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Understanding the Physical Behavior of Volcanoes Steven N Cambridge University Press 0521592542 - Volcanoes and the Environment Edited by Joan Marti and Gerald Ernst Excerpt More information Chapter 1 Understanding the physical behavior of volcanoes Steven N. Carey Introduction Each eruption represents a unique culmination of this complex series of steps, making the predic- Volcanism is a spectacular display of the com- tion of volcanic eruptions one of the most chal- plex way in which energy and materials are lenging tasks in the geosciences. exchanged between three major components of The science of volcanology draws from many our planet: the solid Earth, oceans, and atmo- different fields, including petrology, geochem- sphere. Mankind has long been both fascinated istry, seismology, and sedimentology. It began and terrified by erupting volcanoes. Yet through- with mainly qualitative observations about the out history people have been drawn to their fer- distribution of volcanoes, their eruptive behavior, tile slopes and have developed a unique sym- and the types of products they create. In AD 79, biosis. In many cultures, volcanoes symbolize a Pliny the Younger made the first written descrip- source of tremendous power that must be pla- tion of a volcanic eruption in two letters describ- cated by worship or sacrifice. Volcanologists, on ing the death of his uncle, Pliny the Elder, during the other hand, strive to understand how volca- the eruption of Vesuvius volcano in Italy. During noes work in order to better predict their behav- the past two decades the field of volcanology has iour and reduce the hazards to people who live undergone revolutionary changes in the under- near them. But volcanoes are not merely destruc- standing of volcanic activity. This has resulted tive and need to be viewed as an integral part from a more quantitative approach to the anal- of the dynamic Earth system. They create new ysis of volcanic processes through the applica- land, replenish soil, and provide essential water tion of physics, chemistry, and fluid dynamics. In and other gases to our oceans and atmosphere. many instances, strides in the field were spurred Much of this book will focus on the relationship on by observations of major eruptions such as of volcanoes to the environment and to mankind. Mount St. Helens, USA, in 1980, El Chichon,´ However, before these topics are addressed we Mexico, in 1982, Nevado del Ruiz, Colombia, need to begin with some fundamental concepts in 1985, and Mt. Pinatubo, Philippines, in 1991 about the causes and processes of volcanism. In (described further in Chapter 2). These eruptions this chapter we explore how volcanoes work by often provided the opportunity to observe vol- examining the complex path that must be taken canic processes that had not previously been doc- before an eruption takes place at the Earth’s sur- umented quantitatively and led to the ability to face. This includes the generation of magma at interpret such processes from deposits in the geo- depth, its rise, storage and evolution within the logic record (this will be discussed further in Earth’s crust, and finally the factors that deter- Chapter 3). With the advent of Earth-observing mine the nature of the eruption at the surface. satellites, the remote sensing of volcanoes from Volcanoes and the Environment, eds. J. Mart´ı and G. G. J. Ernst. Published by Cambridge University Press. C Cambridge University Press 2005. © Cambridge University Press www.cambridge.org Cambridge University Press 0521592542 - Volcanoes and the Environment Edited by Joan Marti and Gerald Ernst Excerpt More information 2 UNDERSTANDING THE PHYSICAL BEHAVIOR OF VOLCANOES space has led to a new appreciation for the global the line continues southward all the way to New impact of some large-scale eruptions and pro- Zealand. This remarkable clustering of volcanoes vided volcanologists with new tools to assess the is known as the ‘‘Ring of Fire.” Large earthquakes relationship of volcanoes to the environment (e.g. are also concentrated along this belt adding to Rose et al., 2000). A particularly exciting break- the hazards associated with the volcanoes. Other through has enabled the mapping of the ocean’s significant belts of subaerial volcanoes occur in bathymetry from satellites by measuring subtle the Indonesian Archipelago, the Mediterranean changes in the elevation of the seasurface, allow- region and in the Lesser Antilles islands of the ing for the recognition and mapping of volcanic West Indies. features on the seafloor with high resolution. As It would take one of the major revolutions a result of land-based, marine, and space obser- in the earth sciences, that of plate tectonics, to vations there is now an excellent documenta- provide the unifying explanation for the distri- tion of the distribution of volcanic activity on bution of volcanoes on the Earth. It has now Earth. been demonstrated that the surface of the Earth consists of numerous large plates that are con- stantly in motion