Encyclopedia of Volcanoes

Haraldur Sigurdsson,Bruce Houghton,Hazel Rymer,John Stix,Steve McNutt

• Verlag: Academic Press
• Jahr: 1999
• ISBN:
  9780080547985
• Thema: Scienze fisiche
• Thema: Sismologia e vulcanismo

Volcanoes are unquestionably one of the most spectacular and awe-inspiring features of the physical world. Our paradoxical fascination with them stems from their majestic beauty and powerful, if sometimes deadly, destructiveness. Notwithstanding the tremendous advances in volcanology since ancient times, some of the mystery surrounding volcanic eruptions remains today. The Encyclopedia of Volcanoes summarizes our present knowledge of volcanoes. Through its thematic organization around the melting of the earth, it provides a comprehensive source of information on the multidisciplinary influences of volcanic eruptions--both the destructive as well as the beneficial aspects.The majority of the chapters focus on the geoscience-related aspects of volcanism (radioactive heat source, melting rock, ascent of magma, surface phenomena associated with exiting magma, extraterrestrial volcanism, etc.). In addition, complementary chapters discuss the multidisciplinary aspects of volcanism; these include the history of volcanology, geothermal energy resources, interaction with the oceans and atmosphere, health aspects of volcanism, mitigation of volcanic disasters, post-eruption ecology, and the impact of eruptions on organismal biodiversity. In addition to its appeal to educators, students, and professional and amateur scientists, the Encyclopedia of Volcanoes functions as an important information resource for administrators and officials responsible for developing and implementing volcanic hazard mitigation around the world.* The first and only reference work to cover all aspects of volcanology* More than 80 separate peer-reviewed articles--all original contributions by leading authors from major institutions of science around the world, commissioned for this work* An integrated transition from the volcanic process through hazards, risk, and societal impacts, with an emphasis on how volcanoes have influenced and shaped society* Convenient single-volume format with topics arranged thematically--articles provide coverage of nine different aspects of volcanology* Each entry in the Encyclopedia begins with an outline of the article content and a concise definition of the subject of the article* 3, 000 Glossary entries explain key terms* Further Reading lists appear at the end of each entry* Extensive cross-referencing system links related articles* Sixteen pages of color will convey the science and excitement of this often violent phenomena * Large 8 1/2" x 11" page size, easy-to-read double-column format

INTRODUCTION

HARALDUR. SIGURDSSON

University of Rhode Island

I. The Melt of the Earth

II. Volcanism and Plate Tectonics

III. The Midocean Ridge

IV. The Subduction Factory

V. Volcanoes on Hot Spots

VI. Volcanism on Other Worlds

VII. Volcanic Processes

VIII. The Impact of Volcanism

GLOSSARY

convection The process by which the Earth’s mantle loses heat, producing currents of solid but deformable rock that rise toward the surface and contribute to plate motion.

eclogite A rock type common in the Earth’s mantle, with the same chemical composition as basalt, but with a mineral assemblage that is stable at high pressure.

intensity The rate of flow of magma out of a volcano during eruption, expressed as mass eruption rate (MER) in kilograms per second (kg/s).

magnitude The size of a volcanic eruption, expressed as the volume of material erupted, usually in cubic kilometers (km³).

peridotite The principal rock that forms the Earth’s upper mantle, consisting mainly of the mineral olivine, with lesser amounts of pyroxene, garnet, and/or spinel. It is the source of basaltic magmas, which are formed from peridotite by partial melting.

photodissociation Chemical reactions that occur in the atmosphere, due to ultraviolet radiation.

proterozoic eon The period in Earth’s history that began 2.5 billion years ago and ended 0.57 billion years ago.

pyroclasts Fragmentary material ejected during a volcanic eruption, including pumice, ash, and rock fragments.

radionuclides Chemical elements that undergo spontaneous and time-dependent decay, resulting in the formation of other elements and the release of radioactive energy in the form of heat.

