Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in a straight-line distance from the northernmost part of Western Canada, to New Mexico in the Southwestern United States. Depending on differing definitions between Canada and the U.S., its northern terminus is located either in northern British Columbia's Terminal Range south of the Liard River and east of the Trench, or in the northeastern foothills of the Brooks Range/British Mountains that face the Beaufort Sea coasts between the Canning River and the Firth River across the Alaska–Yukon border. Its southernmost point is near the Albuquerque metropolitan area, adjacent to the Rio Grande rift, and north of the Sandia–Manzano Mountain Range, also near Santa Fe, New Mexico. Being the easternmost portion of the North American Cordillera, the Rockies are distinct from the tectonically younger Cascade Range and Sierra Nevada, which both lie farther to its west.
The Rockies formed 55 million to 80 million years ago during the Laramide orogeny, in which a number of plates began sliding underneath the North American plate. The angle of subduction was shallow, resulting in a broad belt of mountains running down western North America. Since then, further tectonic activity and erosion by glaciers have sculpted the Rockies into dramatic peaks and valleys. At the end of the last ice age, humans began inhabiting the mountain range. After explorations of the range by Europeans, such as Sir Alexander Mackenzie, and Anglo-Americans, such as the Lewis and Clark Expedition, natural resources such as minerals and fur drove the initial economic exploitation of the mountains, although the range itself has never experienced a dense population.
Most of the highest summits of the Rocky Mountains are in Colorado, with the state having an average elevation of. Public parks and forest lands protect much of the mountain range, and they are popular tourist destinations, especially for hiking, camping, mountaineering, fishing, hunting, mountain biking, snowmobiling, skiing, and snowboarding.
Etymology
The name of the mountains is a calque of an Algonquian name, specifically the Plains Cree wikt:ᐊᓯᓃᐘᒋᕀ wikt:asinîwaciy, literally "rocky mountain / alp"; its French form "montagnes de Roche" first attested in Jacques Legardeur de Saint-Pierre's journal in 1752. Another name given to the place by the Cree is ᐊᓭᓂᐓᒉ Aseniwuche.Geography
The Rocky Mountains are the easternmost portion of the expansive North American Cordillera. They are often defined as stretching from the Liard River in British Columbia south to the headwaters of the Pecos River, a tributary of the Rio Grande, in New Mexico. The Rockies vary in width from. The Rocky Mountains contain the highest peaks in central North America. The range's highest peak is Mount Elbert in Colorado at above sea level. Mount Robson in British Columbia, at, is the highest peak in the Canadian Rockies.The eastern edge of the Rockies rises dramatically above the Interior Plains of central North America, including the Sangre de Cristo Mountains of New Mexico and Colorado, the Front Range of Colorado, the Wind River Range and Big Horn Mountains of Wyoming, the Absaroka-Beartooth ranges and Rocky Mountain Front of Montana and the Clark Range of Alberta.
Central ranges of the Rockies include the La Sal Range along the Utah-Colorado border, the Abajo Mountains and Henry Mountains of Southeastern Utah, the Uinta Range of Utah and Wyoming, and the Teton Range of Wyoming and Idaho.
The western edge of the Rockies includes ranges such as the Wasatch near Salt Lake City, the San Juan Mountains of New Mexico and Colorado, the Bitterroots along the Idaho-Montana border, and the Sawtooths in central Idaho. The Great Basin and Columbia River Plateau separate these subranges from distinct ranges further to the west. In Canada, the western edge of the Rockies is formed by the huge Rocky Mountain Trench, which runs the length of British Columbia from its beginning as the Kechika Valley on the south bank of the Liard River, to the middle Lake Koocanusa valley in northwestern Montana.
The Canadian Rockies are defined by Canadian geographers as everything south of the Liard River and east of the Rocky Mountain Trench, and do not extend into Yukon, Northwest Territories or central British Columbia. They are divided into three main groups: the Muskwa Ranges, Hart Ranges and Continental Ranges. Other more northerly mountain ranges of the eastern Canadian Cordillera continue beyond the Liard River valley, including the Selwyn, Mackenzie and Richardson Mountains in Yukon as well as the British Mountains/Brooks Range in Alaska, but those are not officially recognized as part of the Rockies by the Geological Survey of Canada, although the Geological Society of America definition does consider them parts of the Rocky Mountains system as the "Arctic Rockies".
The Continental Divide of the Americas is in the Rocky Mountains and designates the line at which waters flow either to the Atlantic or Pacific Oceans. Triple Divide Peak in Glacier National Park is so named because water falling on the mountain reaches not only the Atlantic and Pacific but Hudson Bay as well. Farther north in Alberta, the Athabasca and other rivers feed the basin of the Mackenzie River, which has its outlet on the Beaufort Sea of the Arctic Ocean.
