Rocky Mountains

Extending 3,000 miles (4,800 km) from central New Mexico to northwest Alaska in the easternmost

Cordillera, the Rocky Mountains are one of the largest mountain belts in North America. The mountains are situated between the Great Plains on the east and a series of plateaus and broad basins on the west. Mount Elbert in Colorado is the highest mountain in the range, reaching 14,431 feet (4,399 m). The continental divide is located along the rim of the Rockies, separating waters that flow to the Pacific and to the Atlantic Oceans. The Rocky Mountains are divided into the Southern, Central, and Northern Rockies in the conterminous United States, Canadian Rockies in Canada, and the Brooks Range in Alaska. Several national parks are located in the system, including Rocky Mountain, Yellowstone, Grand Teton, and Glacier National Parks in the united States, and Banff, Glacier, Yoho, Kootenay, and Mount Revelstoke in Canada. The mountains were a major obstacle to traveling west during the expansion of the united States, but western regions opened up when the oregon trail crossed the ranges through South Pass in Wyoming.

In New Mexico, Colorado, and southern Wyoming the Southern Rockies consist of two north-south ranges of folded mountains that have been eroded to expose Precambrian cores with overlying sequences of layered sedimentary rocks. Three basins are located between these ranges, known as the North, South, and Middle Parks. The southern Rockies are the highest section of the whole range, including many peaks more than 14,000 feet (4,250 m) high.

The Central Rockies in northeastern utah and western Wyoming are lower and more discontinuous than the southern Rockies. Most are eroded down to their Precambrian cores, surrounded by Paleo-zoic-Mesozoic sedimentary rocks. Garnet Peak in the Wind River Range (13,785 feet; 4,202 m) and Grand Teton in the Teton Range (13,766 feet; 4,196 m) are the highest peaks in the Central Rockies.

The Northern Rockies in northeastern Washington, Idaho, and western Wyoming extend from Yellowstone National Park to the Canadian border. This section is dominated by north-south trending ranges separated by narrow valleys, including the Rocky Mountain trench, an especially deep and long valley that extends north from Flathead Lake. The highest peaks in the Northern Rockies include Borah Peak (12,655 feet; 3,857 m) and Leatherman Peak (12,230 feet; 3,728 m) in the Lost River Range.

The Canadian Rockies stretch along the British Columbia-Alberta border and reach their highest point in Canada on Mount Robson (12,972 feet; 3,954 m). The Rocky Mountain trench continues 800 miles (1,290 km) north-northwest from Montana, becomes more pronounced in Canada, and is joined by the Purcell trench in Alberta. In the Northwest Territories (Nunavet), the Rockies expand north eastward in the Mackenzie and Franklin mountains and near the Beaufort Sea pick up as the Richardson Mountains that gain elevation westward into the Brooks Range of Alaska. Mount Chamberlin (9,020 feet; 2,749 m) is the highest peak in the Brooks Range.

The Rocky Mountains are rich in mineral deposits including gold, silver, lead, zinc, copper, and molybdenum. Principal mining areas include the Butte-Anaconda district of Montana, Leadville and Cripple Creek in Colorado, Coeur d'Alene in Idaho, and the Kootenay Trail region of British Colombia. Lumbering is an active industry in the mountains, but it is threatened by growing environmental concerns and tourism in the national park systems.

Mesozoic-Early Cenozoic contractional events produced the Rockies during uplift associated with the Cordilleran orogeny. Evidence for older events and uplifts are commonly referred to as belonging to the ancestral Rocky Mountain system. The Rocky Mountains are part of the larger Cordilleran oro-genic belt that stretches from South America through Canada to Alaska, and it is best to understand the evolution of the Rockies through a wider discussion of events in this mountain belt. The Cordillera is presently active and has been active for the past 350 million years, making it one of the longest-lived oro-genic belts on Earth. In the Cordillera, many of the structures are not controlled by continent/continent collisions as they are in many other mountain belts, since the Pacific Ocean is still open. In this orogen structures are controlled by the subduction/accretion process, collision of arcs, islands, and oceanic plateaus, and strike-slip motions parallel to the mountain belt. Present-day motions and deformation are controlled by complex plate boundaries between the North American, Pacific, Gorda, Cocos, and some completely subducted plates such as the Farallon. In this active tectonic setting the style, orientation, and intensity of deformation and magmatism depend largely on the relative convergence-strike-slip vectors of motion between different plates.