stromatolites The earliest calcium carbonate-secreting organisms on Earth, first appearing during the Proterozoic. Their activity contributed to drawing down carbon dioxide from the atmosphere and fixing it in limestone and other sedimentary rocks, contributing to changing atmospheric chemistry and climate.

subduction The process of sinking of crustal plates into the Earth’s mantle. Subduction causes magma generation and results in buildup of island arcs above subduction zones.

volatiles Those chemical compounds or elements contained in magmas that are generally released as gases to the atmosphere during a volcanic eruption. They include water, carbon dioxide, and sulfur dioxide. They are generally dissolved in the magma prior to eruption or when the magma is under high pressure, but exsolve during ascent of the magma to the surface.

volcanic aerosol Tiny particles of sulfuric acid droplets, formed by reactions between volcanic sulfur dioxide gas and water vapor in the stratosphere. The resulting aerosol dust veil has important effects on backscattering and absorption of solar radiation and leads to a net surface cooling on the Earth, with possible climate change.

I THE MELT OF THE EARTH

Volcanic eruptions are the most awesome and powerful display of nature’s force. The idea that terra firma may explode under our feet and bombard us with glowing hot ejecta seems almost incomprehensible. Every year about 50 volcanoes throughout the world are active above sea level, threatening the lives and property of millions of people. A single eruption can claim thousands of lives in an instant. For example, in the 1902 eruption of Mont Pelée on the Caribbean island of Martinique, a flow of hot ash and gases overwhelmed the city of St. Pierre, killing all but one of its 28,000 inhabitants. More recently, a mudflow triggered by the 1985 eruption of the volcano Nevado del Ruiz in Colombia killed nearly all of the 25,000 inhabitants of the town of Armero.

The relationship between people and volcanoes is as old as the human race. Our earliest ancestors evolved in the volcanic region of the East African Rift, where their activities and remains are preserved by volcanic deposits, such as the stunning 3.7-million-year-old Australopithecus hominid footprints crisscrossing a volcanic ash deposit at Laetoli. In fact, the wealth of information we now have on early hominid evolution has only been made possible because of the rapid burial and excellent preservation of their remains in volcanic deposits. Is it a mere sport of nature that humans evolved in a volcanic region, or was this African volcanic environment especially favorable to human evolution? Was it the abundant game and fertility of the volcanic plains, with their rich soils? We shall probably never know the answer, but humans quickly learned to make use of volcanic rocks in toolmaking and captured fire from volcanoes, to open up the realms of the dark and the cold for further expansion of the race.

When humans first sought explanations for volcanic phenomena, they linked these violent processes to mythology and religion. But with the rise of Western philosophy and learning, Greek scholars in the third century before Christ began the search for the actual physical causes of volcanism, as outlined in Chapter 2.

The Greeks speculated that eruptions were the result of the escape of highly compressed air and gases inside the Earth, but later the Romans proposed that volcanoes were natural furnaces in which combustion of sulfur, bitumen, and coal took place. This search for the actual causes of volcanism on Earth and other planets has continued to our day, but we now have many of the answers.

Volcanology is the study of the origin and ascent of magma through the planet’s mantle and crust and its eruption at the surface. Volcanology deals with the physical and chemical evolution of magmas, their transport and eruption, and the formation of volcanic deposits at the planetary surface. Some volcanic processes constitute a major natural hazard, whereas others are highly beneficial to society. Thus, the study of volcanism has far-ranging significance for society. For most people, volcanology conjures up a picture of an erupting volcano. Volcanoes and their eruptions, however, are merely the surface manifestation of the magmatic processes operating at depth in the Earth, and thus the study of the volcanism is inevitably highly interdisciplinary, most closely linked to geophysics, petrology, and geochemistry.

