Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics
Green River (Colorado River tributary)
The Green River, located in the western United States, is the chief tributary of the Colorado River. The watershed of the river, known as the Green River Basin, covers parts of Wyoming and Colorado; the Green River is 730 miles long, beginning in the Wind River Mountains of Wyoming and flowing through Wyoming and Utah for most of its course, except for 40 miles into western Colorado. Much of the route is through the Colorado Plateau and through some of the most spectacular canyons in the United States, it is only smaller than the Colorado when the two rivers merge, but carries a larger load of silt. The average yearly mean flow of the river at Green River, Utah is 6,121 cubic feet per second; the status of the Green River as a tributary of the Colorado River came about for political reasons. In earlier nomenclature, the Colorado River began at its confluence with the Green River. Above the confluence the Colorado was called the Grand River. In 1921, Colorado U. S. Representative Edward T. Taylor petitioned the Congressional Committee on Interstate and Foreign Commerce to rename the Grand River as the Colorado River.
On July 25, 1921, the name change was made official in House Joint Resolution 460 of the 66th Congress, over the objections of representatives from Wyoming and Utah and the United States Geological Survey which noted that the drainage basin of the Green River was more extensive than that of the Grand River, although the Grand carried a higher volume of water at its confluence with the Green. It rises in western Wyoming, in northern Sublette County, on the western side of the Continental Divide in the Bridger-Teton National Forest in the Wind River Range, it flows south through Sublette County and western Wyoming in an area known as the Upper Green River Valley southwest and is joined by the Big Sandy River in western Sweetwater County. At the town of La Barge, it flows into Fontenelle Reservoir, formed by Fontenelle Dam. Below there, it flows through open sage covered rolling prairie where it is crossed by the Oregon and Mormon emigration trails and further south until it flows past the town of Green River and into the Flaming Gorge Reservoir in Southwestern Wyoming, formed by the Flaming Gorge Dam in northeastern Utah.
Prior to the creation of the reservoir, the Blacks Fork joined the Green River south of Green River, today the mouth of Blacks Fork is submerged by the reservoir. South of the dam it flows eastward, looping around the eastern tip of the Uinta Mountains going from Utah into northwestern Colorado and through Browns Park before turning west and south into Dinosaur National Monument where it passes through the Canyon of the Lodore and is joined by the Yampa River at Steamboat Rock, it turns westward back into Utah along the southern edge of the Uintas in Whirlpool Canyon. In Utah it meanders southwest across the Yampa Plateau and through the Uintah and Ouray Indian Reservation and the Ouray National Wildlife Refuge. Two miles south of Ouray, Utah, it is joined by Duchesne River, three miles downstream by the White River. Ten miles farther downstream it is joined by the Willow River. South of the plateau, it is joined by Nine Mile Creek enters the Roan Cliffs where it flows south through the back-to-back Desolation and Gray canyons, with a combined length of 120 mi.
In Gray Canyon, it is joined by the Price River. South of the canyon it passes the town of Green River, Utah and is joined by the San Rafael River in southern Emery County. In eastern Wayne County it meanders through Canyonlands National Park; the Flaming Gorge Dam in Utah is a significant regional source of water for irrigation and mining, as well as for hydroelectric power. Begun in the 1950s and finished in 1963, it was controversial and opposed by conservationists. A dam was to be built in Whirlpool Canyon, but the conservationist movement traded the Flaming Gorge dam for halting that proposal. Apocryphally, the Sierra Club, a not-for-profit environmental organization, lost its tax-exempt status for political action in opposing the proposed dam; the Green is a large, powerful river. It ranges from 100 to 300 feet wide in the upper course to 300 to 1,500 feet wide in its lower course and ranges from 3 to 50 feet in depth, it is navigable by small craft throughout its course and by large motorboats upstream to Flaming Gorge Dam.
