A mountain pass is a navigable route through a mountain range or over a ridge. Since many of the world's mountain ranges have presented formidable barriers to travel, passes have played a key role in trade and both human and animal migration throughout Earth's history. At lower elevations it may be called a hill pass; the highest vehicle-accessible pass in the world appears to be Mana Pass, located in the Himalayas on the border between India and Tibet, China. Mountain passes make use of a gap, saddle, or col. A topographic saddle is analogous to the mathematical concept of a saddle surface, with a saddle point marking the highest point between two valleys and the lowest point along a ridge. On a topographic map, passes are characterized by contour lines with an hourglass shape, which indicates a low spot between two higher points. Passes are found just above the source of a river, constituting a drainage divide. A pass may be short, consisting of steep slopes to the top of the pass, or may be a valley many kilometres long, whose highest point might only be identifiable by surveying.
Roads have long been built through passes, as well as railways more recently. Some high and rugged passes may have tunnels bored underneath a nearby mountainside to allow faster traffic flow throughout the year; the top of a pass is the only flat ground in the area, is a high vantage point. In some cases this makes it a preferred site for buildings. If a national border follows a mountain range, a pass over the mountains is on the border, there may be a border control or customs station, a military post as well. For instance Argentina and Chile share the world's third-longest international border, 5,300 kilometres long; the border runs north -- south with a total of 42 mountain passes. On a road over a pass, it is customary to have a small roadside sign giving the name of the pass and its elevation above mean sea level; as well as offering easy travel between valleys, passes provide a route between two mountain tops with a minimum of descent. As a result, it is common for tracks to meet at a pass.
Passes traditionally were places for trade routes, cultural exchange, military expeditions etc. A typical example is the Brenner pass in the Alps; some mountain passes above the tree line have problems with snow drift in the winter. This might be alleviated by building the road a few meters above the ground, which will make snow blow off the road. There are many words for pass in the English-speaking world. In the United States, pass is common in the West, the word gap is common in the southern Appalachians, notch in parts of New England, saddle in northern Idaho. Scotland has the Gaelic term bealach. In the Lake District of north-west England, the term hause is used, although the term pass is common—one distinction is that a pass can refer to a route, as well as the highest part thereof, while a hause is that highest part flattened somewhat into a high-level plateau. There are thousands of named passes around the world, some of which are well-known, such as the Great St. Bernard Pass at 2,473 metres in the Alps, the Chang La at 5,360 metres, the Khardung La at 5,359 metres in Jammu and Kashmir, India.
The roads at Mana Pass at 5,610 metres and Marsimik La at 5,582 metres, on and near the China-India border appear to be world's two highest motorable passes. Khunjerab Pass between Pakistan and China at 4,693 metres is a high-altitude motorable mountain pass. Media related to Mountain passes at Wikimedia Commons
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca
Great Basin Divide
The Great Basin Divide in the western United States is the continental divide that separates the Great Basin from the Pacific Ocean watershed. The water divide is the perimeter of the largest set of contiguous endorheic watersheds of North America, including six entire USGS watershed subregions. For example, the San Joaquin River watershed that drains to the Pacific is adjacent to the Great Basin's Mono-Northern Mojave subregion and central Lahontan subregion, their triple point is on the Great Basin Divide
Black Rock Desert
The Black Rock Desert is a semi-arid region, of lava beds and playa, or alkali flats, situated in the Black Rock Desert–High Rock Canyon Emigrant Trails National Conservation Area, a silt playa 100 miles north of Reno, Nevada that encompasses more than 300,000 acres of land and contains more than 120 miles of historic trails. It is in the northern Nevada section of the Great Basin with a lakebed, a dry remnant of Pleistocene Lake Lahontan; the Great Basin, named for the geography in which water is unable to flow out and remains in the basin, is a rugged land serrated by hundreds of mountain ranges, dried by wind and sun, with spectacular skies and scenic landscapes. The average annual precipitation at Gerlach, Nevada is 7.90 inches. The region is notable for its paleogeologic features, as an area of 19th-century Emigrant Trails to California, as a venue for rocketry, as an alternative to the Bonneville Salt Flats in northwestern Utah, for setting land speed records, it is the location for the annual Burning Man event.
