The Arctic Ocean is the smallest and shallowest of the world's five major oceans. The International Hydrographic Organization recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean, it is seen as the northernmost part of the all-encompassing World Ocean. Located in the Arctic north polar region in the middle of the Northern Hemisphere, the Arctic Ocean is completely surrounded by Eurasia and North America, it is covered by sea ice throughout the year and completely in winter. The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes; the summer shrinking of the ice has been quoted at 50%. The US National Snow and Ice Data Center uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years. Human habitation in the North American polar region goes back at least 50,000–17,000 years ago, during the Wisconsin glaciation.
At this time, falling sea levels allowed people to move across the Bering land bridge that joined Siberia to north west North America, leading to the Settlement of the Americas. Paleo-Eskimo groups included the Pre-Dorset; the Dorset were the last major Paleo-Eskimo culture in the Arctic before the migration east from present-day Alaska of the Thule, the ancestors of the modern Inuit. The Thule Tradition lasted from about 200 B. C. to 1600 A. D. around the Bering Strait, the Thule people being the prehistoric ancestors of the Inuit who now live in Northern Labrador. For much of European history, the north polar regions remained unexplored and their geography conjectural. Pytheas of Massilia recorded an account of a journey northward in 325 BC, to a land he called "Eschate Thule", where the Sun only set for three hours each day and the water was replaced by a congealed substance "on which one can neither walk nor sail", he was describing loose sea ice known today as "growlers" or "bergy bits". Early cartographers were unsure whether to draw the region around the North Pole as water.
The fervent desire of European merchants for a northern passage, the Northern Sea Route or the Northwest Passage, to "Cathay" caused water to win out, by 1723 mapmakers such as Johann Homann featured an extensive "Oceanus Septentrionalis" at the northern edge of their charts. The few expeditions to penetrate much beyond the Arctic Circle in this era added only small islands, such as Novaya Zemlya and Spitzbergen, though since these were surrounded by pack-ice, their northern limits were not so clear; the makers of navigational charts, more conservative than some of the more fanciful cartographers, tended to leave the region blank, with only fragments of known coastline sketched in. This lack of knowledge of what lay north of the shifting barrier of ice gave rise to a number of conjectures. In England and other European nations, the myth of an "Open Polar Sea" was persistent. John Barrow, longtime Second Secretary of the British Admiralty, promoted exploration of the region from 1818 to 1845 in search of this.
In the United States in the 1850s and 1860s, the explorers Elisha Kane and Isaac Israel Hayes both claimed to have seen part of this elusive body of water. Quite late in the century, the eminent authority Matthew Fontaine Maury included a description of the Open Polar Sea in his textbook The Physical Geography of the Sea; as all the explorers who travelled closer and closer to the pole reported, the polar ice cap is quite thick, persists year-round. Fridtjof Nansen was the first to make a nautical crossing of the Arctic Ocean, in 1896; the first surface crossing of the ocean was led by Wally Herbert in 1969, in a dog sled expedition from Alaska to Svalbard, with air support. The first nautical transit of the north pole was made in 1958 by the submarine USS Nautilus, the first surface nautical transit occurred in 1977 by the icebreaker NS Arktika. Since 1937, Soviet and Russian manned drifting ice stations have extensively monitored the Arctic Ocean. Scientific settlements were established on the drift ice and carried thousands of kilometers by ice floes.
In World War II, the European region of the Arctic Ocean was contested: the Allied commitment to resupply the Soviet Union via its northern ports was opposed by German naval and air forces. Since 1954 commercial airlines have flown over the Arctic Ocean; the Arctic Ocean occupies a circular basin and covers an area of about 14,056,000 km2 the size of Antarctica. The coastline is 45,390 km long, it is surrounded by the land masses of Eurasia, North America, by several islands. It is taken to include Baffin Bay, Barents Sea, Beaufort Sea, Chukchi Sea, East Siberian Sea, Greenland Sea, Hudson Bay, Hudson Strait, Kara Sea, Laptev Sea, White Sea and other tributary bodies of water
The Phanerozoic Eon is the current geologic eon in the geologic time scale, the one during which abundant animal and plant life has existed. It covers 541 million years to the present, began with the Cambrian Period when animals first developed hard shells preserved in the fossil record, its name was derived from the Ancient Greek words φανερός and ζωή, meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick; the time before the Phanerozoic, called the Precambrian, is now divided into the Hadean and Proterozoic eons. The time span of the Phanerozoic starts with the sudden appearance of fossilized evidence of a number of animal phyla. Plant life on land appeared in the early Phanerozoic eon. During this time span, tectonic forces caused the continents to move and collect into a single landmass known as Pangaea, which separated into the current continental landmasses.