Near the areas where the Oregon Trail crosses, the river is 400 - 500 feet wide and averages about 20 feet deep at normal flow. Archaeological evidence indicates that the tributary canyons and sheltered areas in the river valley were home to the Fremont Culture, which flourished from the 7th century to the 13th century; the Fremont were a semi-nomadic people who lived in pithouses and are best known for the rock art on canyon walls and in sheltered overhangs. In centuries, the river basin was home to the Shoshone and Ute peoples, both nomadic hunters; the Shoshone inhabited the river valley north of the Uinta Mountains, whereas the Utes lived to the south. The current reservation of the Utes is in the Uintah Basin; the Shoshone called the river the Seeds-kee-dee-Agie, meaning "Prairie Hen River." In 1776, the Spanish friars Silvestre Vélez de Escalante and Francisco Atanasio Domínguez crossed the river near present-day Jensen, naming it the Rio de San Buenaventura. The map-maker of the expedition, Captain Bernardo Miera y Pacheco, erroneously indicated that the river flowed southwest to what is now known as Sevier Lake.
Cartographers extended the error, representing the Buenaventura River as flowing into the Pacific Ocean. At least one charted the Buenaventura as draining the Great Salt Lake. Spanish
Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray depending on their mineralogy; the word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar; the term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic.
A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids; the extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive and tough; these properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
Wyoming is a state in the mountain region of the western United States. The state is the 10th largest by area, the least populous, the second most sparsely populated state in the country. Wyoming is bordered on the north by Montana, on the east by South Dakota and Nebraska, on the south by Colorado, on the southwest by Utah, on the west by Idaho and Montana; the state population was estimated at 577,737 in 2018, less than 31 of the most populous U. S. cities including Denver in neighboring Colorado. Cheyenne is the state capital and the most populous city, with an estimated population of 63,624 in 2017; the western two-thirds of the state is covered by the mountain ranges and rangelands of the Rocky Mountains, while the eastern third of the state is high elevation prairie called the High Plains. Half of the land in Wyoming is owned by the U. S. government, leading Wyoming to rank sixth by area and fifth by proportion of a state's land owned by the federal government. Federal lands include two national parks—Grand Teton and Yellowstone—two national recreation areas, two national monuments, several national forests, historic sites, fish hatcheries, wildlife refuges.
Original inhabitants of the region include the Crow, Arapaho and Shoshone. Southwestern Wyoming was in the Spanish Empire and Mexican territory until it was ceded to the United States in 1848 at the end of the Mexican–American War; the region acquired the name Wyoming when a bill was introduced to the U. S. Congress in 1865 to provide a "temporary government for the territory of Wyoming"; the name was used earlier for the Wyoming Valley in Pennsylvania, is derived from the Munsee word xwé:wamənk, meaning "at the big river flat". The main drivers of Wyoming's economy are mineral extraction—mostly coal, natural gas, trona—and tourism. Agricultural commodities include livestock, sugar beets and wool; the climate is semi-arid and continental and windier than the rest of the U. S. with greater temperature extremes. Wyoming has been a politically conservative state since the 1950s, with the Republican Party candidate winning every presidential election except 1964. Wyoming's climate is semi-arid and continental, is drier and windier in comparison to most of the United States with greater temperature extremes.
Much of this is due to the topography of the state. Summers in Wyoming are warm with July high temperatures averaging between 85 and 95 °F in most of the state. With increasing elevation, this average drops with locations above 9,000 feet averaging around 70 °F. Summer nights throughout the state are characterized by a rapid cooldown with the hottest locations averaging in the 50–60 °F range at night. In most of the state, most of the precipitation tends to fall in early summer. Winters are cold, but are variable with periods of sometimes extreme cold interspersed between mild periods, with Chinook winds providing unusually warm temperatures in some locations. Wyoming is a dry state with much of the land receiving less than 10 inches of rainfall per year. Precipitation depends on elevation with lower areas in the Big Horn Basin averaging 5–8 inches; the lower areas in the North and on the eastern plains average around 10–12 inches, making the climate there semi-arid. Some mountain areas do receive a good amount of precipitation, 20 inches or more, much of it as snow, sometimes 200 inches or more annually.
