Encyclopedia of Earth Science PDF

www.ebook3000.com encyclopedia of Earth Science www.ebook3000.com www.ebook3000.com ENCYCLOPEDIA OF Earth Science Timothy Kusky, PH.D. Department of Earth and Atmospheric Sciences, Saint Louis University www.ebook3000.com Encyclopedia of Earth Copyright © 2005 by Timothy All rights reserved. No part of this book may be reproduced means, electronic or mechanical, including photocopying, storage or retrieval systems, without the publisher. For information Facts On File, Inc. 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Kusky, Timothy M. Encyclopedia of earth science p. cm. Includes bibliographical references ISBN 0-8160-4973-4 1. Earth sciences—Encyclopedias. QE5.K85 2004 550′.3—dc22 2004004389 Facts On File books are available at special discounts for businesses, associations, institutions, Special Sales Department in New York at 212/967-8800 You can find Facts On File on the World Wide Text design by Joan Cover design by Cathy Illustrations by Richard Garratt Printed in the United States VB Hermitage 10 9 8 7 This book is printed on www.ebook3000.com Dedicated to G.V. Rao (1934–2004) www.ebook3000.com www.ebook3000.com CONTENTS Acknowledgments xi Introduction xiii Entries A–Z 1 Feature Essays: “Coping with Sea-Level Rise in Coastal Cities” 81 “Gaia Hypothesis” 82 “Desertification and Climate Change” 116 “Earthquake Warning Systems” 131 “Loma Prieta Earthquake, 1989” 132 “Mississippi River Basin and the Midwest Floods of 1927 and 1993” 152 www.ebook3000.com “Age of the Earth” 166 “Formation of the Earth and Solar System” 203 “Galveston Island Hurricane, 1900” 214 “Is There Life on Mars?” 262 “Lahars of Nevado del Ruiz, Colombia, 1985” 266 “History of Ocean Exploration” 302 “The World’s Oldest Ophiolite” 307 “Homo sapiens sapiens and Neandertal Migration and in the Ice Ages” 338 “Is Your Home Safe from Radon?” 350 “Why Is Seawater Blue?” 379 “Seismology and Earth’s Internal Structure” 383 “December 26, 2004: Indian Ocean Earthquake and 437 “Volcanoes and Plate Tectonics” 452 www.ebook3000.com Appendixes: Appendix I Periodic Table of the Elements 477 Appendix II The Geologic Timescale 479 Classification of Species 480 Summary of Solar System Data 480 Evolution of Life and the Atmosphere 480 Index 481 ACKNOWLEDGMENTS Many people have helped me with different aspects Frank Darmstadt, Executive Editor at Facts On File, and figures in the encyclopedia, providing guidance Rose Ganley spent numerous hours as editorial assistant sions of the text and helping prepare figures, tables, assistance in the preparation was provided by Soko Angela Bond. Many sections of the encyclopedia draw doing scientific research in different parts of individually thank the hundreds of colleagues whose have related in this book. Their contributions to ing of this volume are greatly appreciated. I have vant works, or in some cases more recent sources reference lists. Any omissions are unintentional. Finally, I would especially like to thank Carolyn, Shoshana and Daniel for their patience during the preparing this book. Without their understanding possible. xi INTRODUCTION The Encyclopedia of Earth Science is intended to provide the most important subjects in the field of earth encyclopedia include longer entries on the many broad sciences (hydrology, structural geology, petrology, geomorphology, atmospheric sciences, climate, and entries on concepts, theories and hypotheses, places, geological time, history, people who have made significant field, technology and instruments, organizations, The Encyclopedia of Earth Science is intended school and college students, teachers and professors, nalists, general readers, and specialists looking cialty. The encyclopedia is extensively illustrated illustrations including line art, graphs, and tables, on topics of interest to society. The work is extensively indexed to facilitate locating topics of interest. Entries in the Encyclopedia of Earth Science and review of the scientific literature, ranging from specialized fields. Most of the entries include important sources listed as “Further Reading” at the end extensively cross-referenced with related entries. draw from my collected field notes, class notes, and about selected topics and regions, and I have tried coverage of most topics at a similar level. Some of ever, go into deeper levels on topics considered to xiii ENTRIES A–Z A aa lava Basaltic lava flows with blocky broken surfaces. The term is of Hawaiian origin, its name originating from the sound that a person typically makes when attempting to walk across the lava flow in bare feet. Aa lava flows are typically 10–33 feet (3–10 m) thick and move slowly downhill out of the volcanic vent or fissure, moving a few meters per hour. The rough, broken, blocky surface forms as the outer layer of the moving flow cools, and the interior of the flow remains hot and fluid and continues to move downhill. The movement of the interior of the flow breaks apart the cool, rigid surface, causing it to become a jumbled mass of blocks with angular steps between adjacent blocks. The flow front is typically very steep and may advance into new areas by dropping a continu- ous supply of recently formed hot, angular blocks in front of the flow, with the internal parts of the flow slowly overriding the mass of broken blocks. These aa lava fronts are rather noisy places, with steam and gas bubbles rising through the hot magma and a continuous clinking of cooled lava blocks rolling down the lava front. Gaps that open in the lava front, top, and sides may temporarily expose the molten lava within, showing the high temperatures inside the flow. Aa flows are therefore hazardous to property and may bulldoze buildings, forests, or anything in their path, and then cause them to burst into flames as the hot magma comes into contact with combustible material. Since these flows move so slowly, they are not considered hazardous to humans. See also PAHOEHOE LAVA; VOLCANO. abyssal plains Flat, generally featureless plains that form large areas on the seafloor. In the Atlantic Ocean, abyssal plains form large regions on either side of the Mid-Atlantic Ridge, covering the regions from about 435–620 miles (700–1,000 km), and they are broken occasionally by hills and volcanic islands such as the Bermuda platform, Cape 2 accretionary wedge numerous thrust faults, whereas other parts or other wedges are characterized by relatively large semi-coherent or folded packages of rocks. They also host rocks known as tectonic mélanges that are complex mixtures of blocks and thrust slices of many rock types (such as graywacke, basalt, chert, and limestone) typically encased in a matrix of a different rock type (such as shale or serpentinite). Some accretionary wedges contain small blocks or layers of high-pressure low- temperature metamorphic rocks (known as blueschists) that have formed deep within the wedge where pressures are high and temperatures are low because of the insulating effect of the cold subducting plate. These high-pressure rocks were brought to the surface by structural processes. Accretionary wedges grow by the progressive offscraping of material from the trench and subducting plate, which con- stantly pushes new material in front of and under the wedge as plate tectonics drives plate convergence. The type and style of material that is offscraped and incorporated into the wedge depends on the type of material near the surface on the subducting plate. Subducting plates with thin veneers of sediment on their surface yield packages in the accretionary Cross section of typical accretionary wedge showing collapse by normal faulting. Either of these two processes can reduce the slope of the wedge and lead it to become more sta- ble. In addition to finding the evidence for thrust faulting in accretionary wedges, structural geologists have documented many examples of normal faults where the tops of the wedges have collapsed, supporting models of extensional collapse of oversteepened wedges. Accretionary wedges are forming above nearly every subduction zone on the planet. However, these accretionary wedges presently border open oceans that have not yet closed by plate tectonic processes. Eventually, the movements of the plates and continents will cause the accretionary wedges to become involved in plate collisions that will dramatically change the character of the accretionary wedges. They are typically overprinted by additional shortening, faulting, fold- ing, and high-temperature metamorphism, and intruded by magmas related to arcs and collisions. These later events, coupled with the initial complexity and variety, make identifi- cation of accretionary wedges in ancient mountain belts diffi- cult, and prone to uncertainty. See also CONVERGENT PLATE MARGIN PROCESSES; MÉLANGE; PLATE TECTONICS; STRUCTURAL GEOLOGY. Further Reading Kusky, Timothy M., and Dwight C. Bradley. “Kinematics of Mélange Fabrics: Examples and Applications from the McHugh Kenai Peninsula, Alaska.” Journal of Structural 12 (1999): 1,773–1,796. Kusky, Timothy M., Dwight C. Bradley, Peter Haeussler, M. Karl. “Controls on Accretion of Flysch and Mélange Belts at Convergent Margins: Evidence from The Chugach Bay Thrust and Iceworm Mélange, Chugach Terrane, Alaska.” Tectonics 16, no. 6 (1997): 855–878. Adirondack Mountains The Adirondack Mountains occu- py the core of a domal structure that brings deep-seated Late Proterozoic rocks to the surface and represents a southern extension of the Grenville province of Canada. The Late Pro- terozoic rocks are unconformably overlain by the Upper Cam- brian/Lower Ordovician Potsdam Sandstone, dipping away from the Adirondack dome. The late Cenozoic uplift is shown by the anomalous elevations of the Adirondack Highlands compared with the surrounding regions and the relatively young (Tertiary) drainage patterns. Uplift is still occurring on the order of few millimeters per year. Five periods of intrusion and two main periods of defor- mation are recognized in the Adirondacks. The earliest intru- sions are the tonalitic and calc-alkaline intrusions that are approximately 1,350–1,250 million years old. These intrusions were followed by the Elzevirian deformation at approximately 1,210–1,160 million years ago. The largest and most signifi- cant magmatic event was the emplacement of the anorthosites, mangerites, charnockites, and granites, commonly referred to as the AMCG suite. This suite is thought to have been intruded 4 Adirondack Mountains Structural map showing axial traces of folds in