The Proterozoic-Phanerozoic boundary is at 541 million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal fossils but several hundred groups of metazoa of the earlier Proterozoic era have been identified since the systematic study of those forms started in the 1950s. Most geologists and paleontologists would set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef-building animals such as corals and others appear; the three different dividing points are within a few million years of each other. In the older literature, the term Phanerozoic is used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature; the Phanerozoic is divided into three eras: the Paleozoic and Cenozoic, which are further subdivided into 12 periods. The Paleozoic features the rise of fish and reptiles; the Mesozoic is ruled by the reptiles, features the evolution of mammals and more famously, dinosaurs.
The Cenozoic is the time of the mammals, more humans. The Paleozoic is a time in Earth's history when complex life forms evolved, took their first breath of oxygen on dry land, when the forerunners of all life on Earth began to diversify. There are six periods in the Paleozoic era: Cambrian, Silurian, Devonian and Permian; the Cambrian starts from 541 to 485 million years ago. The Cambrian sparked a rapid expansion in evolution in an event known as the Cambrian Explosion during which the greatest number of creatures evolved in a single period in the history of Earth. Plants like algae evolved, the fauna was dominated by armored arthropods, such as trilobites. All marine phyla evolved in this period. During this time, the super-continent Pannotia began to break up, most of which recombined into the super-continent Gondwana; the Ordovician spans from 485 million years to 444 million years ago. The Ordovician was a time in Earth's history in which many species still prevalent today evolved, such as primitive fish and coral.
The most common forms of life, were trilobites and shellfish. More the first arthropods crept ashore to colonize Gondwana, a continent empty of animal life. By the end of the Ordovician, Gondwana had moved from the equator to the South Pole, Laurentia had collided with Baltica, closing the Iapetus Ocean; the glaciation of Gondwana resulted in a major drop in sea level, killing off all life that had established along its coast. Glaciation caused a snowball Earth, leading to the Ordovician-Silurian extinction, during which 60% of marine invertebrates and 25% of families became extinct; this is considered the second deadliest in the history of Earth. The Silurian spans from 444 million years to 419 million years ago, which saw a warming from Snowball Earth; this period saw the mass evolution of fish, as jaw-less fish became more numerous, jawed fish evolved, the first freshwater fish evolved, though arthropods, such as sea scorpions, remained the apex predators. Terrestrial life evolved, which included early arachnids and centipedes.
The evolution of vascular plants allowed plants to gain a foothold on land. These early terrestrial plants are the forerunners of all plant life on land. During this time, there were four continents: Gondwana, Laurentia and Siberia; the recent rise in sea levels provided new habitats for many new species. The Devonian spans from 419 million years to 359 million years ago. Informally known as the "Age of the Fish", the Devonian features a huge diversification in fish, including armored fish like Dunkleosteus and lobe-finned fish which evolved into the first tetrapods. On land, plant groups diversified. By the Middle Devonian, shrub-like forests of primitive plants existed: lycophytes, horsetails and progymnosperm; this event allowed the diversification of arthropod life as they took advantage of the new
The Mississippi River is the second-longest river and chief river of the second-largest drainage system on the North American continent, second only to the Hudson Bay drainage system. Its source is Lake Itasca in northern Minnesota and it flows south for 2,320 miles to the Mississippi River Delta in the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 32 U. S. two Canadian provinces between the Rocky and Appalachian mountains. The main stem is within the United States; the Mississippi ranks as the fifteenth-largest river by discharge in the world. The river either borders or passes through the states of Minnesota, Iowa, Missouri, Tennessee, Arkansas and Louisiana. Native Americans have lived along its tributaries for thousands of years. Most were hunter-gatherers, but some, such as the Mound Builders, formed prolific agricultural societies; the arrival of Europeans in the 16th century changed the native way of life as first explorers settlers, ventured into the basin in increasing numbers.