The state's highest recorded temperature is 114 °F at Basin on July 12, 1900 and the lowest recorded temperature is −66 °F at Riverside on February 9, 1933. The number of thunderstorm days vary across the state with the southeastern plains of the state having the most days of thunderstorm activity. Thunderstorm activity in the state is highest during early summer; the southeastern corner of the state is the most vulnerable part of the state to tornado activity. Moving away from that point and westwards, the incidence of tornadoes drops with the west part of the state showing little vulnerability. Tornadoes, where they occur, tend to be small and brief, unlike some of those that occur farther east; as specified in the designating legislation for the Territory of Wyoming, Wyoming's borders are lines of latitude 41°N and 45°N, longitude 104°3'W and 111°3'W, making the shape of the state a latitude-longitude quadrangle. Wyoming is one of only three states to have borders along only straight latitudinal and longitudinal lines, rather than being defined by natural landmarks.
Due to surveying inaccuracies during the 19th century, Wyoming's legal border deviates from the true latitude and longitude lines by up to half of a mile in some spots in the mountainous region along the 45th parallel. Wyoming is bordered on the north by Montana, on the east by South Dakota and Nebraska, on the south by Colorado, on the southwest by Utah, on the west by Idaho, it is the tenth largest state in the United States in total area, containing 97,814 square miles and is made up of 23 counties. From the north border to the south border it is 276 miles; the Great Plains meet the Rocky Mountains in Wyoming. The state is a great plateau broken by many mountain ranges. Surface elevations range from the summit of Gannett Peak in the Wind River Mountain Range, at 13,804 feet, to the Belle Fourche River val
Pine nuts called piñón or pinoli, are the edible seeds of pines. About 20 species of pine produce seeds large enough to be worth harvesting. In Europe and places with a Mediterranean climate, two species in particular are harvested. Four other species, Siberian pine, Siberian dwarf pine, Chinese white pine and lacebark pine, are used to a lesser extent. Russia is the largest producer of Pinus sibirica nuts in the world followed by Mongolia which produces over 10,000 tonnes of forest grown nuts annually, the majority of harvest is exported to China. Afghanistan is an important source of pine nuts, behind Korea. Pine nuts produced in Europe come from the stone pine, cultivated for its nuts for over 5,000 years, harvested from wild trees for far longer; the Swiss pine is used to a small extent. In North America, the main species are three of the pinyon pines: Colorado pinyon, single-leaf pinyon, Mexican pinyon; the other eight pinyon species are used to a small extent, as are gray pine, Coulter pine, Torrey pine, sugar pine and Parry pinyon.
Here, the nuts themselves are known by the Spanish name for the pinyon pine: piñón. In the United States, pine nuts are harvested by Native American and Hispano communities in the Western United States and Southwestern United States, by the Uto-Aztecan Shoshone, Navajo, Hopi and Hispanos of New Mexico. Certain treaties negotiated by tribes and laws in Nevada guarantee Native Americans' right to harvest pine nuts, the state of New Mexico protects the use of the word piñon for use with pine nuts from certain species of indigenous New Mexican pines. For those seeking to grow edible landscapes, these are the more used species. Old World Pinus armandii – Chinese white pine Pinus bungeana – lacebark pine Pinus cembra – Swiss pine Pinus gerardiana – Chilgoza pine Pinus koraiensis – Korean pine Pinus pinea – Mediterranean stone pine Pinus pumila – Siberian dwarf pine Pinus sibirica – Siberian pine New World Pinus cembroides – Mexican pinyon Pinus culminicola – Potosi pinyon Pinus edulis – Two-needle piñon or Colorado pinyon Pinus johannis – Johann's pinyon Pinus monophylla – Single-leaf pinyon Pinus orizabensis – Orizaba pinyon Pinus quadrifolia – Parry pinyon Pinus remota – Texas pinyon or papershell pinyo The pine nut species will take a time that depends on the exact species to complete its maturity.
For some American species development begins in early spring with pollination. A tiny cone, about the size of a small marble, will form from mid-spring to the end of summer; the cone will commence growth until it reaches maturity near the end of summer. The mature piñon pine cone is ready to harvest ten days. A cone is harvested by placing it in a burlap bag and exposing it to a heat source such as the sun to begin the drying process, it takes about 20 days until the cone opens. Once it is open and dry, the seed can be extracted in various ways; the most common and practical extraction method used is the repeated striking of the burlap bag containing the cone against a rough surface to cause the cone to shatter, leaving just the job of separating by hand the seed from the residue within the bag. Another option for harvesting is to wait until the cone opens on the tree and harvest the cone from the piñon pine, followed by the extracting process mentioned above. Fallen seed can be gathered beneath the trees.