Human population is not very dense in the Rockies, with an average of four people per square kilometer and few cities with over 50,000 people. However, the human population grew rapidly in the Rocky Mountain states between 1950 and 1990. The forty-year statewide increases in population range from 35% in Montana to about 150% in Utah and Colorado. The populations of several mountain towns and communities have doubled in the forty years 1972–2012. Jackson, Wyoming, increased 260%, from 1,244 to 4,472 residents, in those forty years.
Geology
The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock is Precambrian metamorphic rock that forms the core of the North American continent. There is also Precambrian sedimentary argillite, dating back to 1.7 billion years ago. During the Paleozoic, western North America lay underneath a shallow sea, which deposited many kilometers of limestone and dolomite.File:Jackson Glacier terminus.jpg|thumb|right|Glaciers, such as Jackson Glacier in Glacier National Park, Montana, as shown here, have dramatically shaped the Rocky Mountains.
In the southern Rockies, near present-day Colorado, these ancestral rocks were disturbed by mountain building approximately 300 million years ago, during the Pennsylvanian. This mountain-building produced the Ancestral Rocky Mountains. They consisted largely of Precambrian metamorphic rock forced upward through layers of the limestone laid down in the shallow sea. The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock.
Terranes began colliding with the western edge of North America in the Mississippian, causing the Antler orogeny. For 270 million years, the focus of the effects of plate collisions were near the edge of the North American Plate boundary, far to the west of the Rocky Mountain region. It was not until 80 million years ago that these effects began reaching the Rockies.
The current Rocky Mountains arose in the Laramide orogeny from between 80 million and 55 million years ago. For the Canadian Rockies, the mountain building is analogous to pushing a rug on a hardwood floor: the rug bunches up and forms wrinkles. In Canada, the terranes and subduction are the foot pushing the rug, the ancestral rocks are the rug, and the Canadian Shield in the middle of the continent is the hardwood floor.
Further south, an unusual subduction may have caused the growth of the Rocky Mountains in the United States, where the Farallon Plate dove at a shallow angle below the North American Plate. This low angle moved the focus of melting and mountain building much farther inland than the normal. Scientists hypothesize that the shallow angle of the subducting plate increased the friction and other interactions with the thick continental mass above it. Tremendous thrusts piled sheets of crust on top of each other, building the broad, high Rocky Mountain range.
Some researchers have proposed that aspects of Rocky Mountain uplift and deformation are better explained by a “hit-and-run” tectonic collision model, in which oblique and transient plate interactions distributed deformation far inland, rather than through long-lived shallow-angle subduction alone. Variants of this interpretation and related alternatives to traditional Cordilleran tectonic models have also been discussed in synthesis and field-based settings, including a Geological Society of America Penrose Conference held in Idaho.
File:Roxborough.jpg|thumb|left|Tilted slabs of sedimentary rock in Roxborough State Park near Denver
The current southern Rockies were forced upwards through the layers of Pennsylvanian and Permian sedimentary remnants of the Ancestral Rocky Mountains. Such sedimentary remnants were often tilted at steep angles along the flanks of the modern range; they are now visible in many places throughout the Rockies, and are shown along the Dakota Hogback, an early Cretaceous sandstone formation running along the eastern flank of the modern Rockies.
Just after the Laramide orogeny, the Rockies were like Tibet: a high plateau, probably above sea level. In the last 60 million years, erosion stripped away the high rocks, revealing the ancestral rocks beneath, and forming the current landscape of the Rockies.
Periods of glaciation occurred from the Pleistocene Epoch to the Holocene Epoch. These ice ages left their mark on the Rockies, forming extensive glacial landforms, such as U-shaped valleys and cirques. Recent glacial episodes included the Bull Lake Glaciation, which began about 150,000 years ago, and the Pinedale Glaciation, which perhaps remained at full glaciation until 15,000–20,000 years ago.
All of these geological processes exposed a complex set of rocks at the surface. For example, volcanic rock from the Paleogene and Neogene periods occurs in the San Juan Mountains and in other areas. Millennia of severe erosion in the Wyoming Basin transformed intermountain basins into a relatively flat terrain. The Tetons and other north-central ranges contain folded and faulted rocks of Paleozoic and Mesozoic age draped above cores of Proterozoic and Archean igneous and metamorphic rocks ranging in age from 1.2 billion to more than 3.3 billion years.