The geologic history of the North American Cordillera begins with rifting of the present western margin of North America at 750-800 million years ago, which is roughly the same age as rifting along the east coast in the Appalachian orogen. These rifting events reflect the breakup of the supercontinent of Rodinia at the end of the Proterozoic, and they left North America floating freely from the majority of the continental landmass on Earth. Rifting and the subsequent thermal subsidence of the rifted margin led to the deposition of Precambrian clastic rocks of the Windemere supergroup and carbonates of the Belt and Purcell supergroups, in belts stretching from southern California and Mexico to Canada. These are overlain by Cambrian-Devonian carbonates, Carboniferous clastic wedges, Carboniferous-Permian carbonates, and finally Mesozoic clastic rocks.

The Antler orogeny is a Late Devonian-Early Carboniferous (350-400 million-year-old) tectonic event formed during an arc-continent collision, in which deep-water clastic rocks of the Roberts Mountain allochthon in Nevada were thrust from west to east over the North American carbonate bank, forming a foreland basin that migrated onto the craton. This orogenic event, similar to the Taconic orogeny in the Appalachian Mountains, marks the end of passive margin sedimentation in the Cordillera, and the beginning of Cordilleran tectonism.

In the Late Carboniferous (about 300 million years ago), the zone of active deformation shifted to the east with a zone of strike-slip faulting, thrusts, and normal faults near Denver. Belts of deformation formed what is known as the ancestral Rocky Mountains, including the Front Ranges in Colorado, and the Uncompahgre uplift of western Colorado, Utah, and New Mexico. These uplifts are only parts of a larger system of strike-slip faults and related structures that cut through the entire North American cra-ton in the Late Carboniferous, probably in response to compressional deformation that was simultaneously going on along three margins of the continent.

The Late Permian-Early Triassic Sonoma orogeny (260-240 million years ago) refers to events that led to the thrusting of deep-water Paleozoic rocks of the Golconda allochthon eastward over autochthonous shallow-water sediments just outboard (oceanward) of the Roberts Mountain allochthon. The Golconda allochthon in western Nevada includes deep-water oceanic pelagic rocks, an island-arc sequence, and a carbonate-shelf sequence and is interpreted to represent an arc/continent collision.

In the Late Jurassic (about 150 million years ago) a new, northwest-striking continental margin was established by cross cutting the old northeast-striking continental margin. This event, known as the early Mesozoic truncation event, reflects the start of continental margin volcanic and plutonic activity that continues to the present day. There is considerable uncertainty about what happened to the former extension of the old continental margin—it may have rifted and drifted away, or may have moved along the margin along large strike-slip faults.

Pacific margin magmatism has been active intermittently from the Late Triassic (220 million years ago) through the Late Cenozoic and in places continues to the present. This magmatism and deformation is a direct result of active subduction and arc magmatism. since the Late Jurassic, there have been three main periods of especially prolific magmatism, including the Late Jurassic/Early Cretaceous Neva-

dan orogeny (150-130 million years ago), the Late Cretaceous Sevier orogeny (80-70 million years ago), and the Late Cretaceous/Early Cenozoic Laramide orogeny (66-50 million years ago).

Cretaceous events in the Cordillera resulted in the formation of a number of tectonic belts that are still relatively easy to discern. The Sierra Nevada ranges of California and Nevada represent the arc batholith, and contain high-temperature, low-pressure metamorphic rocks characteristic of arcs. The Sierra Nevada is separated from the Coast Ranges by flat-lying generally unmetamorphosed sedimentary rocks of the Great Valley, deposited over ophiol-itic basement in a forearc basin. The Coast Ranges include high-pressure, low-temperature metamorphic rocks, including blueschists in the Franciscan complex. Together, the high-pressure, low-temperature metamorphism in the Franciscan complex with the high-temperature, low-pressure metamorphism in the Sierra Nevada, represent a paired metamorphic belt, diagnostic of a subduction zone setting.