The Black Rock Desert is part of the National Conservation Area, a unit of the Bureau of Land Management National Landscape Conservation System. The NCA is located in northwest Nevada, was established by legislation in 2000, it is a unique combination of desert playa, narrow canyons, mountainous areas. Humans have been in Black Rock Desert since 11,000 BC. In 1300 BC the area was settled by the Paiute people; the large black rock formation was used as a landmark by the Paiute and emigrants crossing the area. The landmark is a conical outcrop composed of interbedded Permian marine limestone and volcanic rocks. At its base is a large hot spring and grassy meadow, an important place for those crossing the desert headed for California and Oregon. In 1843, John Fremont and his party were the first white men to cross the desert, his trail was used by over half the 22,000 gold seekers headed to California after 1849. In 1867, Hardin City, a short-lived silver mill town was established; the Black Rock Desert region is in the northwestern Great Basin.
The playa extends for 100 mi northeast from the towns of Gerlach and Empire, between the Jackson Mountains to the east and the Calico Hills to the west. The Black Rock Desert is separated into two arms by the Black Rock Range, it has an area of about 1,000 sq mi. There are several possible definitions of the extent of the Black Rock Desert. People refer just to the playa surface. Sometimes terrain which can be seen from the playa is included; the widest definition of the Black Rock Desert region is the watershed of the basin that drains into the playa. The intermittent Quinn River is the largest river in the region, starting in the Santa Rosa Range and ending in the Quinn River Sink on the playa south of the Black Rock Range; the watershed covers 11,600 sq mi including the Upper and Lower Quinn River, Smoke Creek Desert, Massacre Lake, Thousand Creek/Virgin Valley watersheds of northwestern Nevada as well as small parts across the borders of California and Oregon. If the playa is wet for a month or so, the shallow waters teem with fairy shrimp, or anostraca born of eggs that lie dormant in the silt crust for long periods of time - sometimes for many years.
The edges of the playa and the Quinn River Sink stay wet longer than the rest of the playa, which concentrates the fairy shrimp and migratory birds in those areas. More than 250 species of neo-tropical migrant birds and many other water birds stop in Black Rock-High Rock Country for varying lengths of time; when wet in spring, the playa is a favorite place for these winged visitors to rest and feed. When it rains, the playa can become sticky, bogging down four-wheel-drive vehicles; some areas of the Black Rock are environmentally closed to all vehicles. Humboldt and Washoe Counties of Nevada intersect at the Black Rock Desert; the following mountain ranges are within. The desert has numerous volcanic and geothermal features of the northwest Nevada volcanic region, including two Black Rock Points at the southern end of the Black Rock Range and which have dark Permian volcanic rocks similar to another Permian black diabase dike formation in Nevada; the portion of the Lake Lahontan lakebed in the Black Rock Desert is flat with Lahontan salt shrub vegetation scattered hot springs, a playa.
In areas of the lakebed along mountains, rain shadow results in desert precipitation levels. The playa of the Black Rock Desert lakebed is ~200 sq mi within an area bounded by the Calico Mountains Wilderness, the Applegate National Historic Trail, the Union Pacific Railroad; the "South Playa" is between Gerlach and the southwest boundary of the National Conservation Area, while the northeast NCA portion of the playa is between the NCA boundary and the Applegate National Historic Trail. A Nobles route between Gerlach and Black Rock Hot Springs extends through the length of the playa; the playa's Quinn River Sink of ~3 sq mi is where the Quinn River discharges/evaporates ~2.75 mi south-southwest of Black Rock Hot Springs. Prospecting and mining has occurred in the Black Rock region since the mid-19th century. US Gypsum Corporation operated a gypsum mine and drywall manufacturing plant in Empire, which
Lake Tahoe is a large freshwater lake in the Sierra Nevada of the United States. Lying at 6,225 ft, it straddles the state line between Nevada, west of Carson City. Lake Tahoe is the largest alpine lake in North America, at 122,160,280 acre⋅ft trails only the five Great Lakes as the largest by volume in the United States, its depth is 1,645 ft, making it the second deepest in the United States after Crater Lake in Oregon. The lake was formed about two million years ago as part of the Lake Tahoe Basin, with the modern extent being shaped during the ice ages, it is known for the panorama of surrounding mountains on all sides. The area surrounding the lake is referred to as Lake Tahoe, or Tahoe. More than 75% of the lake's watershed is national forest land, comprising the Lake Tahoe Basin Management Unit of the United States Forest Service. Lake Tahoe is a major tourist attraction in both California, it is home to winter sports, summer outdoor recreation, scenery enjoyed throughout the year. Snow and ski resorts are a significant part of the area's reputation.
The Nevada side offers large casinos, with highways providing year-round access to the entire area. Lake Tahoe is the second deepest lake in the U. S. with a maximum depth of 1,645 feet, trailing Oregon's Crater Lake at 1,949 ft. Tahoe is the 16th deepest lake in the world, the fifth deepest in average depth, it is about 22 mi long and 12 mi wide and has 72 mi of shoreline and a surface area of 191 square miles. The lake is so large. At lake level the opposing shorelines are below the horizon at its widest parts. Visibility may vary somewhat with atmospheric refraction. Fata Morgana may be responsible for Tahoe Tessie sightings. Two-thirds of the shoreline is in California; the south shore is dominated by the lake's largest city, South Lake Tahoe, which adjoins the town of Stateline, while Tahoe City, California, is located on the lake's northwest shore. Although highways run within sight of the lake shore for much of Tahoe's perimeter, many important parts of the shoreline lie within state parks or are protected by the United States Forest Service.
The Lake Tahoe Watershed of 505 sq mi includes the land area that drains to the lake and the Lake Tahoe drainage divide traverses the same general area as the Tahoe Rim Trail. Lake Tahoe is fed by 63 tributaries; these drain an area about the same size as the lake and produce half its water, with the balance entering as rain or snow falling directly on it. The Truckee River is the lake's only outlet, flowing northeast through Reno, into Pyramid Lake which has no outlet, it accounts for one third of the water that leaves the lake, the rest evaporating from the lake's vast surface. The flow of the Truckee River and the height of the lake are controlled by the Lake Tahoe Dam at the outlet; the natural rim is with a spillway at the dam controlling overflow. The maximum legal limit, to which the lake can be allowed to rise in order to store water, is at 6,229.1 ft. Around New Year 1996/1997 a Pineapple Express atmospheric river melted snow and caused the lake and river to overflow, inundating Reno and surrounding areas.
The Lake Tahoe Basin was formed by vertical motion faulting. Uplifted blocks created the Carson Range on the main Sierra Nevada crest on the west. Down-dropping and block tilting created the Lake Tahoe Basin in between; this kind of faulting is characteristic of the geology of the adjoining Great Basin to the east. Lake Tahoe is the youngest of several extensional basins of the Walker Lane deformation zone that accommodates nearly 0.47 in per year of dextral shear between the Sierra Nevada-Great Valley Block and North America. Three principal faults form the Lake Tahoe basin: the West Tahoe Fault, aligned between Meyers and Tahoe City, and, the local segment of the Sierra Nevada Fault, extending on shore north and south of these localities; the West Tahoe Fault appears to be the most active and hazardous fault in the basin. A study in Fallen Leaf Lake, just south of Lake Tahoe, used seafloor mapping techniques to image evidence for paleoearthquakes on the West Tahoe and revealed the last earthquake occurred between 4,100 and 4,500 years ago.
Subsequent studies revealed submarine landslides in Fallen Leaf Lake and Lake Tahoe that are thought to have been triggered by earthquakes on the West Tahoe fault and the timing of these events suggests a recurrence interval of 3,000–4,000 years. Some of the highest peaks of the Lake Tahoe Basin that formed during the process of Lake Tahoe creation are Freel Peak at 10,891 feet, Monument Peak at 10,067 feet, Pyramid Peak at 9,984 feet, Mount Tallac at 9,735 feet; the north shore boasts three peaks at 10,000+ feet: Mount Rose at 10,785 feet, Houghton and Relay peaks. Mt. Rose is a popular hiking and backcountry skiing destination. Eruptions from the extinct volcano Mount Pluto formed a volcanic
Sierra Nevada (U.S.)
