tions. For example, calderas at Yellowstone National Park, Long Valley (California), Lake Taupo (New Zealand), and Lake Toba (Sumatra) all developed during pyroclastic-flow eruptions with volumes of tens to hundreds of cubic kilometers. General summaries of pyroclastic-flow deposits are provided by Smith (1960a,b) and Ross and Smith (1961); summaries of the concepts of relations between pyroclastic-flow deposits and calderas include those of Williams (1941), Smith (1960a, 1979), Smith and Bailey (1966, 1968), and Fisher and Schminke (1984).

Pyroclastic surge is a type of flow of particulate matter characterized by a relatively low ratio of solids to gas. Consequently surges are less dense, and they tend to be of relatively low temperature; their deposits are commonly sorted and stratified and display assorted bedforms, such as crossbedding. In contrast, typical pyroclastic flows have a high ratio of particulate matter to gas and have relatively high temperatures; deposits are typically nonsorted and nonstratified. Surges tend to be pulsating, whereas other pyroclastic flows are more continuous. Like pyroclastic flows, surges may travel at high speed, commonly in the range of 50 m per second but sometimes exceeding 100 m per second. Large historical surges have traveled as much as 30 km from the source. Surges may occur either separately or together with pyroclastic flows, and sometimes surges precede or follow pyroclastic flows, forming intergradational deposits. For these reasons, pyroclastic surge is here considered as a variant form of pyroclastic flow, even though some authors regard it as a distinct process. Several different types of surge have been described, and the development of the concept and additional references are found in Moore (1967), Sparks (1976), Wohletz and Sheridan (1979), and Fisher and Schminke (1984). Both flows and surges are highly destructive of property, crops, and natural resources; they have taken many human lives. The eruption of Pelée in 1902, for example, which included both flow and surge phenomena, claimed 28,000 lives.

Lahars are dense slurries of water-saturated volcanic debris that travel downslope, occasionally at velocities as high as 40 m/sec. They are sometimes called volcanic mudflows; but because they consist of material of all sizes, including blocks as much as several meters in diameter, the Indonesian term lahar is preferred. They may be generated during eruptions when fragmented volcanic material becomes intermixed with water, such as from a crater lake or any other body of water or from eruption-induced melting of snow and ice or from eruption-induced rainfall. They may also be generated during quiet periods between eruptions when heavy rain or breaching of ponds or lakes mobilizes unconsolidated tephra. Historically, lahars have been one of the most destructive of all volcanic agents, with a high toll of both lives and property. Some historical deposits are tens of cubic kilometers, and some prehistoric deposits are hundreds of cubic kilometers in volume, and large lahars can travel over 100 km from their source. Kelut Volcano, Indonesia, is a notorious example where dozens of historical eruptions have been accompanied by lahars; in 1919 more than 100 villages were destroyed and over 5000 people were killed. Ruiz volcano (Colombia) had a moderate eruption in November 1985; melted snow and ice generated lahars that destroyed cities and towns greater than 50 km from the volcano, and more than 20,000 people were buried.

During volcanic eruptions gases are not only a major product of emission, but they are considered to be the chief agent that propels the eruption. The most abundant volcanic gases include H2O, CO2, CO, SO2, SO3, H2S, HCl, and HF, and minor amounts of many other gases have been identified. Gas emissions often continue between eruptions, and some vents issue volcanic gas continually for years and decades. Several of the gases are poisonous, and some are corrosive; and in developed areas near gas vents humans, animals, plants, and property may be adversely affected. Certain forest trees and agricultural crops may become stunted or fail to survive gas emissions, such as during the 1783 eruption of Laki, Iceland, when fluorine-poisoned crops resulted in a famine that led to 10,000 deaths. At some volcanoes heavy gases have accumulated in basins or flowed down valleys, displacing oxygen and killing humans and animals. At Dieng, Indonesia, such gas emitted during a small eruption in 1979 killed 150 people.

Another important volcanic process, which became much more widely recognized as a result of the 1980 eruption of Mount St. Helens, is the debris avalanche. At Mount St. Helens, an earthquake triggered the unstable, oversteepened north flank of the volcano into motion, and a catastrophic large landslide ensued that deposited some 2.8 km3 of debris in the nearby river courses and lake basin (Voight et al. 1981). Although large landslide deposits had previously been identified at a number of volcanoes, many additional deposits of similar origin have subsequently been recognized throughout the world. The experience at Mount St. Helens demonstrates that not only is the avalanche itself destructive, but the abrupt removal of material can depressurize an underlying phreatomagmatic system, which may then explode with cataclysmic violence (Lipman and Mullineaux, 1981). At Mount St. Helens, the debris avalanche launched a whole array of additional volcanic processes, including pyroclastic surges, flows and falls, and lahars.

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