the synform; OD: Oregon Dome; SD: Snowy Mountain Dome; charnockites, granitic gneisses. Between the Marcy massif and the Carthage-Colton mylonite zone is an area known as the Central Highlands. Here, the rock types consist of AMCG rocks and hornblende gneisses, both of which exhibit variable these metasedimentary rocks may be correlative with rocks in the Northwest Lowlands. The following sections briefly review the important Highland suites. TONALITIC SUITE The tonalitic suite outcrops in the extreme southern Adirondacks where they are highly deformed. These tonalitic rocks are one of the oldest suites in the Adirondacks and have been dated at circa 1.3 billion years. The tonalitic gneiss is thought to be igneous in origin based on the presence of xenoliths from the surrounding rock and the subophitic tex- tures. Strong calc-alkaline trends suggest that these rocks are arc-related; however, this geochemical signature does not dif- ferentiate between an island-arc and an Andean arc-type set- ting. This suite may be correlative with tonalitic rocks in the Green Mountains of Vermont based on age relations and pet- rographic features. They are also similar in composition with the somewhat younger Elzevirian batholith (1.27–1.23 billion years old) in the central metasedimentary belt. Consequently, the tonalitic suite in the Adirondacks is thought to have been emplaced in the early intraoceanic history of the Elzevirian arc, prior to collision at circa 1,200 million years ago. AMCG SUITE The circa 1,555–1,125-million-year-old AMCG suite occurs predominantly in the Adirondack High- lands and central granulite terrain of the Canadian Grenville province. Though highly deformed, the AMCG suite has been characterized as igneous in origin based on the presence of relict igneous textures. Several geologists, pioneered by Jim McLelland, have suggested that the post-collisional delamina- tion of the subcontinental lithospheric mantle generated gab- broic melts that ponded at the mantle-crust boundary. This ponding would have provided a significant source of heat, thereby affecting the lower crust in two ways: it created melts in the lower crust, thus producing a second generation of more felsic magma. This model is supported by the bimodal nature of the AMCG suite. The second effect was weakening of the crust, which provided a conduit for the hot, less dense magmas to ascend to the surface. This hypothetical emplace- ment model is supported by the AMCG suite’s anhydrous nature in conjunction with the shallow crustal levels the magma has invaded. Large-Scale Structural Features The structure of the Adirondack Mountains has puzzled geol- ogists for decades. This is due to the polyphase deformation that complexly deformed the region during the Ottawan orogeny (1.1–1.0 billion years ago). In 1936 J. one of the first investigators who recognized that the stratig- raphy of the Northwest Lowlands is repeated by a series of folds. Later workers, including Ynvar Isachsen, suggested that there are five sets of large-scale folds that occur through- out the Adirondacks. In addition, rocks of the central and 6 Afar Depression, Ethiopia Corrigan, Dave, and Simon Hanmer. “Anorthosites and Related Granitoids in the Grenville Orogen: A Product of the Convective Thinning of the Lithosphere?” Geology 25 (1997): 61–64. Davidson, Anthony. “Post-collisional A-type Plutonism, Southwest Grenville province: Evidence for a Compressional Geological Society of America Abstracts with Programs 28 (1996): 440. ———. “An Overview of Grenville province Geology, Shield.” In “Geology of the Precambrian Superior and Grenville provinces and Precambrian Fossils in North America,” edited by S. B. Lucas and Marc R. St-Onge. Geological Society of America, Geology of North America C-1 (1998): 205–270. Hoffman, Paul F. “Did the Breakout of Laurentia Turn Gondwana- land Inside-Out?” Science 252 (1991): 1,409–1,411. Kusky, Timothy M., and Dave P. Loring. “Structural and U/Pb Chronology of Superimposed Folds, Adirondack Mountains: Implications for the Tectonic Evolution of the Grenville province.” Journal of Geodynamics 32 (2001): McLelland, Jim M., J. Stephen Daly, and Jonathan M. “The Grenville Orogenic Cycle (ca. 1350–1000 Ma): an Adiron- dack perspective.” In Tectonic Setting and Terrane Accretion in Precambrian Orogens, edited by Timothy M. Kusky, Ben A. van der Pluijm, Kent C. Condie, and Peter J. Coney. Tectonophysics 265 (1996): 1–28. McLelland, Jim M., and Ynvar W. Isachsen. “Synthesis of Geology of the Adirondack Mountains, New York, And Their Tectonic Set- ting within the Southwestern Grenville province.” In The Grenville province, edited by J. M. Moore, A. Davidson, J. Baer. Geological Association of Canada Special (1986): 75–94. ———. “Structural Synthesis of the Southern and Central Adiron- dacks: A Model for the Adirondacks as a Whole and Plate Tec- tonics Interpretations.” Geological Society of America Bulletin 91 (1980): 208–292. Moores, Eldredge M. “Southwest United States-East Antarctic (SWEAT) Connection: A Hypothesis.” Geology 19 (1991): 425–428. Rivers, Toby. “Lithotectonic Elements of the Grenville province: Review and Tectonic Implications.” Precambrian (1997): 117–154. Rivers, Toby, and Dave Corrigan. “Convergent Margin ern Laurentia during the Mesoproterozoic: Tectonic tions.” Canadian Journal of Earth Sciences 37 (2000): 359–383. Rivers, Toby, J. Martipole, Charles F. Gower, and Anthony David- son. “New Tectonic Subdivisions of the Grenville province, Southeast Canadian Shield.” Tectonics 8 (1989): 63–84. Afar Depression, Ethiopia One of the world’s largest, deepest regions below sea level that is subaerially the continents, home to some of the earliest known hominid fossils. It is a hot, arid region, where the Awash River drains northward out of the East African rift system, and is evaporat- ed in Lake Abhe before it reaches the sea. It is located Africa in Ethiopia and Eritrea, between Sudan and Somalia, and across the Red Sea and Gulf of Aden from Yemen. The reason the region is so topographically low is that it is located at a tec- tonic triple junction, where three main plates are spreading Landsat Thematic Mapper image of the area where the rift, and the Goba’ad rift, an extension of the drainages like the Awash River terminate in lakes with variable amounts of water molecules. Opal is typically iridescent, displaying changes in color when viewed in differ- ent light or from different angles. Agate and opal typically form colorful patterns including bands, clouds, or dendritic patterns indicating that they grew together from sil- ica-rich fluids. Agate is found in vugs in volcanic rocks and is 8 Aleutian Islands and trench moving air molecules bumping into each other and objects in the atmosphere. The air molecules in the atmosphere are con- stantly moving and bumping into each other with each air molecule averaging a remarkable 10 billion collisions per sec- ond with other air molecules near the Earth’s surface. The density of air molecules is highest near the surface, decreases rapidly upward in the lower 62 miles (100 km) of the atmo- sphere, then decreases slowly upward to above 310 miles (500 km). Air molecules are pulled toward the Earth by gravity and are therefore more abundant closer to the surface. Pressure, including air pressure, is measured as the force divided by the area over which it acts. The air pressure is greatest near the Earth’s surface and decreases with height, because there is a greater number of air molecules near the Earth’s surface (the air pressure represents the sum of the total mass of air above a certain point). A one-square-inch column of air extending from sea level to the top of the atmosphere weighs about 14.7 pounds. The typical air pressure at sea level is therefore 14.7 pounds per square inch. It is commonly measured in units of millibars (mb) or hectopascals (hPa), and also in inches of mercury. Standard air pressure in these units equals mb, 1,013.25 hPa, and 29.92 in of mercury. Air pressure is equal in all directions, unlike some pressures (such as a weight on one’s head) that act in one direction. This explains why objects and people are not crushed or deformed by the pres- sure of the overlying atmosphere. Air pressure also changes in response to temperature and density, as expressed by the gas law: Pressure = temperature × density × constant equal to 2.87 × 106 erg/g K). From this gas law, it is apparent that at the same temper- ature, air at a higher pressure is denser than air at a lower pressure. Therefore, high-pressure regions of the atmosphere are characterized by denser air, with more molecules of air than areas of low pressure. These pressure changes are caused by wind that moves air molecules into and out of a region. When more air molecules move into an area than move out, the area is called an area of net convergence. Conversely, in areas of low pressure, more air molecules are moving out than in, and the area is one of divergence. If the air density is con- stant and the temperature changes, the gas law states that at a given atmospheric level, as the temperature increases, the air pressure decreases. Using these relationships, if either the tem- perature or pressure is known, the other can be calculated. If the air above a location is heated, it will expand and rise; if air is cooled, it will contract, become denser, and sink closer to the surface. Therefore, the air pressure decreases rapidly with height in the cold column of air because the molecules are packed closely to the surface. In the warm col- umn of air, the air pressure will be higher at any in the cold column of air, because the air has expanded and more of the original air molecules are above the specific point source on a mountainside. They represent erosional- depositional systems in which rock material is eroded from mountains and carried by rivers to the foot of the where it is deposited in the alluvial fans. The apex vial fan is the point source from which the river system emerges from the mountains and typically breaks into smaller distributaries forming a braided stream network frequently shifts in position on the fan, evenly distributi alluvial gravels across the fan with time. The shape fans depends on many factors, including tectonic uplift and subsidence, and climatic influences that change the river load-discharge balance. If the discharge decreases time, the river may downcut through part of the fan and emerge partway through the fan surface as a point-source a new cone. This type of morphology