Rocky Mountains, Colorado:

The Rocky Mountains, commonly known as the Rockies, are a major mountain range in western North America. The Rocky Mountains stretch more than 3,000 miles from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States. Within the North American Cordillera, the Rockies are somewhat distinct from the Pacific Coast Ranges and the Cascade Range and Sierra Nevada which all lie farther to the west.The Rocky Mountains were initially formed from 80 million to 55 million years ago during the Laramide orogeny, in which a number of plates began to slide 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 started to inhabit the mountain range. After Europeans, such as Sir Alexander Mackenzie and the Lewis and Clark expedition, started to explore the range, minerals and furs drove the initial economic exploitation of the mountains, although the range itself never became densely populated.Currently, much of the mountain range is protected by public parks and forest lands, and is a popular tourist destination, especially for hiking, camping, mountaineering, fishing, hunting, mountain biking, skiing, and snowboarding.The Rocky Mountains are commonly defined as stretching from the Liard River in British Columbia south to the Rio Grande in New Mexico. Other mountain ranges continue beyond those two rivers, including the Selwyn Range in Yukon, the Brooks Range in Alaska, and the Sierra Madre in Mexico, but those are not part of the Rockies, though they are part of the American cordillera. The United States definition of the Rockies, however, includes the Cabinet and Salish Mountains of Idaho and Montana, whereas their counterparts north of the Kootenai River, the Columbia Mountains, are considered a separate system in Canada, lying to the west of the huge Rocky Mountain Trench, which runs the length of British Columbia from its beginnings in the middle Flathead River valley in western Montana to the south bank of the Liard River.The Rockies vary in width from 70 to 300 miles . Also west of the Rocky Mountain Trench, farther north and facing the Muskwa Range across the trench, are the Stikine Ranges and Omineca Mountains of the Interior Mountains system of British Columbia. A small area east of Prince George, British Columbia on the eastern side of the Trench, the McGregor Plateau, resembles the Rockies but is considered part of the Interior Plateau.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.In the southern Rocky Mountains, near present-day Colorado, these ancestral rocks were disturbed by mountain building approximately 300Ma, 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 started to collide with the western edge of North America in the Mississippian, causing the Antler orogeny. For 270 million years, the effects of plate collisions were focused very near the edge of the North American plate boundary, far to the west of the Rocky Mountain region. It was not until 80 Ma that these effects began to reach the Rockies.The current Rocky Mountains were raised in the Laramide orogeny from between 80 to 55 Ma.For the Canadian Rockies, the mountain building is analogous to a rug being pushed 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, the growth of the Rocky Mountains in the United States was probably caused by an unusual subduction, 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 200 to 300 miles.It is postulated that the shallow angle of the subducting plate greatly 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 extraordinarily broad, high Rocky Mountain range.

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