Geology of the Rocky Mountains
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The Geology of the Rocky Mountains formed a majestic mountain barrier that stretches from Canada through central New Mexico. Although formidable, a look at the topography reveals a discontinuous series of mountain ranges with distinct geological origins.
The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains.
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[edit] The Ancestral Rocky Mountains
The Ancestral Rocky Mountains were an ancient Paleozoic mountain range that existed in western North America in the location of the present-day southern Rocky Mountains in Colorado. The range was formed approximately 300 MYA during the Pennsylvanian by an uplift process known as the Colorado orogeny, itself a result of a period of intense sea-floor spreading. At the time of the uplift, western North America in the area of present-day Colorado was largely covered by a shallow sea. The uplift created two large mountainous islands, known to geologists as Frontrangia and Uncompahgria, located roughly in the current locations of the Front Range and the San Juan Mountains. They consisted largely of Precambrian metamorphic rock forced upward through layers of limestone laid down in the shallow sea throughout the early and late Paleozoic.
The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving a large deposition of debris in the surrounding floodplains that formed layers of sedimentary rock.
[edit] Before the Laramide orogeny
During the last half of the Mesozoic Era, the Age of the Dinosaurs, much of today's California, Oregon, and Washington were added to North America. Western North America suffered the effects of repeated collision as slabs of ocean crust sank beneath the continental edge. Slivers of continental crust, carried along by subducting ocean plates, were swept into the subduction zone and scraped onto North America's edge.
About 200-300 miles inland, magma generated above the subducting slab rose into the North American continental crust. Great arc-shaped volcanic mountain ranges (known as the Sierran Arc) grew as lava and ash spewed out of dozens of individual volcanoes. Beneath the surface, great masses of molten rock were injected and hardened in place.
Mesozoic deposition in the Rockies was a mix of marine, transitional, and continental that varied over time as crustal conditions altered the region. By the close of this era, 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognized formations. The most extensive non-marine formations were deposited in the Cretaceous period when the eastern part of the Cretaceous Seaway (a warm shallow sea that periodically divided North America in that period) covered the region. Their sediment came from rock eroded from a mountain chain east of the seaway interbeded with ash from volcanos west of the seaway in the Sierran Arc.
For 100 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 70 million years ago that these effects began to reach the Rockies.
[edit] Raising the Rockies
The growth of the Rocky Mountains has been one of the most perplexing of geologic puzzles. Normally, mountain building is focused between 200 to 400 miles inland from a subduction zone boundary, yet the Rockies are hundreds of miles farther inland. What geologic processes raise mountains at this scale? Although geologists continue to gather evidence to explain the rise of the Rockies, the answer most likely lies with an unusual subducting slab.
At a 'typical' subduction zone, an oceanic plate typically sinks at a fairly high angle (see image to right). A volcanic arc grows above the subducting plate. During the growth of the Rocky Mountains, the angle of the subducting plate may have been significantly flattened, moving the focus of melting and mountain building much farther inland than is normally expected.
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.
The current 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 and visible in many places throughout the Rockies, including prominently along the Dakota Hogback, an early Cretaceous sandstone formation that runs along the eastern flank of the modern Rockies.
[edit] References
- Geologic Provinces of the United States: Rocky Mountains (HTML). USGS Geology in the Parks. Retrieved on 2006-12-10. (public domain source for article starting content)