The river served first as a barrier, forming borders for New Spain, New France, the early United States, as a vital transportation artery and communications link. In the 19th century, during the height of the ideology of manifest destiny, the Mississippi and several western tributaries, most notably the Missouri, formed pathways for the western expansion of the United States. Formed from thick layers of the river's silt deposits, the Mississippi embayment is one of the most fertile regions of the United States. During the American Civil War, the Mississippi's capture by Union forces marked a turning point towards victory, due to the river's strategic importance to the Confederate war effort; because of substantial growth of cities and the larger ships and barges that replaced steamboats, the first decades of the 20th century saw the construction of massive engineering works such as levees and dams built in combination. A major focus of this work has been to prevent the lower Mississippi from shifting into the channel of the Atchafalaya River and bypassing New Orleans.
Since the 20th century, the Mississippi River has experienced major pollution and environmental problems – most notably elevated nutrient and chemical levels from agricultural runoff, the primary contributor to the Gulf of Mexico dead zone. The word Mississippi itself comes from Misi zipi, the French rendering of the Anishinaabe name for the river, Misi-ziibi. In the 18th century, the river was the primary western boundary of the young United States, since the country's expansion westward, the Mississippi River has been considered a convenient if approximate dividing line between the Eastern and Midwestern United States, the Western United States; this is exemplified by the Gateway Arch in St. Louis and the phrase "Trans-Mississippi" as used in the name of the Trans-Mississippi Exposition, it is common to qualify a regionally superlative landmark in relation to it, such as "the highest peak east of the Mississippi" or "the oldest city west of the Mississippi". The FCC uses it as the dividing line for broadcast call-signs, which begin with W to the east and K to the west, mixing together in media markets along the river.
The Mississippi River can be divided into three sections: the Upper Mississippi, the river from its headwaters to the confluence with the Missouri River. The Upper Mississippi runs from its headwaters to its confluence with the Missouri River at St. Louis, Missouri, it is divided into two sections: The headwaters, 493 miles from the source to Saint Anthony Falls in Minneapolis, Minnesota. The source of the Upper Mississippi branch is traditionally accepted as Lake Itasca, 1,475 feet above sea level in Itasca State Park in Clearwater County, Minnesota; the name "Itasca" was chosen to designate the "true head" of the Mississippi River as a combination of the last four letters of the Latin word for truth and the first two letters of the Latin word for head. However, the lake is in turn fed by a number of smaller streams. From its origin at Lake Itasca to St. Louis, the waterway's flow is moderated by 43 dams. Fourteen of these dams are located above Minneapolis in the headwaters region and serve multiple purposes, including power generation and recreation.
The remaining 29 dams, beginning in downtown Minneapolis, all contain locks and were constructed to improve commercial navigation of the upper river. Taken as a whole, these 43 dams shape the geography and influence the ecology of the upper river. Beginning just below Saint Paul and continuing throughout the upper and lower river, the Mississippi is further controlled by thousands of wing dikes that moderate the river's flow in order to maintain an open navigation channel and prevent the river from eroding its banks; the head of navigation on the Mississippi is the Coon Rapids Dam in Minnesota. Before it was built in 1913, steamboats could go upstream as far as Saint Cloud, depending on river conditions; the uppermost lock and dam on the Upper Mississippi River is the Upper St. Anthony Falls Lock an
North America is a continent within the Northern Hemisphere and all within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, to the southeast by South America and the Caribbean Sea. North America covers an area of about 24,709,000 square kilometers, about 16.5% of the earth's land area and about 4.8% of its total surface. North America is the third largest continent by area, following Asia and Africa, the fourth by population after Asia and Europe. In 2013, its population was estimated at nearly 579 million people in 23 independent states, or about 7.5% of the world's population, if nearby islands are included. North America was reached by its first human populations during the last glacial period, via crossing the Bering land bridge 40,000 to 17,000 years ago; the so-called Paleo-Indian period is taken to have lasted until about 10,000 years ago. The Classic stage spans the 6th to 13th centuries.
The Pre-Columbian era ended in 1492, the transatlantic migrations—the arrival of European settlers during the Age of Discovery and the Early Modern period. Present-day cultural and ethnic patterns reflect interactions between European colonists, indigenous peoples, African slaves and their descendants. Owing to the European colonization of the Americas, most North Americans speak English, Spanish or French, their culture reflects Western traditions; the Americas are accepted as having been named after the Italian explorer Amerigo Vespucci by the German cartographers Martin Waldseemüller and Matthias Ringmann. Vespucci, who explored South America between 1497 and 1502, was the first European to suggest that the Americas were not the East Indies, but a different landmass unknown by Europeans. In 1507, Waldseemüller produced a world map, in which he placed the word "America" on the continent of South America, in the middle of what is today Brazil, he explained the rationale for the name in the accompanying book Cosmographiae Introductio:... ab Americo inventore... quasi Americi terram sive Americam.