Several Cretaceous foreland fold-thrust belts are preserved east of the magmatic belt in the Cordillera, stretching from Alaska to Central America. These belts include the Sevier fold-thrust belt in the United States, the Canadian Rockies fold-thrust belt, and the Mexican fold-thrust belt. They are all characterized by imbricate-style thrust faulting, with fault-related folds dominating the topographic expression of deformation.

The Late Cretaceous-Early Tertiary Laramide Orogeny (about 70-60 million years ago) is surprisingly poorly understood but generally interpreted as a period of plate reorganization that produced a series of basement uplifts from Montana to Mexico. Some models suggest that the Laramide orogeny resulted from the subduction of a slab of oceanic lithosphere at an unusually shallow angle, perhaps related to its young age and thermal buoyancy.

The Late Mesozoic-Cenozoic tectonics of the Cordillera saw prolific strike-slip faulting, with relative northward displacements of terranes along the western margin of North America. The San Andreas fault system is one of the major transform faults formed in this interval, formed as a consequence of the subduction of the Farallon plate. Previous convergence between the Farallon and North American plates stopped when the Farallon plate was subducted, and new relative strike-slip motions between the Pacific and North American plates resulted in the formation of the San Andreas system. Remnants of the Farallon plate are still preserved as the Gorda and Cocos plates.

Approximately 15 million years ago the Basin and Range Province and the Colorado Plateau began uplifting and extending through the formation of rifts and normal faults. Much of the Colorado Plateau stands at more than a mile (1.5-2.0 km) above sea level but has a normal crustal thickness. The cause of the uplift is controversial but may be related to heating from below. The extension is related to the height of the mountains being too great for the strength of the rocks at depth to support, so gravitational forces are able to cause high parts of the crust to extend through the formation of normal faults and rift basins.

See also accretionary wedge; Archean; Cambrian; convergent plate margin processes; cra-ton; deformation of rocks; divergent plate margin processes; economic geology; flysch; Gondwana, Gondwanaland; granite, granite batholith; orogeny; Paleozoic; Pangaea; passive margin; Phanerozoic; Precambrian; Silurian; structural geology; supercontinent cycles.

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, edited by A. W. Bally and A. R. Palmer, 10-78. Boulder, Colo: Geological Society of America, 1989.

Anderson, J. Lawford, ed. The Nature and Origin of Cor-dilleran Magmatism. Special Paper 174, Boulder, Colo.: Geological Society of America, 1990. 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. Bleeker, Wouter, and William Davis, eds. "NATMAP Slave Province Project." Canadian Journal of Earth Sciences 36 (1999).

Bowring, Samuel A., Ian S. Williams, and William Compston. "3.96 Ga Gneisses from the Slave Province, Northwest Territories." Geology 17/11 (1989b): 969-1,064.

Bradley, Dwight C. "Tectonics of the Acadian Orogeny in New England 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. Burchfiel, Bert Clark, and George A. Davis. "Nature and Controls of Cordilleran Orogenesis, Western United States: Extensions of an Earlier Synthesis." American Journal of Science 275 (1975): 363-396.

-. "Structural Framework of the Cordilleran Orogen,

Western United States." American Journal of Science 272 (1972): 97-118. Corcoran, P. L., W. U. Mueller, and Timothy M. Kusky. "Inferred Ophiolites in the Archean Slave Province."

In Precambrian Ophiolites and Related Rocks, Developments in Precambrian Geology, edited by Timothy M. Kusky, 363-404. Amsterdam: Elsevier, 2004.

Helmstaedt, Herart, William A. Padgham, and John A. Brophy. "Multiple Dikes in the Lower Kam Group, Yellowknife Greenstone Belt: Evidence for Archean Sea-floor Spreading?" Geology 14 (1986): 562-566.

Henderson, John B., "Archean Basin Evolution in the Slave Province, Canada." In Precambrian Plate Tectonics, edited by Alfred Kroner, 213-235. Amsterdam: Elsevier, 1981.

Kusky, Timothy M. "Structural Development of an Archean Orogen, Western Point Lake, Northwest Territories." Tectonics 10 (1991): 820-841.

-. "Evidence for Archean Ocean Opening and Closing in the Southern Slave Province." Tectonics 9 (1990): 1,533-1,563.

-. "Accretion of the Archean Slave Province." Geology 17 (1989): 63-67.

Kusky, Timothy M., J. Chow, and Samuel A. Bowring. "Age and Origin of the Boil Mountain Ophiolite and Chain Lakes Massif, Maine: Implications for the Penobscottian Orogeny." Canadian Journal of Earth Sciences 34, (1997): 646-654.

Kusky, Timothy M., and William S. F. Kidd. "Early Silurian Thrust Imbrication of the Northern Exploits Subzone, Central Newfoundland." 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 Bearing on the Post-Taconic Evolution of North-Central Newfoundland." Journal of Geodynamics 7 (1987): 105-133.

Oldow, John S., Albert W. Bally, Hans G. Ave Lallemant, and William P. Leeman. "Phanerozoic Evolution of the North American Cordillera; United States and Canada." In The Geology of North America; The Appalachian-Ouchita Orogen in the United States, Vol. F-2 of Decade of North American Geology, edited by Robert D. Hatcher, Jr., William A. Thomas, and George W. Viele, 179-232. Boulder, Colo.: Geological Society of America, 1989.

Osberg, Phil, James F. Tull, Peter Robinson, Rudolph Hon, and J. Robert Butler. "The Acadian Orogen." Chapter 4 in The Geology of North America, an Overview. Vol. A of Decade of North American Geology, edited by Albert W. Bally and Allison R. Palmer, 139-232. Boulder, Colo.: Geological Society of America, 1989.

Rankin, D. W., Avery Ala Drake, Jr., Lynn Glover III, Richard Goldsmith, Leo M. Hall, D. P. Murray, Nicholas M. Ratcliffe, J. F. Read, Donald T. Secor, Jr., and R. S. Stanley. "Pre-Orogenic Terranes." Chapter 2 in The Geology of North America, The Appalachian-Ouchita Orogen in the United States, Vol. F-2 of Decade of North American Geology, edited by Robert D. Hatcher, Jr., William A. Thomas, and George W.

Viele, 7-100, Boulder, Colo.: Geological Society of America, 1989. Rast, Nick. "The Evolution of the Appalachian Chain." Chapter 12 in The Geology of North America. Vol. A of The Geology of North America, an Overview, edited by A. W. Bally and A. R. Palmer, 323-348. Boulder, Colo.: Geological Society of America, 1989. Rowley, David B., and William S. F. Kidd. "Stratigraphic Relationships 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. "The Acadian Orogeny: Recent Studies in New England, Maritime Canada, and the Autochthonous Foreland." In Geological Society of America Special Paper 275, edited by James W. Skehan, 1993. Sisson, Virginia B., Sarah M. Roeske, and Terry L. Pav-lis. Geology of a Transpressional Orogen Developed During Ridge-Trench Interaction Along the North Pacific Margin. Geological Society of America Special Paper 371, 2003. Socci, Anthony D., James W. Skehan, and Geoffrey W. Smith. "Geology of the Composite Avalon Terrane of Southern New England." Special Paper 245 Geological Society of America, 1990. Stanley, Rolfe S., and Ratcliffe, Nicholas M. "Tectonic Synthesis of the Taconian Orogeny in Western New England." Geological Society of America Bulletin 96 (1985): 1,227-1,250. van Staal, Cees R., and Leslie R. Fyffe. "Dunnage Zone-New Brunswick." In Geology of the Appalachian-Caledonian Orogen in Canada and Greenland, edited by H. Williams, 166-178. The Geology of North America Vol. F-1, Geological Society of America, 1995.

nova A nova is a general name for a type of star that vastly increases in brightness (by up to 10,000 times) over very short periods of time, typically days or weeks. The word nova comes from the Latin for "new" and stems from early astronomers who thought that nova were new stars appearing in the sky, since the parent stars were too faint to be observed from the Earth before powerful telescopes were invented.

