greatest variation in composition of gases prior to eruption, since magma in these volcanoes is ultimately derived from fluids carried to depth by the subducting oceanic lithospheric slabs.

The details of geophysical volcano monitoring are complex and have undergone a rapid explosion in sophistication in recent years. one of the more common techniques in use now involves the use of a dense array or group of very sensitive seismographs called broadband seismometers that can detect a variety of different kinds of earthquakes. Broadband seismometers can detect seismic waves with frequencies of 0.1-100 seconds, a great improvement over earlier short-period seismometers that detected only frequencies between 0.1-1 second. Swarms of small earthquakes, known as harmonic tremors, are sometimes associated with the movement of magma upward or laterally beneath a volcano, and they characteristically increase in number before an eruption. These are different from tectonic earthquakes that generally follow a pattern of main shock-aftershocks. By analysis of the seismic data from the array of seismographs geologists are able to build a three-dimensional image of the area beneath the volcano, much like a tomographic image or a CAT scan, and thereby monitor the distribution and movement of magma beneath the volcano. When the magma gets closer to the surface, an eruption is more likely to occur. Movement of magma is also sometimes associated with explosion-type earthquakes, easily differentiated from earthquakes associated with movement on faults.

Many explosive volcanic eruptions are preceded by swelling, bulging, or other deformation of the ground surface on the volcano, so one method to predict eruptions involves measuring and monitoring this bulging. Ground deformation is commonly measured using a variety of devices. some instruments pre cisely measure shifts in the level surface, others measure tilting, and still others make electronic distance measurements. These types of measurements have recently increased in accuracy with the advent of the use of precise Global Positioning System instruments that allow measurements of latitude, longitude, and elevation to be made that are accurate to less than a half-inch (1 cm), even in very remote locations.

Some observations have been made of phenomena that precede some eruptions, even though their cause is not clearly understood. Electrical and magnetic fields have been observed to show changes at many volcanoes, especially those with basaltic magma that has a high concentration of magnetic minerals. These changes may be related to movement of magma (and the magnetic minerals), changes in heating, movement of gases, or other causes. Recent studies have linked small changes in the microgravity fields around active volcanoes, especially explosive andesitic volcanoes, to movement of magma beneath the cones.

Satellite images are now commonly used to map volcanic deposits and features and to monitor eruptions. There is now a wide range in types of features satellites can measure and monitor, including a large range of the visible and other parts of the electromagnetic spectrum. Changes in the volcanic surface, growth of domes, and opening and closure of fissures on the volcano can be observed from satellites. Some satellites use radar technology that is able to see through clouds and some ash, and thus are particularly helpful for monitoring volcanoes in remote areas, in bad weather, at night, and during eruptions. A technique called radar interferometry can measure ground deformation at the sub-inch (cm) scale, showing bulges and swelling related to buildup of magma beneath the volcano. Some satellites can measure and monitor the temperature of the surface, and others can watch eruption plumes, ash clouds, and other atmospheric effects on a global scale.

Together all these techniques have given seismologists and geologists tools they need to make more accurate predictions of when an eruption may occur, saving lives and property. When many of the techniques are integrated into one monitoring program, then scientists are better able to predict when the next eruption may occur. Several volcano monitoring programs in the United States use many different types of observations to provide for the safety of citizens. These include the Alaskan Volcano Observatory, the Cascades Volcano Observatory, and the Hawaiian Volcano Observatory.

See also convergent plate margin processes; energy in the Earth system; geochemical cycles; magma; plate tectonics; tsunami, generation mechanisms.

FURTHER READING

Blong, Russel J. Volcanic Hazards: A Sourcebook on the Effects of Eruptions. New York: Academic Press, 1984.

Fisher, R. V. Out of the Crater: Chronicles of a Volcanolo-gist. Princeton, N.J.: Princeton University Press, 2000.

Fisher, R. V., G. Heiken, and J. B. Hulen. Volcanoes: Crucibles of Change. Princeton, N.J.: Princeton University Press, 1998.

Oregon Space Grant Consortium. Volcanoworld. Available online. URL: http://volcano.oregonstate.edu/. Accessed October 10, 2008.

Scarpa, Roberto, and Robert I. Tilling. Monitoring and Mitigation of Volcano Hazards. New York: Springer, 1996.

Simkin, T., and R. S. Fiske. Krakatau 1883: The Volcanic Eruption and Its Effects. Washington, D.C.: Smithsonian Institution Press, 1993.

U.S. Geological Survey. Volcano Hazards Program home page. Available online. URL: http://volcanoes.usgs. gov/. Last modified September 11, 2008. Data updated daily or more frequently.

weathering Weathering is the process of mechanical and chemical alteration of rock marked by the interaction of the lithosphere, atmosphere, hydrosphere, and biosphere. The resistance to weathering varies with climate, composition, texture, and how much a rock is exposed to the elements of weather. Weathering processes occur at the lithosphere/atmosphere interface. This is actually a zone that extends down into the ground to the depth that air and water can penetrate—in some regions this is a few feet (meters), in others it is a mile (kilometer) or more. In this zone, the rocks make up a porous network, with air and water migrating through cracks, fractures, and pore space. The effects of weathering can often be seen in outcrops on the sides of roads, where they cut through the zone of alteration into underlying bedrock. These roadcuts and weathered outcroppings of rock show some similar properties. The upper zone near the surface is made of soil or regolith in which the texture of the fresh rock is not apparent, a middle zone in which the rock is altered but retains some of its organized appearance, and a lower zone, of fresh unaltered bedrock.

PROCESSES OF WEATHERING

There are three main types of weathering. Chemical weathering, the decomposition of rocks through the alteration of individual mineral grains, is a common process in the soil profile. Mechanical weathering is the disintegration of rocks, generally by abrasion. Mechanical weathering is common in the talus slopes at the bottom of mountains, along beaches, and along river bottoms. Biological weathering involves the breaking down of rocks and minerals by biological agents. some organisms attack rocks for nutritional purposes; for instance, chitons bore holes through limestone along the seashore, extracting their nutrients from the rock.

Generally, mechanical and chemical weathering are the most important, and they work hand-in-hand to break down rocks into the regolith. The combination of chemical, mechanical, and biological weathering produces soils, or a weathering profile.

Mechanical Weathering

There are several different types of mechanical weathering which may act separately or together to break down rocks. The most common process of mechanical weathering is abrasion, where movement of rock particles in streams, along beaches, in deserts, or along the bases of slopes causes fragments to knock into each other. These collisions cause small pieces of each rock particle to break off, gradually rounding the particles and making them smaller, and creating more surface area for processes of chemical weathering to act upon.

some rocks develop joints, or parallel sets of fractures, from differential cooling, the pressures exerted by overlying rocks, or tectonic forces. Joints are fractures along which no observable movement has occurred. Joints promote weathering in two ways: they are planes of weakness across which the rock can break easily, and they act as passageways for fluids to percolate along, promoting chemical weathering.

Crystal growth may aid mechanical weathering. When water percolates through joints or fractures, it can precipitate minerals such as salts, which grow larger and exert large pressures on the rock along the joint planes. If the blocks of rock are close enough to a free surface such as a cliff, large pieces of rock may be forced off in a rockfall, initiated by the gradual growth of small crystals along joints.

Sheets of granite carved by Tuolumne River and ancient glaciers in Yosemite National Park, California (Joseph H. Bailey/National Geographic/Getty Images)

When water freezes to form ice, its volume increases by 9 percent. Water is constantly seeping into the open spaces provided by joints in rocks. When water filling the space in a joint freezes, it exerts large pressures on the surrounding rock. These forces are very effective agents of mechanical weathering, especially in areas with freeze-thaw cycles. They are responsible for most rock debris on talus slopes of mountains and cracks in concrete in areas with cold climates.

Heat may also aid mechanical weathering, especially in desert regions where the daily temperature range may be extreme. Rapid heating and cooling of rocks sometimes exerts enough pressure on the rocks to shatter them to pieces, thus breaking large rocks into smaller fragments.

Plants and animals may also aid mechanical weathering. Plants grow in cracks and push rocks apart. This process may be accelerated if plants such as trees become uprooted, or blown over by wind, exposing more of the underlying rock to erosion. Burrowing animals, worms, and other organisms bring an enormous amount of chemically weathered soil to the surface, and continually turn the soils over and over, greatly assisting the weathering process.

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