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Geology of the Rocky Mountains

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    Geology of the Rocky Mountains

    During the tertiary period, the Rocky Mountains were gradually uplifted several thousand feet, attaining, probably, a greater height than they at present possess. Out of the water have been heaved the Andes, the backbone of South America, the Alps, and the Himalayas, the mighty crests of Europe and Asia; lakes have been drained; immense plains wrested from the waters; and now the land-surface of the globe becomes very similar in outline to that with which we are familiar.

    —  William Denton, Our Planet, Its Past and future; or, Lectures on Geology. (p.233)

    The Rocky Mountains stretch more than 4,800km (3,000 miles) from Alaska, through Canada and the western United States. They wind through western North America, from Cassiar Mountains near Canada’s border with Alaska to the Sangre de Cristo Mountains of northern New Mexico. They are part of the Cordillera system, the largest mountain chain in North America. Because they form the Continental Divide, which separates rivers that flow west from those that run east, the Rockies are sometimes referred to as the “backbone of North America.” The southern half of the Rocky Mountains (although well over half of the Rockies’ total area) lies within the US.

    The Canadian Rockies

    In Canada, the Rockies span 1,450km (900 miles), separating Alberta from British Columbia and extending from the U.S. border north into Yukon Territory. The Canadian Rockies consist of long thrust faults of sedimentary strata, the old continental shelf of North America that was broken up and pushed eastward starting bout 100 million years ago (Cannings 5). Many mountains show the characteristic form of thrust faults — long western slopes with sharply broke eastern faces. The tallest peak is Mount Robson (3,954m/12,972 ft.). Many others exceed 3,000m (9,840ft).

    The Rockies, in Canada and the US, are quite young, relatively speaking. They were formed millions of years ago, when sediments that had piled up on the beds of ancient seas caused the earth’s surface to shift, lifting and folding enormous blocks of sediment. On the mountainsides in Yoho National Park, paleontologists have found fossils of animals that inhabited the seas 500 million years ago. Over time, erosion, wind, water and ice shaped and sculpted the mountains, producing their angular peaks. The sedimentary rocks that make up the Canadian Rockies are mainly limestone, dolomite, sandstone, and shale. The mountains are grouped into four ranges, based on geological makeup:

    Foothills: To the west of Calgary, there are gentle, rolling foothills, which are the youngest rocks — and primarily formed of sandstone.
    Front Ranges: Just to the west of the foothills are the steeper Front Ranges of the Rocky Mountains. These peaks are composed mainly of limestone and are pale grey in color. Examples include Mt. Brazeau in Jasper National Park, which 3,470m (11,382 ft.) is the tallest peak in the Front Ranges, Roche Miette, also in Jasper, and Mount Rundle in Banff.
    Main Ranges: The Main Ranges, which are east of the Front Ranges, are formed mostly of sandstone, limestone, and shale. The highest mountains in the Canadian Rockies, the Main Ranges include the peaks of the Continental Divide, Mount Robson, near Jasper National Park, and Mount Victoria.
    Western Ranges: To the west of Banff National Park, there are Western Ranges, which run from Golden to Radium Hot Springs, south of Kootenay National Park.

    The US Rockies

    Traveling through the Rockies gives a sense of seeing earth’s geology at work, as a wide range of conditions vividly illustrates this planet’s shaping forces. (Ackland, Freeburg 88)

    Key Rocky Mountains states in the US are Montana, Wyoming and Colorado, but major outliers extend deep into Idaho and Utah. The Rocky Mountains have many diverging outliers with a complex system of ridge and isolated buttes (lone hills or mountains with steep sides). The Rockies’ eastern slope is fringed by the Great Plains, composed of massive deposits of erosion (including glacial) material washed down from the mountains. This seemingly endless expanse of sparse, grassy steppes continues east, gradually dropping elevation to meet the Mississippi River. The Continental Divide – the ridgeline that sheds water either west to eh Pacific or east to the Atlantic – wanders erratically through the Rocky Mountains, often a long way from their highest summits and ranges. This fuels debate as to which mountains should considered part of the Rockies proper.  The US Rockies are divided into three – northern , middle, and southern – sections, which are offset slightly west-to-east and have considerable latitudinal overlap. The Front Range can be a considered as a category in itself.

    Northern Rockies:  The northern US Rockies extend south-southwest from the Canadian border through central-eastern Idaho and western Montana. They are lower than the middle and southern Rockies, with only a few summits rising much above 12,000 ft (3,658 meters), including Mt Borah and Leatherman Peak, both in Idaho’s Lost River Range.  The western ranges of the northern US Rockies form the higher eastern rim of the Columbia Plateau. The ranges consist of the Salmon River and Clear Water Mountains, as well as the more rugged Lost River and Sawtooth Ranges at their south, and are largely drained by the tributaries of the Snake River.  Further east are the Bitterroot Mountains, which form most of the Idaho-Montana border. The Rocky Mountains Trench, which extends 1000 miles (1610 km) south from Canada’s Yukon as far as the Bitterroot Valley in southwestern Montana, divides these western ranges from the mighty Front Ranges of the Greater Glacier region, which stand on, or close to, the Continental Divide. To the south, the Tobacco Root, Anaconda and Pioneer Ranges are cut by broad valleys up to 40 miles (65 km) wide.  The eastern slopes of the northern US Rockies are drained by the Missouri and its upper tributaries, chiefly the Yellowstone River.