The Sierra Nevada is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies in Nevada; the Sierra Nevada is part of the American Cordillera, a chain of mountain ranges that consists of an continuous sequence of such ranges that form the western "backbone" of North America, Central America, South America and Antarctica. The Sierra runs 400 miles north-to-south, is 70 miles across east-to-west. Notable Sierra features include the largest alpine lake in North America; the Sierra is home to three national parks, twenty wilderness areas, two national monuments. These areas include Yosemite and Kings Canyon National Parks; the character of the range is shaped by its ecology. More than one hundred million years ago during the Nevadan orogeny, granite formed deep underground; the range started to uplift four million years ago, erosion by glaciers exposed the granite and formed the light-colored mountains and cliffs that make up the range.
The uplift caused a wide range of elevations and climates in the Sierra Nevada, which are reflected by the presence of five life zones. Uplift continues due to faulting caused by tectonic forces, creating spectacular fault block escarpments along the eastern edge of the southern Sierra; the Sierra Nevada has a significant history. The California Gold Rush occurred in the western foothills from 1848 through 1855. Due to inaccessibility, the range was not explored until 1912; the Sierra Nevada lies in Central and Eastern California, with a small but important spur extending into Nevada. West-to-east, the Sierra Nevada's elevation increases from 1,000 feet in the Central Valley to heights of about 14,000 feet at its crest 50–75 miles to the east; the east slope forms the steep Sierra Escarpment. Unlike its surroundings, the range receives a substantial amount of snowfall and precipitation due to orographic lift; the Sierra Nevada's irregular northern boundary stretches from the Susan River and Fredonyer Pass to the North Fork Feather River.
It represents where the granitic bedrock of the Sierra Nevada dives below the southern extent of Cenozoic igneous surface rock from the Cascade Range. It is bounded on the west by California's Central Valley and on the east by the Basin and Range Province; the southern boundary is at Tehachapi Pass. Physiographically, the Sierra is a section of the Cascade-Sierra Mountains province, which in turn is part of the larger Pacific Mountain System physiographic division; the California Geological Survey states that "the northern Sierra boundary is marked where bedrock disappears under the Cenozoic volcanic cover of the Cascade Range." The range is drained on its western slope by the Central Valley watershed, which discharges into the Pacific Ocean at San Francisco. The northern third of the western Sierra is part of the Sacramento River watershed, the middle third is drained by the San Joaquin River; the southern third of the range is drained by the Kings, Kaweah and Kern rivers, which flow into the endorheic basin of Tulare Lake, which overflows into the San Joaquin during wet years.
The eastern slope watershed of the Sierra is much narrower. From north to south, the Susan River flows into intermittent Honey Lake, the Truckee River flows from Lake Tahoe into Pyramid Lake, the Carson River runs into Carson Sink, the Walker River into Walker Lake. Although none of the eastern rivers reach the sea, many of the streams from Mono Lake southwards are diverted into the Los Angeles Aqueduct which provides water to Southern California; the height of the mountains in the Sierra Nevada increases from north to south. Between Fredonyer Pass and Lake Tahoe, the peaks range from 5,000 feet to more than 9,000 feet; the crest near Lake Tahoe is 9,000 feet high, with several peaks approaching the height of Freel Peak. Farther south, the highest peak in Yosemite National Park is Mount Lyell; the Sierra rises to 14,000 feet with Mount Humphreys near Bishop, California. Near Lone Pine, Mount Whitney is at 14,505 feet, the highest point in the contiguous United States. South of Mount Whitney, the elevation of the range dwindles.
The crest elevation is 10,000 feet near Lake Isabella, but south of the lake, the peaks reach to only a modest 8,000 feet. There are several notable geographical features in the Sierra Nevada: Lake Tahoe is a large, clear freshwater lake in the northern Sierra Nevada, with an elevation of 6,225 ft and an area of 191 sq mi. Lake Tahoe lies between a spur of the Sierra. Hetch Hetchy Valley, Yosemite Valley, Kings Canyon, Kern Canyon are examples of many glacially-scoured canyons on the west side of the Sierra. Yosemite National Park is filled with notable features such as waterfalls, granite domes, high mountains and meadows. Groves of Giant Sequoia