Alaska is a U. S. state in the northwest extremity of North America, just across the Bering Strait from Asia. The Canadian province of British Columbia and territory of Yukon border the state to the east and southeast, its most extreme western part is Attu Island, it has a maritime border with Russia to the west across the Bering Strait. To the north are the Chukchi and Beaufort seas—southern parts of the Arctic Ocean; the Pacific Ocean lies to southwest. It is the largest U. S. state by the seventh largest subnational division in the world. In addition, it is the most sparsely populated of the 50 United States. Half of Alaska's residents live within the Anchorage metropolitan area. Alaska's economy is dominated by the fishing, natural gas, oil industries, resources which it has in abundance. Military bases and tourism are a significant part of the economy; the United States purchased Alaska from the Russian Empire on March 30, 1867, for 7.2 million U. S. dollars at two cents per acre. The area went through several administrative changes before becoming organized as a territory on May 11, 1912.
It was admitted as the 49th state of the U. S. on January 3, 1959. The name "Alaska" was introduced in the Russian colonial period when it was used to refer to the Alaska Peninsula, it was derived from an Aleut-language idiom. It means object to which the action of the sea is directed. Alaska is the northernmost and westernmost state in the United States and has the most easterly longitude in the United States because the Aleutian Islands extend into the Eastern Hemisphere. Alaska is the only non-contiguous U. S. state on continental North America. It is technically part of the continental U. S. but is sometimes not included in colloquial use. S. called "the Lower 48". The capital city, Juneau, is situated on the mainland of the North American continent but is not connected by road to the rest of the North American highway system; the state is bordered by Yukon and British Columbia in Canada, to the east, the Gulf of Alaska and the Pacific Ocean to the south and southwest, the Bering Sea, Bering Strait, Chukchi Sea to the west and the Arctic Ocean to the north.
Alaska's territorial waters touch Russia's territorial waters in the Bering Strait, as the Russian Big Diomede Island and Alaskan Little Diomede Island are only 3 miles apart. Alaska has a longer coastline than all the other U. S. states combined. Alaska is the largest state in the United States by total area at 663,268 square miles, over twice the size of Texas, the next largest state. Alaska is larger than all but 18 sovereign countries. Counting territorial waters, Alaska is larger than the combined area of the next three largest states: Texas and Montana, it is larger than the combined area of the 22 smallest U. S. states. There are no defined borders demarcating the various regions of Alaska, but there are six accepted regions: The most populous region of Alaska, containing Anchorage, the Matanuska-Susitna Valley and the Kenai Peninsula. Rural unpopulated areas south of the Alaska Range and west of the Wrangell Mountains fall within the definition of South Central, as do the Prince William Sound area and the communities of Cordova and Valdez.
Referred to as the Panhandle or Inside Passage, this is the region of Alaska closest to the rest of the United States. As such, this was where most of the initial non-indigenous settlement occurred in the years following the Alaska Purchase; the region is dominated by the Alexander Archipelago as well as the Tongass National Forest, the largest national forest in the United States. It contains the state capital Juneau, the former capital Sitka, Ketchikan, at one time Alaska's largest city; the Alaska Marine Highway provides a vital surface transportation link throughout the area, as only three communities enjoy direct connections to the contiguous North American road system. Designated in 1963; the Interior is the largest region of Alaska. Fairbanks is the only large city in the region. Denali National Park and Preserve is located here. Denali is the highest mountain in North America. Southwest Alaska is a sparsely inhabited region stretching some 500 miles inland from the Bering Sea. Most of the population lives along the coast.
Kodiak Island is located in Southwest. The massive Yukon–Kuskokwim Delta, one of the largest river deltas in the world, is here. Portions of the Alaska Peninsula are considered part of Southwest, with the remaining portions included with the Aleutian Islands; the North Slope is tundra peppered with small villages. The area is known for its massive reserves of crude oil, contains both the National Petroleum Reserve–Alaska and the Prudhoe Bay Oil Field; the city of Utqiagvik known as Barrow, is the northernmost city in the United States and is located here. The Northwest Arctic area, anchored by Kotzebue and containing the Kobuk River valley, is regarded as being part of this region. However, the respective Inupiat of the No
The Appalachian National Scenic Trail known as the Appalachian Trail or the A. T. is a marked hiking trail in the Eastern United States extending between Springer Mountain in Georgia and Mount Katahdin in Maine. The trail is about 2,200 miles long, though the exact length changes over time as parts are modified or rerouted; the Appalachian Trail Conservancy describes the Appalachian Trail as the longest hiking-only trail in the world. More than 2 million people are said to take a hike on part of the trail at least once each year; the idea of the Appalachian Trail came about in 1921. The trail itself was completed in 1937 after more than a decade of work, although improvements and changes continue, it is maintained by 31 trail clubs and multiple partnerships, managed by the National Park Service, United States Forest Service, the nonprofit Appalachian Trail Conservancy. Most of the trail is in forest or wild lands, although some portions traverse towns and farms, it passes through 14 states: Georgia, North Carolina, Virginia, West Virginia, Pennsylvania, New Jersey, New York, Massachusetts, New Hampshire, Maine.
Thru-hikers attempt to hike the trail in its entirety in a single season. The number of thru-hikes per year has increased with 715 northbound and 133 southbound thru-hikes reported for 2017. Many books, documentaries and fan organizations are dedicated to the pursuit; some hike from one end to the other turn around and thru-hike the trail the other way, known as a "yo-yo". An extension known as the International Appalachian Trail continues northeast, crossing Maine and cutting through Canada to Newfoundland, with sections continuing in Greenland, through Europe, into Morocco. Other separate extensions continue the southern end of the Appalachian range in Alabama and continue south into Florida, creating what is known as the Eastern Continental Trail; the Appalachian Trail, the Continental Divide Trail, the Pacific Crest Trail form what is known as the Triple Crown of Hiking in the United States. The trail was conceived by Benton MacKaye, a forester who wrote his original plan—called "An Appalachian Trail, A Project in Regional Planning"—shortly after the death of his wife in 1921.
MacKaye's idea detailed a grand trail that would connect a series of farms and wilderness work/study camps for city-dwellers. In 1922, at the suggestion of Major William A. Welch, director of the Palisades Interstate Park Commission, his idea was publicized by Raymond H. Torrey with a story in the New York Evening Post under a full-page banner headline reading "A Great Trail from Maine to Georgia!" The idea was adopted by the new Palisades Interstate Park Trail Conference as their main project. On October 7, 1923, the first section of the trail, from Bear Mountain west through Harriman State Park to Arden, New York, was opened. MacKaye called for a two-day Appalachian Trail conference to be held in March 1925 in Washington, D. C; this meeting inspired the formation of the Appalachian Trail Conference. A retired judge named Arthur Perkins and his younger associate Myron Avery took up the cause. In 1929, a member of the Connecticut Forest and Park Association and its Blue Blazed Trails committee, found Ned Anderson, a farmer in Sherman, who took on the task of mapping and blazing the Connecticut leg of the trail.
It ran from Dog Tail Corners in Webatuck, New York, which borders Kent, Connecticut, at Ashley Falls, 50 miles through the northwest corner of the state, up to Bear Mountain at the Massachusetts border. Anderson's efforts helped spark renewed interest in the trail, Avery was able to bring other states on board. Upon taking over the ATC, Avery adopted the more practical goal of building a simple hiking trail, he and MacKaye clashed over the ATC's response to a major commercial development along the trail's path. Avery reigned as Chairman of the ATC from 1932 to 1952. Avery became the first to walk the trail end-to-end, though not as a thru-hike, in 1936. In August 1937, the trail was completed to Sugarloaf Mountain in Maine, the ATC shifted its focus toward protecting the trail lands and mapping the trail for hikers. Many of the trail's present highlights were not part of the trail in 1937: Roan Mountain, North Carolina and Tennessee. Except for places where the Civilian Conservation Corps was brought in, the original trail climbed straight up and down mountains, creating rough hiking conditions and a treadway prone to severe erosion.
The ATC's trail crews and volunteer trail-maintaining clubs have relocated or rehabilitated miles of trail since that time. In 1936, a 121-day Maine to Georgia veteran's group funded and supported thru-hike was reported to have been completed, with all but three miles of the new trail cleared and blazed, by six Boy Scouts from New York City and their guides; the completed thru-hike was much recorded and accepted by the Appalachian Long Distance Hikers Association. In 1938, the trail sustained major damage from a hurricane; this happened right before the start of World War II
An avalanche is an event that occurs when a cohesive slab of snow lying upon a weaker layer of snow fractures and slides down a steep slope. Avalanches are triggered in a starting zone from a mechanical failure in the snowpack when the forces of the snow exceed its strength but sometimes only with gradual widening. After initiation, avalanches accelerate and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough, some of the snow may mix with the air forming a powder snow avalanche, a type of gravity current. Slides of rocks or debris, behaving in a similar way to snow, are referred to as avalanches; the remainder of this article refers to snow avalanches. The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches initiated by this process are known as spontaneous avalanches. Avalanches can be triggered by other loading conditions such as human or biologically related activities.
Seismic activity may trigger the failure in the snowpack and avalanches. Although composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks and other surficial material. However, they are distinct from slushflows which have higher water content and more laminar flow, mudslides which have greater fluidity, rock slides which are ice free, serac collapses during an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds. There is no universally accepted classification system for different forms of avalanches. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.
Most avalanches occur spontaneously during storms under increased load due to snowfall. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Other natural causes include rain, earthquakes and icefall. Artificial triggers of avalanches include skiers and controlled explosive work. Contrary to popular belief, avalanches are not triggered by loud sound. Avalanche initiation can start at a point with only a small amount of snow moving initially. However, if the snow has sintered into a stiff slab overlying a weak layer fractures can propagate rapidly, so that a large volume of snow, that may be thousands of cubic meters, can start moving simultaneously. A snowpack will fail; the load is straightforward. However, the strength of the snowpack is much more difficult to determine and is heterogeneous, it varies in detail with properties of the snow grains, density, temperature, water content. These properties may all metamorphose in time according to the local humidity, water vapour flux and heat flux.
The top of the snowpack is extensively influenced by incoming radiation and the local air flow. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time. A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, layering of the seasonal snowpack. Slab avalanches form in snow, deposited, or redeposited by wind, they have the characteristic appearance of a block of snow cut out from its surroundings by fractures. Elements of slab avalanches include the following: a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, a fracture at the bottom called the stauchwall; the crown and flank fractures are vertical walls in the snow delineating the snow, entrained in the avalanche from the snow that remained on the slope. Slabs can vary in thickness from a few centimetres to three metres.
Slab avalanches account for around 90% of avalanche-related fatalities in backcountry users. The largest avalanches form turbulent suspension currents known as powder snow avalanches or mixed avalanches; these consist of a powder cloud. They can form from any type of snow or initiation mechanism, but occur with fresh dry powder, they can exceed speeds of 300 kilometres per hour, masses of 10000000 tonnes. In contrast to powder snow avalanches, wet snow avalanches are a low velocity suspension of snow and water, with the flow confined to the track surface; the low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow. Despite the low speed of travel, wet snow avalanches are capable of generating powerful destructive forces, due to the large mass and density; the body of the flow of a wet snow avalanche can plough through soft snow, can scour boulders, earth and other vegetation.
Tierra del Fuego
Tierra del Fuego is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of the main island, Isla Grande de Tierra del Fuego, with an area of 48,100 km2, a group of many islands, including Cape Horn and Diego Ramírez Islands. Tierra del Fuego is divided between Chile and Argentina, with the latter controlling the eastern half of the main island and the former the western half plus the islands south of Beagle Channel; the southernmost extent of the archipelago is at about latitude 55 S. The earliest known human settlement in Tierra del Fuego dates to around 8,000 BCE. Europeans first explored the islands during Ferdinand Magellan's expedition of 1520. Settlement by those of European descent and the great displacement of the native populations did not begin until the second half of the 19th century, at the height of the Patagonian sheep farming boom and of the local gold rush. Today, petroleum extraction dominates economic activity in the north of Tierra del Fuego, while tourism and Antarctic logistics are important in the south.
The earliest human settlement occurred around 8,000 BCE. The Yaghan were some of the earliest known humans to settle in Tierra del Fuego. Archeological sites with characteristics of their culture have been found at locations such as Navarino Island; the name Tierra del Fuego was given by the Portuguese explorer Ferdinand Magellan while sailing for the Spanish Crown in 1520. He believed he was seeing the many fires of the Yaghan, which were visible from the sea, that the "Indians" were waiting in the forests to ambush his armada. In 1525 Francisco de Hoces was the first to speculate that Tierra del Fuego was one or more islands rather than part of what was called Terra Australis. Francis Drake in 1578 and a Dutch East India Company expedition in 1616 learned more about the geography; the latter expedition named Cape Horn. On his first voyage with HMS Beagle in 1830, Robert FitzRoy picked up four native Fuegians, including "Jemmy Button" and brought them to England; the surviving three were taken to London to meet the King and Queen and were, for a time, celebrities.
They returned to Tierra del Fuego in Beagle with FitzRoy and Charles Darwin, who made extensive notes about his visit to the islands. During the second half of the 19th century, the archipelago began to come under Chilean and Argentine influence. Both countries sought to claim the whole archipelago based on de jure Spanish colonial titles. Salesian Catholic missions were established in Dawson Island. Anglican missions were established by British colonists at Keppel Island in the Falklands in 1855 and in 1870 at Ushuaia on the main island, which continued to operate through the 19th century. Thomas Bridges learned the language and compiled a 30,000-word Yaghan grammar and dictionary while he worked at Ushuaia, it was considered an important ethnological work. An 1879 Chilean expedition led by Ramón Serrano Montaner reported large amounts of placer gold in the streams and river beds of Tierra del Fuego; this prompted massive immigration to the main island between 1883 and 1909. Numerous Argentines and Croatians settled in the main island, leading to increased conflicts with native Selk'nam.
Julius Popper, a Romanian explorer, was one of the most successful entrepreneurs in the region. Granted rights by the Argentine government to exploit any gold deposits he found in Tierra del Fuego, Popper has been identified as a central figure in the Selk'nam genocide. Following contact with Europeans, the native Selk'nam and Yaghan populations were reduced by unequal conflict and persecution by settlers, by infectious diseases to which the indigenous people had no immunity, by mass transfer to the Salesian mission of Dawson Island. Despite the missionaries' efforts, many natives died. Today, only a few Selk'nam remain; some of the few remaining Yaghan have settled in Villa Ukika in Navarino Island. Following the signing of the Boundary Treaty of 1881, Tierra del Fuego was divided between Argentina and Chile; the gold rushes of the late 19th century led to the founding of numerous small settlements by immigrants such as the Argentine settlements of Ushuaia and Río Grande and the Chilean settlements of Porvenir and Puerto Toro.
In 1945 a division of Chilean CORFO engaged in oil exploration made a breakthrough discovery of oil in northern Tierra del Fuego. Extraction began in 1949, in 1950 the state created ENAP to deal with oil extraction and prospecting; until 1960, most oil extracted in Chile came from Tierra del Fuego. During the 1940s Chile and Argentina formulated their Antarctic claims; the governments realized the key role of Tierra del Fuego's geographical proximity in backing their claims as well as in supplying their Antarctic bases. In the 1950s, the Chilean military founded Puerto Williams to counter Ushuaia's monopoly as the only settlement in the Beagle Channel, a zone where Argentina disputed the 1881 borders. In the 1960s and 1970s, sovereignty claims by Argentina over Picton and Nueva Islands in Tierra del Fuego led the two countries in December 1978 to the brink of war. In response to the threat of an Argentine invasion, minefields were deployed and bunkers built on the Chilean side in some areas of Tierra del Fuego.
The threat of war caused the Chilean Pin
Lightning is a violent and sudden electrostatic discharge where two electrically charged regions in the atmosphere temporarily equalize themselves during a thunderstorm. Lightning creates a wide range of electromagnetic radiations from the hot plasma created by the electron flow, including visible light in the form of black-body radiation. Thunder is the sound formed by the shock wave formed as gaseous molecules experience a rapid pressure increase; the three main kinds of lightning are: created either inside one thundercloud, or between two clouds, or between a cloud and the ground. The 15 recognized observational variants include "heat lightning", seen but not heard, dry lightning, which causes many forest fires, ball lightning, observed scientifically. Humans have deified lightning for millennia, lightning inspired expressions like "Bolt from the blue", "Lightning never strikes twice", "blitzkrieg" are common. In some languages, "Love at first sight" translates as "lightning strike"; the details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification.