In the United States, millions of hectares of productive pinyon pine woods have been destroyed due to conversion of lands, in China, destructive harvesting techniques and the removal of trees for timber have led to losses in production capacity. The elevation of the pinyon pine is an important determinant of the quantity of pine cone production, therefore, will determine the amount of pine nuts the tree will yield. American Pinyon pine cone production is most found at an elevation between 6,000 feet and 8,500 feet, ideally at 7,000 feet; this is due to higher temperatures at elevations lower than 6,000 feet during the spring, which dry up humidity and moisture content that provide for the tree throughout the spring and summer, causing little nourishment for pine cone maturity. Although there are several other environmental factors that determine the conditions of the eco-system, without sufficient water the trees tend to abort cones. High humidity encourages cone development. There are certain topographical areas found in lower elevations, such as shaded canyons, where the humidity remains constant throughout the spring and summer, allowing pine cones to mature and produce seed.
At elevations above 8,500 feet, the temperature drops, dras
Indigenous peoples of the Great Basin
The Indigenous Peoples of the Great Basin are Native Americans of the northern Great Basin, Snake River Plain, upper Colorado River basin. The "Great Basin" is a cultural classification of indigenous peoples of the Americas and a cultural region located between the Rocky Mountains and the Sierra Nevada, in what is now Nevada, parts of Oregon, Idaho and Utah; the Great Basin region at the time of European contact was ~400,000 sq mi. There is little precipitation in the Great Basin area which affects the lifestyles and cultures of the inhabitants. Original inhabitants of the region may have arrived by 12,000 BCE. 9,000 BCE to 400 CE marks the Great Basin Desert Archaic Period, following by the time of the Fremont culture, who were hunter-gatherers, as well as agriculturalists. Numic language-speakers, ancestors of today's Western Shoshone and both Northern and Southern Paiute peoples entered the region around the 14th century CE; the first Europeans to reach the area was the Spanish Dominguez-Escalante Expedition, who passed far from present day Delta, Utah in 1776.
Great Basin settlement was free of non-Native settlers until the first Mormon settlers arrived in 1847. Within ten years, the first Indian reservation was established, in order to assimilate the native population; the Goshute Reservation was created in 1863. The attempted acculturation process included sending children to Indian schools and limiting the landbases and resources of the reservations; because their contact with European-Americans and African-Americans occurred comparatively late, Great Basin tribes maintain their religion and culture and were leading proponents of 19th century cultural and religious renewals. Two Paiute prophets and Wovoka, introduced the Ghost Dance in a ceremony to commune with departed loved ones and bring renewal of buffalo herds and precontact lifeways; the Ute Bear Dance emerged on the Great Basin. The Sun Dance and Peyote religion flourished in the Great Basin, as well. In 1930, the Ely Shoshone Reservation was established, followed by the Duckwater Indian Reservation in 1940.
Conditions for the Native American population of the Great Basin were erratic throughout the 20th century. Economic improvement emerged as a result of President Franklin Roosevelt's Indian New Deal in the 1930s, while activism and legal victories in the 1970s have improved conditions significantly; the communities continue to struggle against chronic poverty and all of the resulting problems: unemployment. Today self-determination, beginning with the 1975 passage of the Indian Self-determination and Education Assistance Act, has enabled Great Basin tribes to develop economic opportunities for their members. Different ethnic groups of Great Basin tribes share certain common cultural elements that distinguish them from surrounding groups. All but the Washoe traditionally speak Numic languages, tribal groups, who lived peacefully and shared common territories, have intermingled considerably. Prior to the 20th century, Great Basin peoples were predominantly gatherers. "Desert Archaic" or more "The Desert Culture" refers to the culture of the Great Basin tribes.
This culture is characterized by the need for mobility to take advantage of seasonally available food supplies. The use of pottery was rare due to its weight, but intricate baskets were woven for containing water, cooking food, winnowing grass seeds and storage—including the storage of pine nuts, a Paiute-Shoshone staple. Heavy items such as metates would be cached rather than carried from foraging area to foraging area. Agriculture was not practiced within the Great Basin itself, although it was practiced in adjacent areas; the Great Basin tribes had no permanent settlements, although winter villages might be revisited winter after winter by the same group of families. In the summer, the largest group was the nuclear family due to the low density of food supplies. In the early historical period the Great Basin tribes were expanding to the north and east, where they developed a horse-riding bison-hunting culture; these people, including the Bannock and Eastern Shoshone share traits with Plains Indians.