For Waldseemüller, no one should object to the naming of the land after its discoverer. He used the Latinized version of Vespucci's name, but in its feminine form "America", following the examples of "Europa", "Asia" and "Africa". Other mapmakers extended the name America to the northern continent, In 1538, Gerard Mercator used America on his map of the world for all the Western Hemisphere; some argue that because the convention is to use the surname for naming discoveries, the derivation from "Amerigo Vespucci" could be put in question. In 1874, Thomas Belt proposed a derivation from the Amerrique mountains of Central America. Marcou corresponded with Augustus Le Plongeon, who wrote: "The name AMERICA or AMERRIQUE in the Mayan language means, a country of perpetually strong wind, or the Land of the Wind, and... the can mean... a spirit that breathes, life itself." The United Nations formally recognizes "North America" as comprising three areas: Northern America, Central America, The Caribbean.
This has been formally defined by the UN Statistics Division. The term North America maintains various definitions in accordance with context. In Canadian English, North America refers to the land mass as a whole consisting of Mexico, the United States, Canada, although it is ambiguous which other countries are included, is defined by context. In the United States of America, usage of the term may refer only to Canada and the US, sometimes includes Greenland and Mexico, as well as offshore islands. In France, Portugal, Romania and the countries of Latin America, the cognates of North America designate a subcontinent of the Americas comprising Canada, the United States, Mexico, Greenland, Saint Pierre et Miquelon, Bermuda. North America has been referred to by other names. Spanish North America was referred to as Northern America, this was the first official name given to Mexico. Geographically the North American continent has many subregions; these include cultural and geographic regions. Economic regions included those formed by trade blocs, such as the North American Trade Agreement bloc and Central American Trade Agreement.
Linguistically and culturally, the continent could be divided into Latin America. Anglo-America includes most of Northern America and Caribbean islands with English-speaking populations; the southern North American continent is composed of two regions. These are the Caribbean; the north of the continent maintains recognized regions as well. In contrast to the common definition of "North America", which encompasses the whole continent, the term "North America" is sometimes used to refer only to Mexico, the United States, Greenland; the term Northern America refers to the northern-most countries and territories of North America: the United States, Bermuda, St. Pierre and Miquelon and Greenland. Although the term does not refer to a unifie
The Great Plains is the broad expanse of flat land, much of it covered in prairie and grassland, that lies west of the Mississippi River tallgrass prairie in the United States and east of the Rocky Mountains in the U. S. and Canada. It embraces: The entirety of the U. S. states of Kansas, North Dakota, South Dakota Parts of the states of Colorado, Minnesota, New Mexico, Oklahoma and Wyoming The southern portions of the Canadian provinces of Alberta and SaskatchewanThe region is known for supporting extensive cattle ranching and dry farming. The Canadian portion of the Plains is known as the Prairies, it covers much of Alberta and southern Saskatchewan, a narrow band of southern Manitoba. Despite covering a small geographic area, the Prairies are home to the majority of each of the three provinces' respective populations; the term "Great Plains" is used in the United States to describe a sub-section of the more vast Interior Plains physiographic division, which covers much of the interior of North America.
It has currency as a region of human geography, referring to the Plains Indians or the Plains States. In Canada the term is little used. There is no region referred to as the "Great Plains" in The Atlas of Canada. In terms of human geography, the term prairie is more used in Canada, the region is known as the Prairie Provinces or "the Prairies." The North American Environmental Atlas, produced by the Commission for Environmental Cooperation, a NAFTA agency composed of the geographical agencies of the Mexican and Canadian governments, uses the "Great Plains" as an ecoregion synonymous with predominant prairies and grasslands rather than as physiographic region defined by topography. The Great Plains ecoregion includes five sub-regions: Temperate Prairies, West-Central Semi-Arid Prairies, South-Central Semi-Arid Prairies, Texas Louisiana Coastal Plains, Tamaulipas-Texas Semi-Arid Plain, which overlap or expand upon other Great Plains designations; the region is about 500 mi east to 2,000 mi north to south.