A nova forms when a white dwarf star has a companion, such as in a binary star system, and the companion contributes matter to the white dwarf after its initial death. In a simple system where a white dwarf exists alone, it will cool off indefinitely, approach absolute zero, and be invisible in space. However, in some binary star systems the large gravitational field of the white dwarf can pull material, predominantly hydrogen and helium, away from the companion main sequence star and accrete this material onto the white dwarf. As this gas builds up on the white dwarf surface it heats up and becomes denser until its temperature exceeds 100,000,000 K, at which point the hydrogen ignites and rapidly fuses into helium. This causes a sudden and dramatic flare-up of the surface of the white dwarf over a period of a few days, rapidly burning some of the fuel and expelling the rest of it to space in a nova event. After a few months the star's luminosity and surface temperature return to normal.

In white dwarf-binary star systems in which the companion star is a main sequence star, the material that is transferred off the main sequence star is affected by the rotation of the binary system and the gravitational field between the stars, and this material is forced to swirl around and orbits the white dwarf before it accretes to the surface. This forms what is known as an accretion disk. As the material in the accretion disk orbits the white dwarf, it is heated by friction and begins to glow and emit radiation in the visible, ultraviolet, and X-ray wavelengths and may become brighter than the star itself.

White dwarf-binary systems have the possibility of repeating the cycle of becoming novas many times, and some stars have done this hundreds of times. Such recurrent nova have been known since ancient times, and include systems such as RS Ophiuchi, located about 5,000 light years away in the constellation Ophiuchus; T Coronae Borealis, nicknamed the blaze star, in the constellation Coronos Borealis; and T Pyxidis, located about 6,000 light years from Earth in the constellation Pyxis.

See also astronomy; astrophysics; binary star systems; constellation; dwarfs (stars); stellar evolution; supernova.

FURTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Dibon-Smith, Richard. The Constellations Web Page. Available online. URL: http://www.dibonsmith.com/ index.htm. Last updated November 8, 2007. Prialnik, Dina. "Novae." In Encyclopedia of Astronomy and Astrophysics, edited by Paul Murdin, 1,8461,856. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

ocean basin The surface of the Earth is divided into two fundamentally different types of crust, including relatively light quartz and plagioclase-rich sial, forming the continental regions, and relatively dense olivine and pyroxene-rich sima underlying the ocean basins. The ocean basins are submarine topographic depressions underlain by oceanic (simatic) crust. Ocean basins are quite diverse in size, shape, depth, characteristics of the underlying seafloor topography, and types of sediments deposited on the oceanic crust. The largest ocean basins include the Pacific, Atlantic, Indian, and Arctic Oceans and the Mediterranean Sea, whereas dozens of smaller ocean basins are located around the globe.

The ocean basins' depths were first extensively explored by scientists aboard the H.M.S. Challenger in the 1800s, using depth reading from a weight attached to a several-mile (kilometer) long cable that was dropped to the ocean floor. Results from these studies suggested that the oceans were generally about three to four miles (five to six km) in depth. Later, with the development of echo-sounding technologies and war-induced mapping efforts, the variety of sea floor topography became appreciated. Giant submarine mountain chains were recognized where the depth is reduced to 1.7 miles (2.7 km), and these were later interpreted to be oceanic ridges where new oceanic crust is created. Deep-sea trenches with depths exceeding five miles (eight km) were delineated, and later recognized to be subduction zones where oceanic crust sinks back into the mantle. Other anomalous regions of thick oceanic crust (and reduced depths) were recognized, including large oceanic plateaus where excessive volcanism produced thick crust over large regions, and smaller seamounts (or guyots) where smaller, off-ridge volca-

nism produced isolated submarine mountains. some of these rose above sea level, were eroded by waves, and grew thick reef complexes as they subsided with the cooling of the oceanic crust. Charles Darwin made such guyots and coral atolls famous in his study of coral reefs of the Pacific Ocean basin.