The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward and temperatures range from −15 to −25 °C, see figure to the right. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets, small ice crystals, graupel; the updraft carries the super-cooled cloud droplets and small ice crystals upward. At the same time, the graupel, larger and denser, tends to fall or be suspended in the rising air; the differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. See figure to the left; the updraft carries. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm; the result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm. In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures. A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km tall, from within the cloud to the ground's surface; the actual discharge is the final stage of a complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere.
However, it can occur during dust storms, forest fires, volcanic eruptions, in the cold of winter, where the lightning is known as thundersnow. Hurricanes generate some lightning in the rainbands as much as 160 km from the center; the science of lightning is called fulminology, the fear of lightning is called astraphobia. Lightning is not distributed evenly around the planet. On Earth, the lightning frequency is 44 times per second, or nearly 1.4 billion flashes per year and the average duration is 0.2 seconds made up from a number of much shorter flashes of around 60 to 70 microseconds. Many factors affect the frequency, distribution and physical properties of a typical lightning flash in a particular region of the world; these factors include ground elevation, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC, CC and CG lightning may vary by season in middle latitudes; because human beings are terrestrial and most of their possessions are on the Earth where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types though IC and CC are more common types of lightning.
Lightning's relative unpredictability limits a complete explanation of how or why it occurs after hundreds of years of scientific investigation. About 70 % of lightning occurs over land in the tropics; this occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, it happens at the boundaries between them. The flow of warm ocean currents past drier land masses, such as the Gulf Stream explains the elevated frequency of lightning in the Southeast United States; because the influence of small or absent land masses in the vast stretches of the world's oceans limits the differences between these variants in the atmosphere, lightning is notably less frequent there than over larger landforms. The North and South Poles are limited in their coverage of thunderstorms and theref
The cougar commonly known by other names including catamount, mountain lion and puma, is a large felid of the subfamily Felinae native to the Americas. Its range, from the Canadian Yukon to the southern Andes of South America, is the widest of any large wild terrestrial mammal in the Western Hemisphere. An adaptable, generalist species, the cougar is found in most American habitat types, it is the biggest cat in North America, the second-heaviest cat in the New World after the jaguar. Secretive and solitary by nature, the cougar is properly considered both nocturnal and crepuscular, although daytime sightings do occur; the cougar is more related to smaller felines, including the domestic cat, than to any species of subfamily Pantherinae, of which only the jaguar is native to the Americas. The cougar is an ambush predator. Primary food sources are ungulates deer, it hunts species as small as insects and rodents. This cat prefers habitats with dense underbrush and rocky areas for stalking, but can live in open areas.
The cougar survives at low population densities. Individual territory sizes depend on terrain and abundance of prey. While large, it is not always the apex predator in its range, yielding prey it has killed to lone jaguars, American black bears, grizzly bears, to groups of gray wolves, it is reclusive and avoids people. Fatal attacks on humans are rare, but have been increasing in North America as more people enter cougar territories. Intensive hunting following European colonization of the Americas and the ongoing human development of cougar habitat has caused populations to drop in most parts of its historical range. In particular, the North American cougar was extirpated in eastern North America in the beginning of the 20th century, except for the isolated Florida panther subpopulation. Transient males have been verified in Minnesota, Iowa, Michigan and Illinois, in at least one instance, observed as far east as coastal Connecticut. Reports of eastern cougars still surface, although it was declared extirpated in 2011.
P. concolor holds the Guinness record for the animal with the greatest number of names, with over 40 in English alone. With its vast range across the length of the Americas, P. concolor has dozens of names and various references in the mythology of the indigenous Americans and in contemporary culture. Scientists refer to it as "puma", as do the populations in 21 of the 23 countries in the Americas; the first English record of "puma" was in 1777, where it had come from the Spanish, who had in turn borrowed it from the Peruvian Quechua language in the 16th century, where it means "powerful". Although "puma" is the common name in Spanish or Portuguese-speaking countries, the cat has many local or regional names in the United States and Canada, of which cougar and mountain lion are popular, it was called gato monte by the early Spanish explorers of the Americas. "Mountain lion" was a term first used in writing in 1858 from the diary of George Andrew Jackson of Colorado. Other names include catamount, mountain screamer, painter.