In this volume, we examine volcanology in the widest sense, covering not only the traditional aspects of the generation of magmas (traditionally the domain of petrology and geochemistry) and their transport and eruption (the field of traditional volcanology), but also the multitude of effects that volcanism has on the environment and on our society and culture. Volcanism is the best way to probe the interior of Earth. Adopting an anthropomorphic view, we could regard magma as the sweat of the Earth, resulting from the labors of moving the great crustal plates around on the planet’s surface and maintaining the Earth’s mantle well stirred. The analogy is not that far fetched, because magma and volcanism in general are also a means of heat loss for the Earth. For the Earth scientist, these fluids emerging from the Earth carry valuable clues about the internal constitution of our planet. Just as the medical doctor analyzes the various fluids of the human body, the geochemist samples and analyzes the magmatic liquids that issue from Earth’s volcanoes.

For the human race, the Earth is a blessed planet, because of its position in the solar system and because of its physical dimensions and vigorous dynamic internal processes, among which volcanism plays a fundamental role. Our Earth is just sufficiently far away from the Sun to benefit from its heat, but not so close as to lose its crucial oceans by rapid evaporation or its precious water by photodissociation at the top of the atmosphere and subsequent escape to space. The Earth is cool enough that liquid water stays on its surface—but not so cool that all water freezes. Earth’s gravity is sufficiently high to exceed the escape velocity of water and carbon dioxide molecules, allowing it to retain a unique atmospheric composition, with enough carbon dioxide to create a comfortable greenhouse and to provide the building blocks for life, as well as to shield us against harmful solar ultraviolet radiation. Earth’s internal heat reservoir is not so hot that it makes life unbearable because of continuous volcanic eruptions, but is sufficient to drive mantle convection and plate tectonics. The volcanism resulting from plate tectonics continuously recycles volatile elements such as water, carbon dioxide, and sulfur between the inner Earth and its surface reservoirs: the oceans and atmosphere. It is a very ancient cycle. In the distant history of the primitive Earth, the original atmosphere and oceans probably resulted from the early degassing of the interior of the globe, largely through volcanic activity.

Volcanism is flux of energy and matter. It is an expression of the storehouse of Earth’s inner energy, derived in part from cooling of an originally hot planet and in part from heat resulting from the radioactive decay of naturally occurring uranium, potassium, thorium, and other radionuclides present deep in the Earth. Thus, volcanic eruptions are the surface expression of these deep Earth processes. When viewed on a geologic timescale, the motions of the inner Earth are veritable storms raging within the planet, with thunderheads rising up through the mantle to form plumes that break the surface as great volcanic hot spots. Other internal storms also lead to convective rollover of the mantle, pulling and pushing along the great crustal plates at the surface, resulting in volcanic activity where plates converge or are pulled apart. When we compare Earth to the other planets in the solar system, we may wonder why our home planet is so prone to volcanic eruptions. The logical question is, however, why does the Earth have such vigorous plate tectonics, the driving force of volcanism?

II VOLCANISM AND PLATE TECTONICS

Earth may be unique among the planets in the solar system in that its outer rigid skin—the crust and lithosphere—is continuously being destroyed and regenerated. It has active plate tectonics, where the heat and smoke of volcanism rise from the two main battlefields between the plates: the rifts or ocean ridges and the subduction zones. The most important consequence of plate tectonics, discussed in Chapter 6, is geologic recycling of materials, turning the Earth into an immense chemical factory where volcanism plays a crucial role. As great crustal plates are pulled apart in the ocean basins, the solid but mobile Earth’s mantle below the rift responds to the decrease in overburden and rises upward to fill in the rift. When the rising peridotite mantle experiences a decrease in pressure, it spontaneously undergoes melting, without addition of heat. The reader who does not have a background in petrology or volcanology should pause at this point and ponder the last sentence, for here lies the key to an understanding of the vast majority of volcanic processes: decompression melting, as described further in Chapter 4. It may be difficult at first to comprehend that rock may melt, without addition of heat, simply because the pressure acting on it decreases, but this is the most common mel...