Much of the region was home to American bison herds until they were hunted to near extinction during the mid/late-19th century. It has an area of 500,000 sq mi. Current thinking regarding the geographic boundaries of the Great Plains is shown by this map at the Center for Great Plains Studies, University of Nebraska–Lincoln; the term "Great Plains", for the region west of about the 96th and east of the Rocky Mountains, was not used before the early 20th century. Nevin Fenneman's 1916 study Physiographic Subdivision of the United States brought the term Great Plains into more widespread usage. Before that the region was invariably called the High Plains, in contrast to the lower Prairie Plains of the Midwestern states. Today the term "High Plains" is used for a subregion of the Great Plains; the Great Plains are the westernmost portion of the vast North American Interior Plains, which extend east to the Appalachian Plateau. The United States Geological Survey divides the Great Plains in the United States into ten physiographic subdivisions: Coteau du Missouri or Missouri Plateau, glaciated – east central South Dakota and eastern North Dakota and northeastern Montana.
The Great Plains consist of a broad stretch of country underlain by nearly horizontal strata extends westward from the 97th meridian west to the base of the Rocky Mountains, a distance of from 300 to 500 miles. It extends northward from the Mexican boundary far into Canada. Although the altitude of the plains increases from 600 or 1,200 ft on the east to 4,000–5,000 or 6,000 feet near the mountains, the local relief is small; the semi-arid climate opens far-reaching views. The plains are by no means a simple unit, they are of various stages of erosional development. They are interrupted by buttes and escarpments, they are broken by valleys. Yet on the whole, a broadly extended surface of moderate relief so prevails that the name, Great Plains, for the region as a whole is well-deserved; the western boundary of the plains is well-defined by the abrupt ascent of the mountains. The eastern boundary of the plains is more climatic than topographic; the line of 20 in. of annual rainfall trends a little east of northward near the 97th meridian.
If a boundary must be drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies. The plains may be described in northern, intermediate and southern sections, in relation to certain peculiar features; the northern section of the Great Plains, north of latitude 44°, including eastern Montana, north-eastern Wyomi
The Acasta Gneiss is a tonalite gneiss in the Slave craton in Northwest Territories, Canada. The rock body is exposed on an island about 300 kilometres north of Yellowknife; the rock of the outcrop was metamorphosed 3.58 to 4.031 billion years ago and is the oldest known intact crustal fragment on Earth. First described in 1989, it was named for the nearby Acasta River east of Great Bear Lake; the Acasta outcrop is found in a remote area of the Tłı̨chǫ people land settlement. It is the oldest known exposed rock in the world; the metamorphic rock exposed in the outcrop was a granitoid that formed 4.03 billion years ago, an age based on radiometric dating of zircon crystals at 4.031 Ga. The Acasta Gneiss is important in establishing the early history of the continental crust. Acasta Gneiss was formed in the Basin Groups unofficial period of the Hadean eon, which came before the Archean: see Timetable of the Precambrian. In 2008 an age of 4.28 billion years was reported for an outcrop in the Nuvvuagittuq Greenstone Belt on the eastern shores of Hudson Bay, 40 kilometres south of Inukjuak, Canada.
However, the dating method used did not involve similar radiometric dating of zircon crystals and it remains somewhat contentious whether the reported date represents the age that the rock itself formed or a residual isotopic signature of older material that melted to form the rock. Mafic rocks from the Nuvvuagittuq Greenstone Belt have recorded isotopic compositions that can only be produced in the Hadean and the complete isotopic study of all the lithologies included in the Nuvvuagittuq Greenstone Belt suggests that it was formed nearly 4.4 billion years ago. In 2003 a team from the Smithsonian Institution collected a four-tonne boulder of Acasta Gneiss for display outside the National Museum of the American Indian in Washington, D. C. Another sample is on display in the Museu de Geociências of the University of Brazil. In October 2016, a sample of Acasta Gneiss was placed on public display at the Clark Planetarium in Salt Lake City, Utah, USA alongside samples of stromatolite and banded iron formation.
In 2006, Peter Skinner and Bert Cervo contributed a small sample of the rock to the Six String Nation project. Part of that material was inlaid into the first fret of Voyageur, the guitar at the heart of the project. Age of the Earth Stern, R. A. Bleeker, W. 1998. Age of the world's oldest rocks refined using Canada's SHRIMP; the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, pp. 27–31