    Middle Rockies: The middle Rockies (not the same as central Rockies) extend south-southwest from Greater Yellowstone and northern Wyoming’s isolate Bighorn Mountains into northeastern Utah. The middle Rockies’ western slopes mark the eastern rim of the vast Great Basin (not to be confused with Wyoming’s Great Divide Basin), a high, semiarid plateau extending west to the Sierra Nevada. The middle Rockies are higher than the mountains to their north but are also much less continuous, with their major ranges often separated by relatively low hills or even rolling prairies. The middle Rockies’ northwestern edge includes the volcanic Yellowstone Plateau and the Absaroka Range to its east. To the south are the granitic Teton and Wind River Ranges, with middle Rockies’ highest summits — Grand Teton (13,777ft) and Gannet Peak (13,804ft). Further south, the Wasatch Range fronts Great Salt Lake, intersecting with the Uinta Mountains — the Rockies’ only west-to-east mountain chain — to form an ‘L’ where Utah meets south-west Wyoming.

    Southern Rockies: The extent of the southern Rockies is from Wyoming’s Great Divide Basin – a broad, undrained depression completely enclosed by the Continental Divide — to as far south as northern New Mexico. They form the widest and highest section of the US Rockies, with numerous peaks rising to well above 14,000 ft (4268 meters). The southern Rockies are the source of the eastward-flowing Rio Grande and Colorado River, which drain into the Pacific. The southern Rockies’ western slopes gradually go over into eh Colorado Plateau, a vast basin dissected by deep canyons stretching through southern Utah into northern Arizona. The southern Rockies are dominated by two broad bands of mountains separated by several large basins, or ‘parks’ including South Park (at the headwaters of South Platte River) and the San Luis Valley (at around 7,500 ft (2286 meters) the widest valley of its height in the world). The western band consists of the Sierra Madre, Elk West Elk and Sawatch Ranges (the latter has the Rockies’ highest summit, Mt, Elbert at 14, 233 ft or 4339 meters); its south-west end being formed by the San Juan Mountains, an irregular and rugged plateau of volcanic origin crossed by high ranges with a dozen peak over 14,000 ft. The eastern band takes in the Laramie, Medicine Bow, Wet and Sangre de Cristo Ranges, as well as the Front Range.

    Front Range:  It extends along the eastern edge of the Rocky Mountains through Montana, Wyoming and Colorado. Geologically, the Front Range marks the easternmost extent of vertical uplift in the ongoing mountain-building process. The Front Range can appear as little more than a high ridge among low foothills or it may rise like a massive blue wall almost directly out of the flat plain — the abrupt Colorado Front is particularly dramatic. In Utah, the western edge of the Rockies is characterized by the spectacular Wasatch Front range.

    The Formation of the Rockies – I

    About three hundred million years ago the curtain rose on the Ancestral Rockies, thrust up out of an inland sea. The “everlasting mountain” is a myth. As that older range reached for the sky, it immediate came under attack by rain, wind, frost, and snow…  By two hundred million years ago, erosion held high ground and the Ancestral Rockies were history. (Muench et al. 105)

    The creation of the modern Rockies began around 100 million years ago as the supercontinent of Pangaea began to break apart. The newly separated North American plate drifted west (as it continues to do), gathering in its path isolated island of earth crust (which compose California and much of the West Coast today). It was the time of the dinosaurs, and about the time flowering plants first arose; long before the first horse and much longer before the advent of first humans. At the dawn of the modern Rockies, only a few isolated hills disturbed the flat expanse of western North America. Earlier mountains (including “ancestral Rockies”) had eroded away, and shallow seas had come and gone. Sediments had filled the basins, leaving a monotonous erosion surface.

    The stage was in fact set for mountain building about 75 million years ago on the west coast, which was then in Idaho. In the geologic event known as Laramide Orogeny, the North American plate started to collide with the Pacific plate. Under the enormous force, its thick, solid continental crust rode over the thinner and more brittle oceanic crust of the Pacific plate, forcing the latter downward in a process known as subduction. Due to this subduction of the Pacific Plate under the North American Plate, a volcanic land mass being carried on the Pacific Plate collided with the coast accreting the land mass onto the continent. Over millions of years, pressure from that accretion radiated eastward, causing rock in the upper layer of the earth’s crust to fold and rumple.