also develops in places where the basin is being uplifted relative to the mountains. In places where the mountains are being uplifted relative to the basin containing the fan, the alluvial fan typically several, progressively steeper surfaces toward the many places, several alluvial fans merge together at the foot of a mountain and form a continuous depositional surface known as a bajada, alluvial apron, or alluvial slope. The surface slope of alluvial fans may be as steep as 10° near the fan apex and typically decreases in the down-fan direction toward the toe of the fan. Most fans have a concave upward profile. The slope of the fan at the apex is typically the same as that of the river emerging from the mountains, showing that deposition on the fans is not controlled by a sudden decrease in gradient along the river profile. Alluvial fans that form at the outlets of large drainage basins are larger than alluvial fans that form at the outlets of smaller drainage basins. The exact relationships between fan size and drainage basin size is dependent on time, climate, type of rocks in the source terrain in the drainage basin, structure, slope, tectonic setting, and the space the fan to grow into. Alluvial fans are common sights along mountain fronts in arid environments but also form in all other types of cli- matic conditions. Flow on the fans is typically confined single or a few active channels on one part of the fan, and shifts to other parts of the fan in flood events in humid envi- ronments or in response to the rare flow events in arid envi- ronments. Deposition on the fans is initiated when the flow leaves the confines of the channel, and the flow velocity and depth decrease dramatically. Deposition on the fans may also be induced by water seeping into the porous gravel and sand on the fan surface, which has the effect of decreasing the flow discharge, initiating deposition. In arid environments it is common for the entire flow to seep into the porous fan before it reaches the toe of the fan. The sedimentary deposits on alluvial fans include fluvial gravels, sands, and overbank muds, as well as debris flow and mudflow deposits on many fans. The debris flows are 10 altimeter but the rocks record a longer history of deformation and events extending back at least into the Mesozoic. Closure of the Tethys Ocean was complex, involving contraction of the older Permian-Triassic Paleo-Tethys Ocean at the same time that a younger arm of the ocean, the Neo-Tethys, was opening in Triassic and younger times. In the late Triassic, carbonate platforms covered older evaporites, and these platforms began foundering and were buried under deepwater pelagic shales and cherts in the early Jurassic. Cretaceous flysch covered convergent margin foreland basins, along with felsic magma- tism and high-grade blueschist facies metamorphism. Conti- nent-continent collision-related events dominate the Eocene-Oligocene, with the formation of giant nappes, thrusts, and deposition of syn-orogenic flysch. Late Tertiary events are dominated by late orogenic uplift, erosion, and deposition of post-orogenic molasse in foreland basins. Defor- mation continues, mostly related to post-collisional extension. See also CONVERGENT PLATE MARGIN PROCESSES; PLATE TECTONICS; STRUCTURAL GEOLOGY. altimeter An instrument, typically an aneroid barometer, that is used for determining the elevation or height above sea level. Aneroid barometer style altimeters operate by precisely measuring the change in atmospheric pressure, that decreases with increasing height above sea level, since there is less air exerting pressure at a point at higher elevations than at lower elevations. Altimeters need to be calibrated each day at a known elevation, to account for weather-related changes in atmospheric pressure. Before 1928 there was no possible way for pilots to know how far above the ground they were. The German inventor Paul Kollsman invented the first reliable and accurate barometric altimeter. The altimeter measured altitude by baro- metric pressure. Pilots still use the barometric altimeter today. In 1924 Lloyd Espenschied invented the first radio altimeter. In 1938 Bell Labs demonstrated the first radio altimeter. A radio altimeter uses radio signals that bounce off of the ground and back to the receiver in the plane showing pilots the altitude of the aircraft. A radar altimeter works much in the same way except it bounces the signal off of an object in the air thus telling the height of the object above the ground. A laser altimeter can measure the distance from a spacecraft or satellite to a fixed position on Earth. The mea- surement when compiled with radial orbit knowledge can provide the topography of the Earth. Altiplano A large, uplifted plateau in the Bolivian and Peruvian Andes of South America. The plateau has an area of about 65,536 square miles (170,000 km2), and an average elevation of 12,000 feet (3,660 m) above sea level. The Alti- plano is a sedimentary basin caught between the mountain ranges of the Cordillera Oriental on the east and the Cordillera Occidental on the west. Lake Titicaca, the largest la HOOCCH2CH2COOH), and has highly variable C:H:O ratios. Amber of Oligocene age seems particularly abundant, although it is known from as old as the Cretaceous and includes all ages since sap-producing trees have proliferated on Earth. Many species of fossil insects and plants have been iden- tified in amber, particularly from the spectacular amber deposits found along the southeastern shores of the Baltic Sea. There, yellow, brown, orange, and even blue amber is rich in contained fossils, though most of the amber was mined by the end of Roman times. Amber has retained a sort of mystical quality since early times, probably because it has some unusual properties. Amber stays warm whereas miner- als often feel cool to the touch, and amber burns giving off a scent of pine sap (from which it is derived). Even more astounding to early people was that when rubbed against wool or silk, amber becomes electrically charged and gives off sparks. This feature led the early Greeks to call amber “electron.” Many theories were advanced for the origin of amber, ranging from tears of gods to solidified sunshine. The origin of amber was first appreciated by Pliny the Elder, who, in his famous work Historia Naturalis (published in C.E. 77), suggested that amber is derived from plants. The Romans mined the amber deposits of the Baltic Sea because they thought amber had medicinal qualities that enabled it to ward off fever, tonsillitis, ear infections, and poor eyesight. Decorative amber has been used for burial rituals and to ward off evil spirits for thousands of years, and in Europe it has been found in graves as old as 10,000 years. Amber was widely transported on the ancient silk roads and in ancient Europe, where figurines, beads, and other decorative items were among the most valuable items in the markets. Further Reading Zahl, P. A. “Golden Window on the Past.” National Geographic 152, no. 3 (1977): 423–435. American Geological Institute (AGI) The American Geo- logical Institute (http://www.agiweb.org) was founded in 1948. It plays a major role in strengthening geoscience educa- tion and increasing public awareness of the vital role that geo- sciences play in society. AGI supports its programs and initiatives through sales of its publications and services, royal- ties, contracts, grants, contributions, and affiliated society dues. AGI’s staff provides professional and informational ser- vices related to government affairs; earth-science education, outreach, human resources, and scholarships; the bibliograph- ic database GeoRef and its Document Delivery Service; and the monthly newsmagazine Geotimes and other publications. The Member Society Council meets twice a year in conjunc- tion with the annual meetings of the American Association of Petroleum Geologists and the Geological Society of America. 12 Andes igneous environments where water is available. Amphibole is best known, however, as a metamorphic mineral indicative of medium grade pressure-temperature metamorphism of mafic rocks. When basalt, gabbro, or similar rocks are heated to 930°F–1,300°F (500°C–700°C) at 3–10 kilobars pressure (equivalent to 6 to 20-mile or 10 to 30-km depth), the prima- ry mineral assemblage will commonly turn to an assemblage of amphibole+plagioclase feldspar. Many field geologists will call such a rock an “amphibolite,” although this term should be reserved for a description of the metamorphic conditions (known as facies) at which these rocks formed. See also MINERALOGY. Andes A 5,000-mile (8,000-km) long mountain range in western South America, running generally parallel to the coast, between the Caribbean coast of Venezuela in the north and Tierra del Fuego in the south. The mountains merge with ranges in Central America and the West Indies in the north, and with ranges in the Falklands and Antarctica in the south. Many snow-covered peaks rise more than 22,000 feet (6,000 m), making the Andes the second largest mountain belt world, after the Himalayan chain. The highest range in the Andes is the Aconcagua on the central and northern Argen- tine-Chile border. The high cold Atacama desert is located in the northern Chile sub-Andean range, and the high Altiplano Plateau is situated along the great bend in the Andes in Bolivia and Peru. The southern part of South America consists of a series of different terranes added to the margin of Gondwana in the late Proterozoic and early Proterozoic. Subduction and the accretion of oceanic terranes continued through the Paleozoic, forming a 155-mile (250-km) wide accretionary wedge. The Andes formed as a continental margin volcanic arc system on the older accreted terranes, formed above a complex system of subducting plates from the Pacific Ocean. They are geological- ly young, having been uplifted mainly in the Cretaceous and Tertiary, with active volcanism, uplift, and earthquakes. specific nature of volcanism, plutonism, earthquakes, and uplift is found to be strongly segmented in the Andes, and related to the nature of the subducting part of the plate, including its dip and age. Regions above places where ducting plate dips more than 30 degrees have active volcan- ism, whereas regions above places where the subduction zone is sub-horizontal do not have active volcanoes. See also CONVERGENT PLATE MARGIN PROCESSES; PLATE TECTONICS. andesite A fine-grained, dark-colored intermediate vol- canic rock, andesite typically has phenocrysts