relative to one another and rel- Distribution of volcanic activity: ative to the deep interior of the Earth (Fig. 1.1). the plate tectonic framework The theory of plate tectonics provides the fun- damental linkage between our understanding The distribution of volcanoes on the Earth’s sur- of the Earth’s surface and its interior (Keary face provides important information about the and Vine, 1990; Canon-Tapia and Walker, 2003). A underlying causes of volcanism, yet making a cross-section through the planet reveals a stron- global inventory of volcanoes is not a simple gly layered internal structure (Fig. 1.2). Much of task. Simkin (1993) notes that it is difficult to our knowledge of the Earth’s interior has been define precisely the number of active volcanoes derived from the study of how earthquakes are on Earth. During historic times there have been transmitted in different types of materials. By some 538 with documented eruptions and over studying the arrival of different types of seismic 1300 have erupted during the last 10 000 years. waves, such as P- and S-types, at various positions However, two-thirds of the planet’s surface is cov- around the globe, it is possible to infer the nature ered by water and thus observations of volcanic of the deep Earth. This remote-sensing approach activity are strongly biased towards the conti- is necessary because direct sampling of the deep nents and islands. We now know that virtually all Earth by drilling is usually limited to depths of of the seafloor is composed of volcanic rock and about 5 km. Some deeper samples are brought to that most volcanic activity occurs far removed the surface naturally by volcanic eruptions. These from sight in the depths of the ocean. xenoliths have been critical for determining the Early workers who documented the distribu- chemical composition and physical nature of the tion of volcanoes realized that they did not occur source areas of volcanism. randomly but tended to run in belts or linear From the center of the Earth to a distance segments. This is particularly evident when one of 1228 km lies the inner core, a dense solid looks at the present location of volcanism on consisting mostly of iron with lesser amounts of the Earth’s surface (Fig. 1.1). A large majority of nickel and sulfur. Surrounding the solid inner the Earth’s active and conspicuous volcanoes can core out to a distance of 3500 km is a liquid be found along a belt that rims the Pacific Ocean outer core, also consisting of iron and sulfur. beginning at the southern tip of South America, The presence of a liquid region deep in the proceeding north along its western coast across Earth is inferred from the blocking of earthquake Mexico into the western United States (Fig. 1.1). shear waves (S-waves), which are unable to prop- From there it winds into Alaska, across the agate through a liquid. Within this layer, convec- Aleutian islands to the western Pacific, where tive movement of material is responsible for the © Cambridge University Press www.cambridge.org © CambridgeUniversityPress More information Excerpt Edited byJoanMartiandGeraldErnst 0521592542 -VolcanoesandtheEnvironment Cambridge UniversityPress www.cambridge.org Fig. 1.1. Distribution of active volcanic centers (solid circles) and the boundaries between major lithospheric plates. Arrows indicate directions of relative plate motion. Modified from Sparks et al.(1997). Cambridge University Press 0521592542 - Volcanoes and the Environment Edited by Joan Marti and Gerald Ernst Excerpt More information 4 UNDERSTANDING THE PHYSICAL BEHAVIOR OF VOLCANOES Fig. 1.2. Cross-section of the Earth’s interior showing the major concentric regions. Arrows indicate the motions of the major lithospheric plates. Thickness of the lithosphere is exaggerated for viewing purposes. Modified from Frankel (1996). generation of the Earth’s magnetic field. At a dis- the lithosphere, a combination of crustal rocks tance of 3500 km from the Earth’s center there and upper mantle material (Fig. 1.2). They are of is an abrupt transition to the largest volumet- the order of 100 km thick and are considered ric component of the solid Earth, the mantle. to behave in a generally rigid form. Lithospheric Extending to less than 100 km from the sur- plates ride over an underlying asthenosphere face, the mantle consists of silicate minerals and that consists of mantle rocks at higher temper- is the source of most magma for volcanism on atures and pressures. These conditions allow the the planet. As a result of gradients in pressure asthenosphere material to deform under stress in and temperature throughout the mantle, the a ductile fashion and to accommodate the move- assemblage of minerals changes as a function of ment of the overriding plates. depth. In the lower mantle, dense phases such as The boundaries between plates can be subdi- perovskite, magnesiowustite, and stishovite pre- vided into three main types: divergent, conver- dominate, whereas above 670 km (upper man- gent, and transform (Fig.
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