    Compressed within the earth’s semifluid mantle, molten sections of the subducted oceanic crust now pushed back upward. Unable to withstand this enormous pressure, the continental crust itself uplifted in numerous places. Associated buckling and fracturing created the Rockies complex system of ranges and caused it older sediments to slide over younger rock strata. Major volcanic eruptions occurred as molten mantle was forced through cracks in the continental crust, raising the mountains further as whole valleys were filled in by mineral-rich deposits. The intense volcanic activity seen today in Yellowstone National Park, which sits above an unusually large bubble of molten magma (known as a ‘hot spot’) lying just below the earth’s surface, shows that this process is continuing.

    By about 45 million years ago, pressure from accretion had stopped and the land began to relax, creating extension fault that cause huge block of rock to drop between some ranges. While the original folding and faulting were going on, another set of forces was at work shaping the Rockies. Between 75 million and 60 million years ago, the subducting Pacific Plate had become hot enough to melt continental rocks above it. Enormous volumes of the resultant magma rose like colossal bubbles, intruding into upper continental crust in Idaho, western Montana, and British Columbia. Those upswellings of magma cooled underground to form a group of batholiths-huge masses of granitic rock that may be 100 or more square miles across and 10 to 20 miles thick.

    Already rumpled by folding and faulting, the earth’s surface was bulged higher by the batholiths. In some cases, bulges grew big enough for the overlying surface to break into giant slabs. A 10-mile-thick hunk of crust known as the Sapphire block broke off the Idaho batholith and move about 50 miles east into Montana. The Bitterroot Valley marks the trailing edge of that block. After the process of folding, relaxing, bulging, and spewing had given the mountains shape, the ranges continued to gain elevation. In fact, most of the gain occurred within the last 5 million years, possibly as a result of the earth’s crust floating up to find some geologic equilibrium (isostatic adjustment). Rivers and streams in existence before this recent uplift cut downward as fast the land rose, creating such deep river canyons as Royal Gorge, near Canon City, Colorado. the Colorado River was another tool of erosion. Geologists think that much of the Grand Canyon was carved in just the last 2 million to 3 million years. Still other forces joined in on the mountain construction. About 50 million years ago, widely scattered volcanic eruptions spewed rocks and lava over large parts of the northern Rockies and into south-central Idaho.

    The Formation of the Rockies – II

    While the formation of the Rocky Mountains progressed, an ancient sea on their eastern side was also gradually uplifted and filled in by erosion debris from the mountains. This process exposed the thick marine sedimentation now widespread in the Rockies of Montana and more isolated ranges further south, such as the Maroon Bells of Colorado. As it shrank, the eastern sea gave way to the vast swampy flood plain that supports diverse vegetation and wildlife. The thick deposits of organic matter laid down during the period are the origin of the abundant coal and oil resources found on the eastern plains. The steady rise of the Rockies, however, formed an increasingly impenetrable barrier to rain clouds form the Pacific, and eastern plains gradually turned into the dry, sparsely vegetated prairies seen today.

    The Rocky Mountains were transformed by the (Pleistocene) cycle of ice ages, beginning some two million years ago. Enormous glaciers affected all but the higher ranges, grinding away at the bedrock to create deep, U-shaped valleys. Starting some 10,000 years ago, the large glaciers began to melt and the glacier debris (moraine) they left as the receded dammed up the rivers to create tens of thousands of lakes and tarns. Today, the northern U.S. and Canadian Rockies have many glaciers that continue to wear down the high mountains (Maynard 9).

    The Rockies’ rugged features are a result of their young geological age and recent massive erosion. By contrast, the Appalachian Mountains to the east which were created more than 150 million years earlier by a similar process are lower and more rounded, showing the signs of greater erosion over the eons. The Rockies’ position as a secondary range back from the point of contact between the Pacific and North American tectonic plates has also given the Rocky Mountains a more complex topography than that of the Cascade-Sierra Nevada Ranges along the Pacific coast. Numerous ancient and recent secondary seismic faults run through the Rockies. Localized earthquakes occur as tension in the earth is released by the sudden slide of a relatively minor fault. The Rockies continue to rise, as evidenced by occasional measurable growth spurts gained during earthquakes.

    References:

    Ackland, Diana; Freeburg, Janie.  “Insight Guide: The Rockies (U.S.A.).” Singapore : APA Publications, 1996

    Cannings, Richard. “The Rockies: A Natural History.” Vancouver, BC : Greystone Books, 2005.

    Denton, William. “Our Planet, Its Past and Future; or, Lectures on Geology.” Michigan Historical Reprint Series. Scholarly Publishing Office, University of Michigan Library, 2005.

    Maynard, Charles William. “Mountain Ranges of the World: The Rocky Mountains.”  New York : The Rosen Publishing Group, 2004

    Muench, David; Udall, James R.; Muench, Marc. “The Rockies.” Portland, OR : Graphic Arts Center Publishing Company, 1997

     

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