Great Basin Native Artists, a collective of indigenous artists from the Great Basin Great Basin artwork in Infinity of Nations, National Museum of the American Indian
A cirque is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are cwm. A cirque may be a shaped landform arising from fluvial erosion; the concave shape of a glacial cirque is open on the downhill side, while the cupped section is steep. Cliff-like slopes, down which ice and glaciated debris combine and converge, form the three or more higher sides; the floor of the cirque ends up bowl-shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens: hence it experiences somewhat greater erosion forces, is most overdeepened below the level of the cirque's low-side outlet and its down slope valley. If the cirque is subject to seasonal melting, the floor of the cirque most forms a tarn behind a dam which marks the downstream limit of the glacial overdeepening: the dam itself can be composed of moraine, glacial till, or a lip of the underlying bedrock; the fluvial cirque or makhtesh, found in karst landscapes, is formed by intermittent river flow cutting through layers of limestone and chalk leaving sheer cliffs.
A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more eroded. Glacial cirques are found amongst mountain ranges throughout the world. Situated high on a mountainside near the firn line, they are partially surrounded on three sides by steep cliffs; the highest cliff is called a headwall. The fourth side forms the lip, threshold or sill, the side at which the glacier flowed away from the cirque. Many glacial cirques contain tarns dammed by a bedrock threshold; when enough snow accumulates it can flow out the opening of the bowl and form valley glaciers which may be several kilometers long. Cirques form in conditions; these areas are sheltered from heat. The process of nivation follows, whereby a hollow in a slope may be enlarged by ice segregation weathering and glacial erosion. Ice segregation erodes the rock vertical rock face and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier.
This hollow may become large enough that glacial erosion intensifies. The enlarging of this open ended concavity creates a larger leeward deposition zone, furthering the process of glaciation. Debris in the ice may abrade the bed surface; the hollow may become a large bowl shape in the side of the mountain, with the headwall being weathered by ice segregation, as well as being eroded by plucking. The basin will become deeper as it continues to be eroded by ice abrasion. Should ice segregation and abrasion continue, the dimensions of the cirque will increase, but the proportion of the landform would remain the same. A bergschrund forms when the movement of the glacier separates the moving ice from the stationary ice forming a crevasse; the method of erosion of the headwall lying between the surface of the glacier and the cirque’s floor has been attributed to freeze-thaw mechanisms. The temperature within the bergschrund changes little, studies have shown that ice segregation may happen with only small changes in temperature.
Water that flows into the bergschrund can be cooled to freezing temperatures by the surrounding ice allowing freeze-thaw mechanisms to occur. If two adjacent cirques erode toward one another, an arête, or steep sided ridge, forms; when three or more cirques erode toward one another, a pyramidal peak is created. In some cases, this peak will be made accessible by one or more arêtes; the Matterhorn in the European Alps is an example of such a peak. Where cirques form one behind the other, a cirque stairway results as at the Zastler Loch in the Black Forest; as glaciers can only originate above the snowline, studying the location of present-day cirques provides information on past glaciation patterns and on climate change. Although a less common usage, the term cirque is used for amphitheatre-shaped, fluvial-erosion features. For example, an 200 square kilometres anticlinal erosion cirque is at 30°35′N 34°45′E on the southern boundary of the Negev highlands; this erosional cirque or makhtesh was formed by intermittent river flow in the Makhtesh Ramon cutting through layers of limestone and chalk, resulting in cirque walls with a sheer 200 metres drop.
The Cirque du Bout du Monde is another such a feature, created in karst terraine in the Burgundy region of the department of Côte-d'Or in France. Yet another type of fluvial erosion formed cirque is found on Réunion island, which includes the tallest volcanic structure in the Indian Ocean; the island consists of an active shield-volcano and an extinct eroded volcano. Three cirques have eroded there in a sequence of agglomerated, fragmented rock and volcanic breccia associated with pillow-lavas overlain by more coherent, solid lavas. A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant