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PART IV Explosive Volcanism  Explosive volcanic eruptions are among the most spectacular displays of Nature. The nature and products of explosive volcanism are treated in eighteen chapters as Part IV of the Encyclopedia of Volcanoes. These chapters follow a pattern that parallels the evolutionary structure of the entire encyclopedia. We discuss first mechanisms for volcanic explosions and the styles of eruption that result in the opening seven chapters of the section, then processes of transport and deposition and their products in the following eight chapters. Finally these deposits are woven together in the final three chapters of the section to build the framework of the three great classes of explosive volcano.The chapters Magmatic Fragmentation and Phreatomagmatic Fragmentation set the stage, defining and modeling the two mechanisms for producing volcanic explosions. Magmatic Fragmentation describes how the rapid release of dissolved gases from swiftly ascending magma "drives" most volcanic explosions. Magma is transformed from a foam of gas bubbles in silicate liquid into a rapidly accelerating stream of gas carrying "pyroclasts" (liquid and solid particles). The violent interaction of magma and surface water (lakes or seas) or ground water is the focus of Phreatomagmatic Fragmentation. The spectrum of activity ranging from passive quenching of the magma to explosive ejection of pyroclasts reflects the hydrology of the environment in which magma comes into contact with water. In the sequence of five chapters that follow, we describe five classical styles of explosive eruptions. The spectacular pyrotechnics of strombolian and hawaiian lava fountaining are described and modeled in the first of these chapters. These eruptions are the least violent yet best understood and most majestic form of volcanism. Their subdued intensity and power are the consequences of the low viscosity of the erupted magmas, allowing gas to escape with relative ease. Next is Vulcanian Eruptions; these are short (seconds to minutes) discrete explosions, capable of ejecting incandescent lava bombs and blocks of fragmented rock to distances < 5 km. During the past two decades, understanding of the causes and timing of vulcanian eruptions has improved dramatically. Following this is Plinian and Subplinian Eruptions. These eruptions are characterized by the formation of high eruption plumes resulting in atmospheric ash and particle injection, and dispersal by winds over huge areas. The name for these eruptions is taken from the 79 AD eruption of Vesuvius that provoked the death of Pliny the Elder. The dynamics of these awesome events are modeled in this chapter, drawing from the classical eruptions of Vesuvius and plinian events at Mount St Helens in 1980 and El Chichon in 1982. The classical eruption of the Icelandic volcano Surtsey in 1963 gave rise to the term surtseyan volcanism. The chapter Surtseyan and Related Phreatomagmatic Eruptions examines this event and other examples of fluid basaltic magma mingling with external water. In phreatoplinian eruptions, silicic magma interacts violently with abundant water often residing in caldera lakes. This chapter re-examines the deposits of classical eruptions of this type in Iceland and New Zealand and the role of the water in the formation of clusters and aggregates of ash particles in the eruption plumes. An explosive volcanic explosion produces a jet of gas and pyroclasts, which may entrain air and atmospheric moisture and continue to rise as a billowing, buoyant eruption plume. Large volcanic plumes that penetrate the tropopause may trigger global environmental effects and produce short-term regional changes of climate. Elegant models for eruption plumes are presented in the chapter Volcanic Plumes. The authors of Pyroclast Transport and Deposition use four parameters (particle trajectory, particle concentration, the extent to which the concentration fluctuates with time, and presence or absence of cohesion during deposition) to explain the characteristics of the great proportion of subaerial pyroclastic deposits. Important clues about eruption processes come from a study of the textures and fabrics of pyroclastic fall deposits, as described in the next chapter. These deposits, produced by the rain-out of clasts through the atmosphere, are the simplest of pyroclastic products, and their value is in the ease with which their properties can be used to infer eruption parameters. The perception of pyroclastic density currents as a highly dangerous and devastating type of explosive volcanic activity was brought about dramatically by the 1902 eruptions of Mont. Pelee, Martinique, which killed 30.000 people. Since then several destructive pyroclastic-flow eruptions have occurred every decade. Pyroclastic surges and blasts, described in the next chapter, are a class of pyroclastic density current that have low particle concentrations, and are turbulent and often pulsating and produce deceptively thin, bedded and cross-bedded to massive deposits. Depositing flows of high particle concentration (10s of volume %) produce ignimbrites and block- and ash- flow deposits. The architecture and characteristics of these units are described in the following chapter. The next chapter is Lahars, which occur when large masses of volcanic sediment, and water, sweep down and off volcano slopes incorporating additional sediment. This chapter describes how both liquid and solid interactions determine the unique behavior of lahars and distinguish them from debris avalanches and floods. The rock fragments carried by the flows make them especially destructive. The next chapter describes the architecture and internal fabric of debris avalanche deposits and the models used to describe their emplacement. A debris avalanche is the product of a large-scale collapse of a sector of a volcanic edifice often triggered either by intrusion of new magma, or a phreatic explosion or an earthquake. The 1792 debris avalanche at Unzen volcano killed 15,190 people. Calderas are described next as large volcanic craters, more or less circular, with diameters many times greater than that of the volcanic vents they enclose. Formation of calderas by roof collapse over an underlying shallow magma reservoir is now recognized as accompanying most eruptions that involve magmatic volumes greater than a few cubic kilometers. Caldera-forming explosive eruptions probably are the most catastrophic geologic events that affect the earth's surface, other than rare large meteorite impacts. Ask a small child to draw a volcano and chances are that he or she will draw a composite cone. Human populations are attracted to these magnificent landforms by the fertile soils generated on the cones and yet threatened by the hazards associated with their frequent eruptions. The chapter Composite Volcanoes builds an elegant model for the life spans, geometry, and internal structure of these volcanoes. Small basaltic volcanoes, scoria cones, tuff rings, tuff cones and maars are the most common volcano type on land, yet each center is tiny compared to calderas or composite cones. Scoria Cones and Tuff Rings explains the behavior of these systems in terms of the eruption of small batches of hot fluid magma. Bruce Houghton | |