Pelagic sediments are deposited in the ocean basins and generally form a blanket of sediments draping over preexisting topography. Carbonate rocks produced mainly by the tests of foraminifera and nannofossils may be deposited on the ocean ridges and guyots that are above the carbonate compensation depth (CCD), above which the sea water is saturated with CaCO3, and below which it dissolves in the water. Below this, sediments comprise red clays and radiolarian and diatomaceous ooze. Manganese nodules are scattered about on some parts of the ocean floor.

The abyssal plains are relatively flat, generally featureless parts of the ocean basins where the deep parts of the seafloor topography have been filled in with sediments, forming flat plains, broken occasionally by hills and volcanic islands such as the Bermuda platform, Cape Verde Islands, and the Azores. some of these submarine plains are quite large, such as the vast 386,100-square mile (1 million-km2) Angolan abyssal plain in the south Atlantic, and the 1,428,578-square mile (3.7 million-km2) abyssal plain in the Antarctic Ocean basin. Other abyssal plains are much smaller, such as the 1,003-square mile (2,600-km2) Alboran Sea in the Mediterranean. Different deep-sea plains may also be characterized and distinguished on the basis of their sediment composition, their geometry, depth, and volume and thickness of the sediments they contain. 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 mile (1 km) above the seafloor. For this reason 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 landform 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 carried to the deep sea by turbidity currents or by 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 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 oceanic plateau; ophiolites; passive margin; plate tectonics.

FURTHER READING

Erickson, Jon. Marine Geology; Exploring the New Frontiers of the Ocean. Rev. ed. New York: Facts On File, 2003.

Moores, Eldridge M., and Robert Twiss. Tectonics. New York: W. H. Freeman, 1995.

ocean currents Like the atmosphere, the ocean is constantly in motion. Ocean currents are defined by the movement paths of water in regular courses, driven by the wind and thermohaline forces across the ocean basins. The wind primarily drives shallow currents, but the Coriolis force systematically deflects them to the right of the atmospheric wind directions in the Northern Hemisphere, and to the left of the prevailing winds in the Southern Hemisphere. Therefore, shallow water currents tend to be oriented about 45° from the predominant wind directions.

Thermohaline effects, the movement of water driven by differences in temperature and salinity, primarily drive deep-water currents. The Atlantic and Pacific Ocean basins both show a general clockwise rotation in the Northern Hemisphere, and a counterclockwise spin in the Southern Hemisphere, with the strongest currents in the midlatitude sectors. The pattern in the Indian Ocean is broadly similar but seasonally different and more complex because of the effects of the monsoon. Antarctica is bound on all sides by deep water, and has a major clockwise current surrounding it known as the Antarctic Circumpolar Current, lying between 40° and 60° south. This is a strong current, moving at 1.6-5 feet per second (0.5-1.5 m/s) and has a couple of major gyres in it at the Ross Ice Shelf and near the Antarctic Peninsula. The Arctic Ocean has a complex pattern, because it is sometimes ice-covered and is nearly completely surrounded by land with only one major entry and escape route east of Greenland, called Fram Strait. Circulation patterns in the Arctic ocean are dominated by a slow, 0.4-1.6-inch-per-second (1-4-cm/s) transpolar drift from Siberia to the Fram Strait and by a thermohaline-induced anticyclonic spin known as the Beaufort Gyre that causes ice to pile up on the Greenland and Canadian coasts. Together the two effects in the Arctic ocean bring numerous icebergs into North Atlantic shipping lanes and send much of the cold deep water around Greenland into the North Atlantic ocean basin.

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  • Samira
    Is the rocky mountains divergent or convergent?
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    What are the plates for the rocky mountains?
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    What does the rocky mountain province have to do with plate tectonics?
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    Are the rocky mountains tectonically active?
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