Lexicographers regard painter as a upper-Southern US regional variant on panther."Cougar" is borrowed from the Portuguese çuçuarana, via French. A current form in Brazil is suçuarana. In the 17th century, German naturalist Georg Marcgrave named the cat the cuguacu ara. Marcgrave's rendering was reproduced in 1648 by his associate, Dutch naturalist Willem Piso. Cuguacu ara was adopted by English naturalist John Ray in 1693; the French naturalist Georges-Louis Leclerc, Comte de Buffon in 1774 converted the cuguacu ara to cuguar, modified to "cougar" in English. Cougars are the largest of the small cats, they are placed in the subfamily Felinae, although their physical characteristics are similar to those of the big cats in the subfamily Pantherinae. The family Felidae is believed to have originated in Asia about 11 million years ago. Taxonomic research on felids remains partial, much of what is known about their evolutionary history is based on mitochondrial DNA analysis, as cats are poorly represented in the fossil record, significant confidence intervals exist with suggested dates.
In the latest genomic study of the Felidae, the common ancestor of today's Leopardus, Puma and Felis lineages migrated across the Bering land bridge into the Americas 8.0 to 8.5 million years ago. The lineages subsequently diverged in that order. North American felids invaded South America 2–4 Mya as part of the Great American Interchange, following formation of the Isthmus of Panama. Linnaeus placed the cougar in the genus which includes the domestic cat; the cougar is now placed in Puma, is most related to the jaguarundi, as well as the modern cheetah of Africa and western Asia, but the relationship is unresolved. The cheetah lineage is suggested by some studies to have diverged from the Puma lineage in the Americas and migrated back to Asia and Africa, while other research suggests the cheetah diverged in the Old World itself. A high level of genetic similarity has been found among North American cougar populations, suggesting they are all recent descendants of a small ancestral group. Culver et al. propose the original North American population of P. concolor was extirpated during the Pleistocene extinctions some 10,000
The term landslide or, less landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients: from mountain ranges to coastal cliffs or underwater, in which case they are called submarine landslides. Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability which produce specific conditions that make a slope prone to failure. In many cases, the landslide is triggered by a specific event, although this is not always identifiable. Landslides occur when the slope undergoes some processes that change its condition from stable to unstable; this is due to a decrease in the shear strength of the slope material, to an increase in the shear stress borne by the material, or to a combination of the two. A change in the stability of a slope can be caused by a number of factors, acting alone.
Natural causes of landslides include: saturation by rain water infiltration, snow melting, or glaciers melting. Slope material that becomes saturated with water may develop into a debris mud flow; the resulting slurry of rock and mud may pick up trees and cars, thus blocking bridges and tributaries causing flooding along its path. Debris flow is mistaken for flash flood, but they are different processes. Muddy-debris flows in alpine areas cause severe damage to structures and infrastructure and claim human lives. Muddy-debris flows can start as a result of slope-related factors and shallow landslides can dam stream beds, resulting in temporary water blockage; as the impoundments fail, a "domino effect" may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid–liquid mixture can reach densities of up to 2,000 kg/m3 and velocities of up to 14 m/s; these processes cause the first severe road interruptions, due not only to deposits accumulated on the road, but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel.
Damage derives from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are destroyed by the impact force of the flow because their span is calculated only for a water discharge. For a small basin in the Italian Alps affected by a debris flow, estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge, was 19 m3/s, a value about 40 times lower than that calculated for the debris flow that occurred. An earthflow is the downslope movement of fine-grained material. Earthflows can move at speeds within a wide range, from as low as 1 mm/yr to 20 km/h. Though these are a lot like mudflows, overall they are more slow moving and are covered with solid material carried along by flow from within, they are different from fluid flows. Clay, fine sand and silt, fine-grained, pyroclastic material are all susceptible to earthflows; the velocity of the earthflow is all dependent on how much water content is in the flow itself: the higher the water content in the flow, the higher the velocity will be.
These flows begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a rolling motion; as these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken; the bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are derived from the slumping at the source. Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material which creates the intrusion of water into the earthflows. Water increases the pore-water pressure a