of plagioclase, and biotite, hornblende, or pyroxene. It has 56–63 percent silica, although basaltic andesites contents down to 52 percent have a composition that is tran- sitional with basalts. Andesite is the extrusive equivalent of for the formation of oil. Oil and gas tend to migrate upward in geologic structures, and if they find a layer with significant porosity and permeability, the oil and gas may become trapped in the anticlinal structure. The broader and gentler the upwarp, the larger the area that the hydrocarbons may be trapped in, and the larger the oil or gas field. Some famous oil and gas fields that are located in anticlinal structures include those of the Newport-Inglewood trend in California and the Zagros Mountains of Iran. See also FOLD; STRUCTURAL GEOLOGY. Appalachians A mountain belt that extends for 1,600 miles (1,000 km) along the east coast of North America, stretching from the St. Lawrence Valley in Quebec, Canada, to Alabama. Many classifications consider the Appalachians to continue through Newfoundland in maritime Canada, and before the Atlantic Ocean opened, the Appalachians were continuous with the Caledonides of Europe. The Appalachi- ans are one of the best-studied mountain ranges in the world, and understanding of their evolution was one of the factors that led to the development and refinement of the paradigm of plate tectonics in the early 1970s. Rocks that form the Appalachians include those that were deposited on or adjacent to North America and thrust upon the continent during several orogenic events. For the length of the Appalachians, the older continental crust con- sists of Grenville Province gneisses, deformed and metamor- phosed about 1.0 billion years ago during the Grenville orogeny. The Appalachians grew in several stages. After Late Precambrian rifting, the Iapetus Ocean evolved and hosted island arc growth, while a passive margin sequence was deposited on the North American rifted margin in Cambrian- Ordovician times. In the Middle Ordovician, the collision of an island arc terrane with North America marks the Taconic orogeny, followed by the Mid-Devonian Acadian orogeny, which probably represents the collision of North America with Avalonia, off the coast of Gondwana. This orogeny formed huge molassic fan delta complexes of the Catskill Mountains and was followed by strike-slip faulting. The Late Paleozoic Alleghenian orogeny formed striking folds and faults in the southern Appalachians but was dominated by strike-slip faulting in the northern Appalachians. This event appears to be related to the rotation of Africa to close the remaining part of the open ocean in the southern Appalachi- ans. Late Triassic-Jurassic rifting reopened the Appalachians, forming the present Atlantic Ocean. The history of the Appalachians begins with rifting of the one-billion-year-old Grenville gneisses and the formation of an ocean basin known as Iapetus approximately 800–570 million years ago. Rifting was accompanied by the formation of normal-fault systems and grabens and by the intrusion of swarms of mafic dikes exposed in places in the Appalachians such as in the Long Range dike swarm on Newfoundland’s 14 Appalachians Tectonic map of the Appalachian Mountains showing HAT: Humber Arm terrane; CLT: Chain Lakes terrane; Raleigh-Goochland belt; KMB: Kings Mountain belt; Also close to the western edge of the orogen is a discon- tinuous belt of mafic and ultramafic rocks comprising an ophiolite suite, interpreted to be remnants of the ocean floor of the Iapetus Ocean that closed during the Taconic orogeny. Spectacular examples of these ophiolites occur in Newfound- land, including the Bay of Islands ophiolite complex along Newfoundland’s western shores. Further east in the Taconic orogen are rocks of the Bron- son Hill anticlinorium or terrane, which are strongly deformed and metamorphosed and have been affected by both the Taconic and Acadian orogenies. These rocks have proven very difficult to map and have been of controversial signifi- cance for more than a century. Perhaps the best interpretation is that they represent rocks of the Taconic island arc that col- lided with North America to produce the Taconic orogeny. The Piscataquis volcanic arc is a belt of Devonian vol- canic rocks that extends from central Massachusetts to the Gaspe Peninsula. These rocks are roughly coextensive with the Ordovician arc of the Bronson Hill anticlinorium and include basalts, andesites, dacites, and rhyolites. Both sub- aerial volcanics and subaquatic pillow lavas are found in the belt. The Greenville plutonic belt of Maine (including Mount Kathadin) is included in the Piscataquis arc, and interpreted by some workers to be post-Acadian, but is more typical of syn-tectonic arc plutons. The eastern part of the Taconic oro- genic belt was also deformed by the Acadian orogeny and contains some younger rocks deposited on top of the eroded Taconic island arc, then deformed in the Acadian orogeny. The Taconic allochthons turn out to be continental rise sediments that were scraped off the North American continen- tal margin and transported on thrusts for 60–120 miles (100–200 km) during the Taconic arc continent collision. A Maine basin (Merrimack trough) preserves Silurian deepwa- ter sedimentary rocks, preserved in accretionary prisms, and is the most likely site where the Acadian Ocean closed. The Fredrickton trough is a continuation of the Merrimack trough, and the Avalon Composite terrane (coastal volcanic arc) contains Silurian–Early Devonian shallow marine vol- canics built upon Precambrian basement of Avalonia. Synthesizing the geology of these complex belts, the tec- tonics of the Acadian orogeny in the Appalachian Mountains can be summarized as follows. The Grenville gneisses and some of the accreted Taconic orogen were overlain by a Pale- ozoic platform sequence, and by mid-Devonian times the region was buried beneath thick clastics of the Acadian fore- land basin, best preserved in the Catskill Mountains. Nearly two miles (3 km) of fluvial sediments were deposited in 20 million years, derived from mountains to the east. Molasse and red beds of the Catskills once covered the Adirondack Mountains and pieces are preserved in a diatreme in Montre- al, and they are exposed along strike as the Old Red Sand- stone in Scotland and on Spitzbergen Island. The Connecticut Valley–Gaspe trough is a complex basin developed over the Taconic suture and was active from Silurian through Early Devonian. It is an extensional basin containing shallow marine sedimentary rocks and may have formed from oblique strike-slip after the Taconic collision, with subsidence Map of the northern Appalachians showing the main 16 aquifer ducting on both sides, forming accretionary wedges of oppo- site vergence, and forming the Coastal and Piscataquis volcanic arcs. The Connecticut Valley–Gaspe trough is a zone of active strike-slip faulting and pull-apart basin formation behind the Piscataquis arc. In the Devonian, the accretionary plexes collided, and west-directed overthrusting produced a migrating flexural basin of turbidite deposition, including the widespread Seboomook and Littleton Formations. The colli- sion continued until the Late Devonian, then more plutons intruded, and dextral strike-slip faulting continued. Acadian plutons intrude all over the different tectonic zones and are poorly understood. Some are related to arc magmatism, some to crustal thickening during collision. Late transpression in the Carboniferous includes abundant dextral strike-slip faults, disrupted zones, and formed pull-apart basins with local accumulations of several miles of About 200 miles (300 km) of dextral strike-slip offsets are estimated to have occurred across the orogen. The Late Paleozoic Alleghenian orogeny in the Carbonif- erous and Permian included strong folding and thrusting in the southern Appalachians and formed a fold/thrust belt with a ramp/flat geometry. In the southern Appalachians the foreland was shortened by about 50 percent during this event, with an estimated 120 miles (200 km) of shortening. The rocks highest in the thrust belt have been transported the farthest and are the most allochthonous. At the same time, motions in the northern Appalachians were dominantly dextral strike-slip in nature. In the Late Triassic–Jurassic, rifting and were associated with the formation of many small basins and the intrusion of mafic dike swarms, related to the opening of the present-day Atlantic Ocean. See also CALEDONIDES; PENOBSCOTTIAN OROGENY; PLATE TECTONICS. Further Reading Ala Drake, A., A. K. Sinha, Jo Laird, and R. E. Guy. “The Taconic Orogen.” Chapter 3 in “The Geology of North America, vol. A: The Geology of North America, an Overview.” Geological Soci- ety of America (1989): 101–178. Bird, John M., and John F. Dewey. “Lithosphere Plate-Continental Margin Tectonics and the Evolution of the Appalachian Orogen.” Geological Society of America Bulletin 81 (1970): 1,031–1,060. Bradley, Dwight C. “Tectonics of the Acadian Orogeny in New Eng- land and Adjacent Canada.” Journal of Geology 91 (1983): 381–400. Bradley, Dwight C., and Timothy M. Kusky. “Geologic Methods of Estimating Convergence Rates during Arc-Continent Collision.” Journal of Geology 94 (1986): 667–681. Dewey, John F., Michael J. Kennedy, and William S. F. Kidd. “A Geo- traverse through the Appalachian of Northern Newfoundland.” Profiles of Orogenic Belts, edited by N. Rast and AGU/Geological Society of America, Geodynamics series 10, 1983. Hatcher, Robert D., Jr., William A. Thomas, Peter A. Geiser, Arthur W. Snoke, Sharon Mosher, and David V. Wiltschko. “Alleghenian Orogeny.” Chapter 5 in “The Geology of North America, vol. A: ing methods. The quality of an aquifer depends on two main quantities, porosity and permeability. Porosity is a measure of the total amount of open void space in the material. Perme- ability is a term that refers to the ease at which a fluid can move through the open pore spaces, and it depends in part on the size, the shape, and how connected individual pore spaces are in the material. Gravels and sandstone make good aquifers, as do fractured rock bodies. Clay is so impermeable that it makes bad aquifers or even aquicludes, which stop the movement of water. There are several main types of aquifers. In meable rock and soil masses, aquifers will form as a uniform layer below the water table. In these simple situations, wells fill with water simply because they intersect the water table. However, the rocks below the surface are not always homo- geneous and uniform, which can result in a complex type of water table known as a perched water table. This results from discontinuous impermeable rock or soil bodies in the subsurface, which create domed pockets of water at eleva- tions higher than the main water table, resting on top of the impermeable layer. When the upper boundary of the groundwater in an aquifer is the water table, the aquifer is said to be unconfined. In many regions, a saturated permeable layer, typically sand- stone, is confined between two impermeable beds, creating a confined aquifer. In these systems, water only enters the sys- tem in a small recharge area, and if this is in the mountains, then the aquifer may be under considerable pressure. This is known as an artesian system. Water that escapes the system from the fracture or well reflects the pressure difference between the elevation of the source area and the discharge area (hydraulic gradient), and it rises above the aquifer as an artesian spring, or artesian well. Some of these wells have made fountains that have spewed water 200 feet (60 m) high. See also FRACTURE ZONE AQUIFERS; GROUNDWATER. Arabian shield The Arabian shield comprises the core of the Arabian Peninsula, a landmass of near trapezoidal shape bounded by three water bodies. The Red Sea bounds it from the west, the Arabian Sea and the Gulf of Aden from the south, and the Arabian Gulf and Gulf of Oman on the east. The Arabian Peninsula can be classified into two major geo- logical provinces, including the Precambrian Arabian shield and the Phanerozoic cover. The Precambrian shield is located along the western and central parts of the peninsula. It narrows in the north and the south but widens in the central part of the peninsula. The shield lies between latitudes 12° and 30° north and between longitudes 34° and 47° east. The Arabian shield is considered as part of the Arabian-Nubian shield that was formed in the upper Proterozoic Era and stabilized in the Late Proterozoic around 600 million years ago. The shield has since subsided Science Kusky, Ph.D. or utilized in any form or by any recording, or by any information permission in writing from contact: Data / Timothy Kusky. and index. I. Title. when purchased in bulk quantities or sales promotions. Please call our or 800/322-8755. Web at http://www.factsonfile.com M. Toro Rincon and Facts On File, Inc. of America 6 5 4 3 2 1 acid-free paper. Relations Tsunami” of preparing this encyclopedia. reviewed and edited all text and consistency throughout. correcting different ver- and photographs. Additional Made, Justin Kanoff, and from my own experiences the world, and it is not possible to collaborations and work I the science that allowed the writ- tried to reference the most rele- that have more extensive my wife, and my children long hours spent at my desk this work would not have been a broad view of some of sciences. The topics covered in the subdisciplines in the earth isotope geology, geochemistry, oceanography), along with events, the major periods of contributions to the and other subjects. to be a reference for high scientists, librarians, jour- for information outside their spe- with photographs and other and contains 19 special essays cross-referenced and are based on extensive research the general science to very scientific references and of each section, and the entries are Some parts of the encyclopedia files of scientific reprints to provide uniformly detailed the more lengthy entries, how- be of great importance. Verde Islands, and the Azores. The deep abyssal areas in the Pacific Ocean are characterized by the presence of more abundant hills or seamounts, which rise up to 0.6 miles (1 km) above the seafloor. Therefore, the deep abyssal region of the Pacific is generally referred to as the abyssal hills instead of the abyssal plains. Approximately 80–85 percent of the Pacific Ocean floor lies close to areas with hills and seamounts, making the abyssal hills the most common land- form on the surface of the Earth. Many of the sediments on the deep seafloor (the abyssal plain) are derived from erosion of the continents and are car- ried to the deep sea by turbidity currents, wind (e.g., volcanic ash), or released from floating ice. Other sediments, known as deep-sea oozes, include pelagic sediments derived from marine organic activity. When small organisms die, such as diatoms in the ocean, their shells sink to the bottom and over time can create significant accumulations. Calcareous ooze occurs at low to middle latitudes where warm water favors the growth of carbonate-secreting organisms. Calcareous oozes are not found in water that is more than 2.5–3 miles (4–5 km) deep, because this water is under such high pressure that it contains dissolved CO2 that dissolves carbonate shells. Siliceous ooze is produced by organisms that use silicon to make their shell structure. See also CONTINENTAL MARGIN. accretionary wedge Structurally complex parts of subduc- tion zone systems, accretionary wedges are formed on the landward side of the trench by material scraped off from the subducting plate as well as trench fill sediments. They typical- ly have wedge-shaped cross sections and have one of the most complex internal structures of any tectonic element known on Earth. Parts of accretionary wedges are characterized by numerous thin units of rock layers that are repeated by 1 wedge dominated by basalt and chert rock types, whereas subducting plates with thick sequences of graywacke sedi- ments yield packages in the accretionary wedge dominated by graywacke. They may also grow by a process known as underplating, where packages (thrust slices of rock from the subducting plate) are added to the base of the accretionary wedge, a process that typically causes folding of the overlying parts of the wedge. The fronts or toes of accretionary wedges are also characterized by material slumping off of the steep slope of the wedge into the trench. This material may then be recycled back into the accretionary wedge, forming even more complex structures. Together, the processes of offscrap- ing and underplating tend to steepen structures and rock lay- ers from an orientation that is near horizontal at the toe of the wedge to near vertical at the back of the wedge. The accretionary wedges are thought to behave mechani- cally somewhat as if they were piles of sand bulldozed in front of a plow. They grow a triangular wedge shape that increases its slope until it becomes oversteepened and mechanically unstable, which will then cause the toe of the wedge to advance by thrusting, or the top of the wedge to material being offscraped at the toe of the wedge and underplated beneath the wedge Adirondack Mountains 3 about 1,155–1,125 million years ago. This magmatism was followed by two more magmatic events; hornblende granites and leucogranites at approximately 1,100–1,090 million years ago (Hawkeye suite) and 1,070–1,045 million years ago (Lyon Mountain granite), respectively. The most intense metamor- phic event was the Ottawan orogeny, which occurred 1,100–1,000 million years ago, with “peak” metamorphism occurring at about 1,050 million years ago. The Adirondacks are subdivided into two provinces: the Northwest Lowlands and the Highlands, separated by the Carthage-Colton mylonite zone. Each province contains dis- tinct rock types and geologic features, both of which have clear affinities related to the Canadian Grenville province. The Northwest Lowlands The Northwest Lowlands are located in the northwest por- tion of the Adirondack Mountains. On the basis of litholo- gies, the Lowlands are closely related to the Frontenac terrane of the Canadian metasedimentary belt and are thought to be connected via the Frontenac Arch. The North- west Lowlands are smaller in area, have lower topographic relief than the Highlands, and are dominated by metasedi- mentary rocks interlayered with leucocratic gneisses. Both lithologies are metamorphosed to upper amphibolite grade. The metasedimentary rocks are mostly marbles but also con- Complex, tain units of quartzites and mica schists, suggesting a plat- Geology 21, no. form sedimentary sequence provenance. The protoliths of the and Susan leucocratic gneisses are controversial. Some geologists consid- er the leucocratic gneisses to be basal rhyolitic and dacitic ash-flow tuff deposits that have been metamorphosed, based on geochemical signatures and the absence of xenoliths in the formations. However, others question this interpretation and suggest that the leucocratic bodies are intrusive in nature, based on crosscutting field evidence and geothermometry. The geothermometry on the leucocratic gneiss yields a tem- perature of 1,436°F–1,490°F (780°C–810°C). This is an anomalously high metamorphic temperature compared with other rocks in the region, suggesting that they may be igneous crystallization temperatures. The Highlands The Highlands are correlative with the central granulite terrain of the Canadian Grenville province. The Green Mountains of Vermont may also be correlative with the Highlands, although other Proterozoic massifs in the northern Appalachians such as the Chain Lakes massif may be exotic to Laurentia. The High- lands are dominated by meta-igneous rocks, including abun- dant anorthosite bodies. The largest anorthosite intrusion is the Mount Marcy massif located in the east-central Adiron- dacks; additional anorthosite massifs are the Oregon and Snowy Mountain domes that lie to the south-southwest of Mount Marcy. The anorthosite bodies are part of the suite of rocks known as the AMCG suite; anorthosites, mangerites, Adirondack Mountains: AMA: Arab Mountain antiform; G: Gore Mountain; LM: Little Moose Mountain WM: Wakeley Mountain nappe amounts of deformation. The Southern Highlands are com- prised of granitic gneisses from the AMCG suite with infolded metasedimentary rocks that are strongly deformed. Within the Southeastern Highlands, metasedimentary rocks are found; Adirondack Mountains 5 southern Adirondacks are strongly foliated and lineated. The large-scale folds and rock fabrics suggest northwest directed tectonic transport, which is consistent with other kinematic indicators in the rest of the Grenville province. Even the most generalized geologic maps of the Adiron- dacks reveal that this region possesses multiple large-scale folds. Delineating the various fold sets is difficult, due to the fold interference patterns, but at least five sets of folds are recognized. The timing of these fold sets has remained obscure, but at least some are related to the Ottawan oroge- ny. It is also not clear whether these folds formed as a pro- gressive event or as part of distinct events. Fold nomenclature, i.e., anticline and syncline, is based on structural evidence found in the eastern parts of the Adirondacks. The shapes of igneous plutons and orientation of igneous compositional layering have aided structural geolo- gists to determine fold superposition in this region. The earli- est fold set (F1 folds) are reclined to recumbent folds. Mainly minor, intrafolial F1 folds have been documented, with rare outcrop-scale examples. The presence of larger F1 folds is sus- pected based on rotated foliations associated with F1 folding in the hinge areas of F2 folds. Many F1 folds may have eluded detection because of their extremely large size. The F2 folds are the earliest mappable folds in the Adirondacks, an example being the Wakely Mountain nappe. In general the F2 folds are recumbent to reclined, isoclinal folds. The F2 folds are coaxial with the F1 folds and have fold axes that trend northwest to east-west. Both of these fold sets have been suggested to be associated with thrust nappes. The F3 folds are large, upright-open folds that trend west- northwest to east-west. Therefore, they are considered coaxial with F1 and F2 folds. F3 folds are best developed in the south- central Adirondack Highlands. Examples of these folds are the Piseco anticline and the Glens Falls syncline. Northwest trending F4 folds are best developed in the Northwest Low- lands and are rare in the Highlands, except in the southern regions. North-northeast trending F5 folds are open, upright folds except near Mount Marcy where they become tight. F5 folds are better developed in the eastern parts of the Adiron- dacks. Due to the spatial separation of F4 and F5 folds, distin- guishing relative timing between the two is difficult. See also GRENVILLE PROVINCE; PROTEROZOIC; STRUC- TURAL GEOLOGY; SUPERCONTINENT CYCLE. Further Reading Brown, John S. “Structure and Primary Mineralization of the Zinc Mine at Balmat, New York.” Economic Geology 31, no. 3 (1936): 233–258. S. Brown was Buddington, Arthur F. “Adirondacks Igneous Rocks and Their Meta- morphism.” Geological Society of America Memoir 7 (1939): 1–354. Chiarenzelli, Jeffrey R., and Jim M. McLelland. “Age and Regional Relationships of Granitoid Rocks of the Adirondack Highlands.” Journal of Geology 99 (1991): 571–590. apart, causing regional subsidence. The Arabian plate is moving northeast away from the African plate, and the Somali plate is moving, at a much slower rate, to the southeast away from Africa. The southern Red Sea and north-central Afar Depres- Setting.” sion form two parallel north-northwest-trending rift basins, separated by the Danakil Horst, related to the separation of Canadian Arabia from Africa. Of the two rifts, the Afar depression is exposed at the surface, whereas the Red Sea rift floor is sub- merged below the sea. The north-central Afar rift is complex, consisting of many grabens and horsts. The Afar Depression merges southward with the northeast-striking Main Ethiopian Rift, and eastward with the east-northeast-striking Gulf of Aden. The Ethiopian Plateau bounds it on the west. Pliocene volcanic rocks of the Afar stratoid series and the Pleistocene to Recent volcanics of the Axial Ranges occupy the floor of the Afar Depression. Miocene to recent detrital and chemical sedi- 395–418. ments are intercalated with the volcanics in the basins. McLelland. The Main Ethiopian and North-Central Afar rifts are part of the continental East African Rift System. These two kinematically distinct rift systems, typical of intracontinental rifting, are at different stages of evolution. In the north and east, the continental rifts meet the oceanic rifts of the Red Sea and the Gulf of Aden, respectively, both of which have propa- gated into the continent. Seismic refraction and gravity studies indicate that the thickness of the crust in the Main Ethiopian and Alec Rift is less than or equal to 18.5 miles (30 km). In Afar the Paper 31 thickness varies from 14 to 16 miles (23–26 km) in the south to 8.5 miles (14 km) in the north. The plateau on both sides of the rift has a crustal thickness of 21.5–27 miles (35–44 km). Rates of separation obtained from geologic and geodetic studies indicate 0.1–0.2 inches (3–6 mm) per year across the northern sector of the Main Ethiopian Rift between the African and Somali plates. The rate of spreading between Africa and Arabia across the North-Central Afar rift is rela- tively faster, about 0.8 inches (20 mm) per year. Paleomagnet- Research 86 ic directions from Cenozoic basalts on the Arabian side of the Gulf of Aden indicate 7 degrees of counterclockwise rotation on Southeast- Implica- of the Arabian plate relative to Africa, and clockwise rota- tions of up to 11 degrees for blocks in eastern Afar. The initia- tion of extension on both sides of the southernmost Red Sea Rift, Ethiopia, and Yemen appear coeval, with extension start- ing between 22 million and 29 million years ago. See also DIVERGENT OR EXTENSIONAL BOUNDARIES; RIFT. Further Reading exposed on Tesfaye, Sansom, Dave Harding, and Timothy Kusky. “Early Conti- nental Breakup Boundary and Migration of the Afar Triple Junc- tion, Ethiopia.” Geological Society of America Bulletin 115 (2003): 1,053–1,067. in eastern agate An ornamental, translucent variety of quartz, known for its spectacular colors and patterns. It is extremely fine- grained (or cryptocrystalline), and mixed with layers of opal, which is another variety of colored silica that has combined air pressure 7 Ethiopian rift segment of the East African rift meets the Tendaho rift, an extension of the Red Sea Gulf of Aden rift system. Note the dramatic change in orientation of fault-controlled ridges and how internal such as Lake Abhe, where the water evaporates. commonly sold at rock and mineral shows as polished slabs of ornamental stone. See also MINERALOGY. moss-like air pressure The weight of the air above a given level. This weight produces a force in all directions caused by constantly height than in the cold column. Therefore, warm air masses at height are generally associated with high-pressure systems, whereas cold air aloft is generally associated with low pres- sure. Heating and cooling of air above a location causes the air pressure to change in that location, causing lateral varia- tion in air pressure across a region. Air will flow from high- pressure areas to low-pressure areas, forming winds. The daily heating and cooling of air masses by the Sun can in some situations cause the opposite effect, if not over- whelmed by effects of the heating and cooling of the upper atmosphere. Over large continental areas, such as the south- western United States, the daily heating and cooling cycle is associated with air pressure fall and rise, as expected from the gas law. As the temperature rises in these locations the pressure decreases, then increases again in the night when the temperature falls. Air must flow in and out of a given vertical column on a diurnal basis for these pressure changes to occur, as opposed to having the column rise and fall in response to the temperature changes. See also ATMOSPHERE. 1,013.25 Aleutian Islands and trench Stretching 1,243 miles (2,000 km) west from the western tip of the Alaskan Peninsu- la, the Aleutian Islands form a rugged chain of volcanic islands that stretch to the Komandorski Islands near the Kamchatka Peninsula of Russia. The islands form an island arc system above the Pacific plate, which is subducted in the Aleutian trench, a 5-mile (8-km) deep trough ocean-ward of the Aleutian Islands. They are one of the most volcanically (gas constant, active island chains in the world, typically hosting several eruptions per year. The Aleutians consist of several main island groups, including the Fox Islands closest to the Alaskan mainland, then moving out toward the Bering Sea and Kamchatka to the Andreanof Islands, the Rat Islands, and the Near Islands. The climate of the Aleutians is characterized by nearly con- stant fog and heavy rains, but generally moderate tempera- tures. Snow may fall in heavy quantities in the winter months. The islands are almost treeless but have thick grass- es, bushes, and sedges, and are inhabited by deer and sheep. The local Inuit population subsists on fishing and hunting. The first westerner to discover the Aleutians was the Dan- ish explorer Vitus Bering, when employed by Russia in 1741. Russian trappers and traders established settlements on the islands and employed local Inuit to hunt otters, seals, and fox. The Aleutians were purchased by the United States along with the rest of Alaska from Russia in 1867. The only good harbor in the Aleutian is at Dutch Harbor, used as a transshipping port, a gold boomtown, and as a World War II naval base. See also PLATE TECTONICS. height than alluvial fans Fan- or cone-shaped deposits of fluvial grav- el, sand, and other material radiating away from a single Alps 9 mountains, of an allu- several that ng of alluvial relative with for Alluvial fan, Death Valley, California. Recent channels are light-colored, whereas older surfaces are coated with a dark desert varnish. (Photo by Timothy Kusky) displays fan apex. In characterized by large boulders embedded in a fine-grained, typically mud-dominated matrix. These deposits shift lateral- ly across the fan, although the debris and mudflow deposits tend to be confined to channels. The fan surface may exhibit a microtopography related to the different sedimentary facies and deposit types. The development of fan morphology, the slope, relative aggradation versus downcutting of channels, and the growth or retreat of the toe and apex of the fan are complex phe- nomena dependent on a number of variables. Foremost among these are the climate, the relative uplift and subsi- dence of the mountains and valleys, base level in the valleys, and the sediment supply. See also DESERT; DRAINAGE BASIN; GEOMORPHOLOGY. Further Reading available for Bull, William B. “Alluvial Fans.” Journal of Geologic Education 16 (1968): 101–106. Ritter, Dale F., R. Craig Kochel, and Jerry R. Miller. Process Geo- morphology, 3rd ed. Boston: WCB-McGraw Hill, 1995. to a Alps An arcuate mountain system of south central Europe, about 497 miles (800 km) long and 93 miles (150 km) wide, stretching from the French Riviera on the Mediterranean coast, through southeastern France, Switzerland, southwest- ern Germany, Austria, and Yugoslavia (Serbia). The snow line in the Alps is approximately 8,038 feet (2,450 m), with many peaks above this being permanently snowcapped or hosting glaciers. The longest glacier in the Alps is the Aletsch, but many landforms attest to a greater extent of glaciation in the Pleistocene. These include famous landforms such as the Matterhorn and other horns, aretes, U-shaped valleys, errat- ics, and moraines. The Alps were formed by plate collisions related to the closure of the Tethys Ocean in the Oligocene and Miocene, high-altitude lake in the world, is located at the northern end of the Altiplano. The Altiplano is a dry region with sparse vegetation, and scattered salt flats. Villagers grow potatoes and grains, and a variety of minerals are extracted from the plateau and sur- rounding mountain ranges. See also ANDES. Amazon River The world’s second longest river, stretching 3,900 miles (6,275 km) from the foothills of the Andes to the Atlantic Ocean. The Amazon begins where the Ucayali and Maranon tributaries merge and drains into the Atlantic near the city of Belem. The Amazon carries the most water and has the largest discharge of any river in the world, averaging 150 feet (45 m) deep. Its drainage basin amounts to about 35 percent of South America, covering 2,500,000 square miles (6,475,000 km2). The Amazon lowlands in Brazil include the largest tropical rainforest in the world. In this region, the Amazon is a muddy, silt-rich river with many channels that wind around numerous islands in a complex maze. The delta region of the Amazon is marked by numerous fluvial islands and distributaries, as the muddy waters of the river get dis- persed by strong currents and waves into the Atlantic. A strong tidal bore, up to 12 feet (3.7 m) high runs up to 500 miles (800 km) upstream. The Amazon River basin occupies a sediment-filled rift basin, between the Precambrian crystalline basement of the Brazil and Guiana Shields. The area hosts economic deposits of gold, manganese, and other metals in the highlands, and detrital gold in lower elevations. Much of the region’s econo- my relies on the lumber industry, with timber, rubber, veg- etable oils, Brazil nuts, and medicinal plants sold worldwide. Spanish commander Vincent Pinzon was probably the first European in 1500 to explore the lower part of the river basin, followed by the Spanish explorer Franciso de Orellana in 1540–41. De Orellana’s tales of tall strong female warriors gave the river its name, borrowing from Greek mythology. Further exploration by Pedro Teixeira, Charles Darwin, and Louis Agassiz led to greater understanding of the river’s course, peoples, and environment, and settlements did not appear until steamship service began in the middle 1800s. amber A yellow or yellowish brown translucent fossil plant resin derived from coniferous trees. It is not a mineral but an organic compound that often encases fossil insects, pollen, and other objects. It is capable of taking on a fine pol- ish and is therefore widely used as an ornamental jewelry piece and is also used for making beads, pipe mouthpieces, or bookshelf oddities. Amber is found in many places, including soils, clays, and lignite beds. It is well known from locations including the shores of the Baltic Sea and parts of the Dominican Republic. Amber contains high concentrations of succinic acid (a crystalline dicarboxylic acid, with the formu- amphibole 11 American Geophysical Union (AGU) AGU (http://www. agu.org), a nonprofit scientific organization, was established in 1919 by the National Research Council. AGU is supplying an organizational framework within which geophysicists have created the programs and products needed to advance their science. AGU now stands as a leader in the increasingly interdisciplinary global endeavor that encompasses the geo- physical sciences. AGU’s activities are focused on the organization and dis- semination of scientific information in the interdisciplinary and international field of geophysics. The geophysical sci- ences involve four fundamental areas: atmospheric and ocean sciences; solid-Earth sciences; hydrologic sciences; and space sciences. AGU has a broad range of publications and meet- ings and educational and other activities that support research in the Earth and space sciences. American Meteorological Association (AMS) The Amer- ican Meteorological Society (http://www.ametsoc.org) was founded in 1919. The society’s initial publication, the Bulletin of the American Meteorological Society, serves as a supplement to the Monthly Weather Review, which was initially published by the U.S. Weather Bureau. The role of the AMS is serving the atmospheric and related sciences. The AMS now publishes in print and online nine well-respected scientific journals and an abstract journal. The AMS administers two professional certi- fication programs, the Radio and Television Seal of Approval and the Certified Consulting Meteorologist (CCM) programs, and also offers an array of undergraduate scholarships and graduate fellowships to support students pursuing careers in the atmospheric and related oceanic and hydrologic sciences. amphibole A group of dark-colored ferromagnesian sili- cate minerals with the general chemical formula: A2–3 B5 (Si, Al)8O22(OH)2 where A = Mg, Fe+2, Ca, or Na, and B = Mg, Fe+3, Fe+2or Al. Amphiboles contain continuous double chains of cross-linked double silicate tetrahedra. The chains are bound together by cations such as Ca, Mg, and Fe, which satisfy the negative charges of the polymerized tetrahedra. Most amphiboles are monoclinic, but some crystallize in the orthorhombic crystal system. They have good prismatic cleavage intersecting at 56° and 124° and typically form columnar or fibrous prismatic crystals. Amphiboles are very common constituents of meta- morphic and igneous rocks and have a chemical composition similar to pyroxenes. Some of the common amphibole miner- als include hornblende, tremolite, actinolite, anthophyllite, cummingtonite, riebeckite, and glaucophane. Amphibole is a fairly common mineral in intermediate to mafic igneous rocks such as granodiorite, diorite, and gab- bro, forming up to 25 percent of these rocks in some cases. Since amphibole is a hydrous mineral, it typically forms in diorite and is characteristic of volcanic belts formed above subduction zones that dip under continents. The name was coined by Buch (1826) for rocks in the Andes Mountains of South America. Andesite is generally associated with continental margin or Andean-type magmatic arcs built on continental crust above subduction zones. Their composition is thought to reflect a combination of processes from the melting of the mantle wedge above the subducting plate, plus some contam- ination of the magmas by partial melting of the continental crust beneath the arc. The average composition of the continental crust is approximately andesitic to dacitic. Many models for the formation and growth of continents therefore invoke the formation of andesitic to dacitic magmas at convergent margins, with the andesitic arcs colliding to form larger continental masses. This is known as the andesite model of continental growth. See also CONVERGENT PLATE MARGIN PROCESSES; PLATE TECTONICS; VOLCANO. in the anticline Folds in rocks in which a convex upward warp contains older rocks in the center and younger rocks on the sides. They typically occur along with synclines in alternating anticline-syncline pairs forming a fold train. Their geometry is defined by several artificial geometric surfaces, known as the fold axial surface, which divides the fold into two equal limbs, and a fold hinge, parallel to the line of maximum cur- vature on the folded layers. The anticlines may be of any size, ranging from micro- scopic folds of thin layers to large mountain-scale uplifts. Regional parts of mountain ranges that are characterized by generally uplifted rocks in the center are known as anticlinoria. Anticlines and broad upwarps of strata make particular- ly good oil and gas traps if the geologic setting is appropriate The the sub- zoned sodic with silica Anticline in the Canadian Rockies, near McConnel (Photo by Timothy Kusky) Appalachians 13 Long Range Peninsula. Rifting was also accompanied by the deposition of sediments, first in rift basins, and then as a Cambrian transgressive sequence that prograded onto the North American craton. This unit is generally known as the Potsdam Sandstone and is well-exposed around the Adiron- dack dome in northern New York State. Basal parts of the Potsdam sandstone typically consist of a quartz pebble con- glomerate and a clean quartzite. Overlying the basal Cambrian transgressive sandstone is a Cambrian-Ordovician sequence of carbonate rocks deposit- ed on a stable carbonate platform or passive margin, known in the northern Appalachians as the Beekmantown Group. Deposition on the passive margin was abruptly terminated in the Middle Ordovician when the carbonate platform was progressively uplifted above sea level from the east, then migrated to the west, and then suddenly dropped down to water depths too great to continue production of carbonates. In this period, black shales of the Trenton and Black River Groups were deposited, first in the east and then in the west. During this time, a system of normal faults also migrated across the continental margin, active first in the east and then in the west. The next event in the history of the continental margin is deposition of coarser-grained clastic rocks of the Austin Glen and correlative formations, as a migrating clastic wedge, with older rocks in the east and younger ones in the west. Together, these diachronous events represent the first stages of the Taconic orogeny, and they represent a response to the emplacement of the Taconic allochthons on the North American continental margin during Middle Ordovician arc- continent collision. The Taconic allochthons are a group of Cambrian through Middle Ordovician slates resting allochthonously on the Cambro-Ordovician carbonate platform. These allochthons are very different from the underlying rocks, implying that there have been substantial displacements on the thrust faults beneath the allochthons, probably on the order of 100 miles (160 km). The allochthons structurally overlie wild flysch breccias that are basically submarine slide breccias and mudflows derived from the allochthons. Eastern sections of the Taconic aged rocks in the Appalachians are more strongly deformed than those in the west. East of the Taconic foreland fold-thrust belts, a chain of uplifted basement with Grenville ages (about one billion years) extends discontinuously from Newfoundland to the Blue Ridge Mountains and includes the Green Mountains of Vermont. These rocks generally mark the edge of the hinter- land of the orogen, and the transition into greenschist and higher metamorphic facies. Some of these uplifted basement gneisses are very strongly deformed and metamorphosed, and they contain domal structures known as gneiss domes, with gneisses at the core and strongly deformed and metamor- phosed Cambro-Ordovician marbles around their rims. These rocks were deformed at great depths. the distribution of major lithotectonic terranes. Abbreviations as follows: HBT: Hare Bay terrane; SLK: St. Lawrence klippe; TK: Taconic klippe; HK: Hamburg klippe; BT: Brunswick terrane; RGB: TT: Talladega terrane; PMT: Pine Mountain terrane (belt) clastic wedge (Austin Glen and Normanskill Formations) was deposited during emplacement of the allochthons, by their ero- sion, and spread out laterally in the foreland. As Taconic defor- mation proceeded, the clastic wedge and underlying carbonates and Grenville basement became involved in the deformation, rotating them, forming the Taconic angular unconformity. The Acadian orogeny has historically been one of the most poorly understood aspects of the regional geology of the Appalachians. Some of the major problems in interpreting the Acadian orogeny include understanding the nature of pre- Acadian, post-Taconic basins such as the Kearsage–Central Maine basin, Aroostook-Matapedia trough, and the Connecti- cut Valley–Gaspe trough. The existence and vergence of Aca- dian subduction zones is debated, and the relative importance of post-Acadian strike-slip movements is not well-constrained. Examining the regional geology of the northern Appalachians using only the rocks that are younger than the post-Taconic unconformity yields a picture of several distinc- tive tectonic belts, including different rock types and struc- tures. The North American craton includes Grenville gneisses and Paleozoic carbonates. The foreland basin includes a thick wedge of Devonian synorogenic clastic rocks, such as the Catskill Mountains, that thicken toward the mountain belt. The Green Mountain anticlinorium is a basement thrust slice, and the Connecticut Valley–Gaspe trough is a post-Taconic basin with rapid Silurian subsidence and deposition. The Bronson Hill–Boundary Mountain anticlinorium (Piscataquis volcanic arc) is a Silurian–Mid-Devonian volcanic belt formed along the North American continental margin. The Aroostook-Matapedia trough is a Silurian extensional basin, and the Miramichi massif represents remnants of a high- standing Ordovician (Taconic) arc. The Kearsarge–Central Appalachians 15 in pull-apart basins. The Aroostook-Matapedia trough is an Ordovician-Silurian turbidite belt, probably a post-Taconic extensional basin, and perhaps a narrow oceanic basin. The Miramichi massif contains Ordovician arc rocks intruded by Acadian plutons and is part of the Taconic arc that persisted as a high area through Silurian times and became part of the Piscataquis volcanic arc in Silurian-Devo- nian times. The coastal volcanic arc (Avalon) is exposed in eastern Massachusetts though southern New Brunswick and includes about 5 miles (8 km) of basalt, andesites, rhyolite, and deep and shallow marine sediments. It is a volcanic arc that was built on Precambrian basement that originated in the Avalonian or Gondwana side of the Iapetus Ocean. The Kearsage–Central Maine basin (Fredericton trough) is the location of a major post-Taconic, pre-Acadian ocean that closed to produce the Acadian orogeny. It contains poly- deformed deepwater turbidites and black shales, mostly Sil- urian. The regional structural plunge results in low grades of metamorphism in Maine, high grades in New Hampshire, Massachusetts, and Connecticut. There are a few dismem- bered ophiolites present in the belt, structurally incorporated in about 3 miles (5 km) of turbidites. Volcanic belts on either side of the Merrimack trough are interpreted to be arcs built over contemporaneous subduction zones. In the Late Silurian, the Acadian Ocean basin was sub- Early Paleozoic tectonic terranes The Geology of North America, an Overview.” Geological Soci- ety of America (1989): 233–319. Kusky, Timothy M., J. Chow, and Samuel A. Bowring. “Age and Ori- gin of the Boil Mountain Ophiolite and Chain Lakes Massif, Maine: Implications for the Penobscottian Orogeny.” Canadian prism com- Journal of Earth Sciences 34, no. 5 (1997): 646– 654. Kusky, Timothy M., and William S. F. Kidd. “Early Silurian Thrust Imbrication of the Northern Exploits Subzone, Central New- foundland.” Journal of Geodynamics 22 (1996): 229–265. Kusky, Timothy M., William S. F. Kidd, and Dwight C. Bradley. “Displacement History of the Northern Arm Fault, and Its Bear- ing on the Post-Taconic Evolution of North-Central Newfound- land.” Journal of Geodynamics 7 (1987): 105–133. Manspeizer, Warren, Jelle Z. de-Boer, John K. Costain, Albert J. Froelich, Cahit Coruh, Paul E. Olsen, Gregory J. McHone, John H. Puffer, and David C. Prowell. “Post-Paleozoic Activity.” Chap- ter 6 in “The Geology of North America, vol. A: The Geology of sediments. North America, an Overview.” Geological Society of America (1989): 319–374. McKerrow, W. Stuart, and A. M. Ziegler. “Paleozoic Oceans.” Nature 240 (1972): 92–94. Neuman, Robert B., A. R. Palmer, and J. Thomas Dutro. “Paleonto- logic Contributions to Paleozoic Paleographic Reconstructions of the Appalachians.” Chapter 7 in “The Geology of North Ameri- ca, vol. A: The Geology of North America, an Overview.” Geo- logical Society of America (1989): 375–384. Osberg, Phil, James F. Tull, Peter Robinson, Rudolph Hon, and J. Robert Butler. “The Acadian Orogen.” Chapter 4 in “The Geolo- gy of North America, vol A: The Geology of North America, an Overview.” Geological Society of America (1989): 179–232. normal faulting Rankin, D. W., Avery Ala Drake, Jr., Lynn Glover III, Richard Gold- smith, Leo M. Hall, D. P. Murray, Nicholas M. Ratcliffe, J. F. Read, Donald T. Secor, Jr., and R. S. Stanley. “Pre-Orogenic Ter- ranes.” Chapter 2 in “The Geology of North America, vol. A: The Geology of North America, an Overview.” Geological Soci- ety of America (1989): 7–100. Rast, Nick. “The Evolution of the Appalachian Chain.” Chapter 12 in “The Geology of North America, vol. A: The Geology of North America, an Overview.” Geological Society of America (1989): 323–348 Rowley, David B., and William S. F. Kidd. “Stratigraphic Relation- ships and Detrital Composition of the Medial Ordovician Flysch of Western New England: Implications for the Tectonic Evolution of the Taconic Orogeny.” Journal of Geology 89 (1981): 199–218. Roy, D., and James W. Skehan. The Acadian Orogeny. Geological Society of America Special Paper 275, 1993. Socci, Anthony D, James W. Skehan, and Geoffrey W. Smith. Geolo- gy of the Composite Avalon Terrane of Southern New England. The Geological Society of America Special Paper 245, 1990. Stanley, Rolfe S., and Nicholas M. Ratcliffe. “Tectonic Synthesis of the Taconian Orogeny in Western New England.” Geological Society of America Bulletin 96 (1985): 1,227–1,250. In F. M. Delany. aquifer Any body of permeable rock or regolith saturated with water through which groundwater moves. The term aquifer is usually reserved for rock or soil bodies that contain economical quantities of water that are extractable by exist- Arabian shield 17 tal shelf sediments along the margins of the Tethys Ocean. Later in the Tertiary the Arabian-Nubian shield was rifted into two fragments by the Red Sea rift system. Phanerozoic cover overlies the eastern side of the Arabi- an shield unconformably and dips gently toward the east. Parts of the Phanerozoic cover are found overlying parts of the Precambrian shield, such as the Quaternary lava flows of Harrat Rahat in the middle and northern parts of the shield as well as some sandstones, including the Saq, Siq, and Wajeed sandstones in different parts of the shield. uniform, per- History of Tectonic Models The Arabian shield includes an assemblage of Middle to Late Proterozoic rocks exposed in the western and central parts of the Arabian Peninsula and overlapped to the north, east, and south by Phanerozoic sedimentary cover rocks. Several parts of the shield are covered by Tertiary and Quaternary lava flows that were extruded concurrently with rifting of the Red Sea. Rocks of the Arabian shield may be divided into assem- blages of Middle to Late Proterozoic stratotectonic units, vol- cano-sedimentary, and associated mafic to intermediate intrusive rocks. These rocks are divided into two major cate- gories, the layered rocks and the intrusive rocks. Researchers variously interpret these assemblages as a result of volcanism and magmatism in ensialic basins or above subduction zones. More recent workers suggested that many of these assem- blages belong to late Proterozoic volcanic-arc systems that comprise distinct tectonic units or terranes, recognized follow- ing definitions established in the North America cordillera. Efforts in suggesting models for the evolution of the Ara- bian shield started in the 1960s. Early workers suggested that the Arabian shield experienced three major orogenies in the Late Proterozoic Era. They also delineated four classes of plu- tonic rocks that evolved in chemistry from calc-alkaline to peralkaline through time. In the 1970s a great deal of research emerged concerning models of the tectonic evolution of the Arabian shield. Two major models emerged from this work, including mobilistic plate-tectonic models, and a non- mobilistic basement-tectonic model. The main tenet of the plate-tectonic model is that the evo- lution of the Arabian shield started and took place in an oceanic environment, with the formation of island arcs over subduction zones in a huge oceanic basin. On the contrary, the basement-tectonic model considers that the evolution of the Arabian shield started by the rifting of an older craton or con- tinent to form intraoceanic basins that became the sites of island arc systems. In both models, late stages of the formation of the Arabian-Nubian shield are marked by the sweeping together and collision of the island arcs systems, obduction of the ophiolites, and cratonization of the entire orogen, forming one craton attached to the African craton. Most subsequent investigators in the 1970s supported one of thes

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