tion is perhaps the more familiar. The size, shape, and structure of volcanoes vary widely with the physical and chemical characteristics and amount of the emitted material, and they are classified according to these various factors. Examples of large volcanic edifices include composite cones and shield volcanoes. Composite cones are typically steep-sided structures, some of great beauty like Mayon in the Philippines and Fuji in Japan. Most large volcanoes associated with compressive plate boundaries are composite cones. They are built of lava flows alternating with layers of pyroclastic fall, pyroclastic flow, and other fragmental material. Shield volcanoes, in contrast, are broad, gently sloping structures, built of many overlapping tongues of lava that had great fluidity; Mauna Loa and Kilauea in Hawaii are examples. Descriptions of these and other volcanic landforms are in such textbooks on volcanoes as those by Macdonald (1972), Bullard (1976), Williams and McBirney (1979), and Decker and Decker (1981).
Material erupted by volcanoes is of widely diverse type and character. Eruptions cover the spectrum of size, violence, and rates of ejection and travel of the material. Hazards vary accordingly. Volcanic products assume a variety of forms, including lava flow, pyroclastic fall, pyroclastic flow, lahar, volcanic gas, and debris avalanche and a wide assortment of intermixtures and intergradations.
Lava flows originate by quiet welling from a vent or by more vigorous lava fountains, and they vary from fluid and mobile to viscous and slow. Fluid flows may travel long distances, they may be either sheetlike or lobate depending on topography, and they are thin relative to their length and breadth. On steep slopes, speeds may reach several tens of kilometers per hour. Viscous flows travel slowly and only short distances, and they are thick relative to fluid flows of comparable lateral dimensions. Speeds are typically a few meters to a few hundred meters per hour. Lava flows commonly destroy property, arable land, buildings, and other structures, but relatively few have taken human lives. A rare exception was the 1977 eruption of the African volcano Nyiragongo, from which a highly fluid, fast-moving lava flow overwhelmed several villages and killed about 300 people.
Pyroclastic falls result from violent ejection of fragmented or pulverized rock that travels through the air and falls to the ground. “Tephra” is a widely used term for this material. Mechanisms that cause fragmentation include explosive eruptions and vigorous streaming of gas from a vent. Fallout may be directly from eruption columns or from clouds produced by convective columns and transported by the wind. Convective clouds may rise above pyroclastic flows (see below) and be transported and deposited as pyroclastic falls. Pyroclastic falls probably constitute the most common of all volcanic hazards. A small to moderate amount of tephra, with thicknesses of up to a few centimeters, is a nuisance and causes damage and inconvenience by clogging machinery and covering buildings, roads, and vegetation. It causes breathing difficulties for humans and animals, and it can abrade tooth enamel. Voluminous tephra falls may be highly destructive; eruptions during historical and recent times have produced deposits many meters thick that collapsed structures and buried towns and farms. Fine material may be carried tens to hundreds of kilometers; some large prehistoric eruptions have deposits that can be recognized thousands of kilometers from their source.
Pyroclastic flows, also known in various forms as ash flows, pumice flows, glowing avalanches, and nuées ardentes, are gravity-controlled ground-hugging masses of rapidly moving hot particulate matter. Their high mobility is the result of fluidization of the mass of particles, thought to be caused by both dissolved gases escaping from the hot particles during travel and the rapid expansion of suddenly heated air engulfed by the advancing mass. The size and shape of source vents, volume-rate of ejection, temperature of material, and mechanism of transport all vary widely, and each is a factor in determining the characteristics of the resulting deposit. Pyroclastic-flow deposits may range from narrow and lobate to broad and sheetlike, and thicknesses may vary from a fraction of a meter to hundreds of meters. Particularly high-temperature and/or voluminous pyroclastic flows may result in the flattening and welding of particles within parts of the deposit to form welded tuffs.
The rapid ejection of large volumes of material from a magma reservoir may cause the overlying surface area to collapse, producing a caldera. Calderas are circular to elongate depressions that are from one to several tens of kilometers across with walls that may be hundreds of meters high. Crater Lake, Oregon, is an example of this process. Relatively few caldera-forming eruptions have occurred during historical time, but they include some of the more notable eruptions such as Tambora (in 1815); Krakatau (in 1883), and Katmai (in 1912). These voluminous eruptions represent perhaps the most severe of all known volcanic hazards, and localities throughout the circum-Pacific belt and elsewhere have the potential for producing such eruptions. Their frequency of occurrence, fortunately, is very low by human time standards. Several times during the past million years, however, calderas have formed during truly immense eruptions of pyroclastic flows with volumes one or two orders of magnitude larger than those of historic erup-