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Extending the Content
Unit : The Dynamic Earth
Chapter 20: Mountain Building  

The Rocky Mountains
The majestic peaks of the Rockies form a massive rampart to the west of the Great Plains. They were a formidable barrier to the westward movement of settlers and contain hidden treasures of gold, silver and other resources. But how and when were they formed?

The major range of mountains that make up the Rocky Mountains extends from Alaska south into Mexico. The Rockies are not a single chain or range of mountains, but a complex belt with an equally complex geologic history. Geologists refer to the mountain ranges that formed in western North America during this time as the Cordillera, which means "mountain range" in Spanish. Major deformation began in the Cordillera during the Triassic Period, over 240 million years ago, due to subduction along the western margin of North America. The subduction formed a chain of volcanoes along the western edge of the continent. In contrast to the passive margin that existed along the eastern margin of North America during the Mesozoic Era, the margin along the western coast continued to be active. Subduction along the western margin of North America continued through the Middle Triassic.

Rocky Mountains

Pangaea Breaks Up
Deformation along the western margin of North America increased substantially when Pangaea broke apart. Some geologists theorize that the breakup of Pangaea caused the North American Plate to be pushed over the oceanic plates in the Pacific region, causing an increase in subduction. As a result, the Cordillera has been active since the Mesozoic Era. Three major episodes of mountain building occurred along the western margin of North America during the Mesozoic. These are the Nevadan, Sevier, and Laramide Orogenies. Different types of deformation characterize each of these orogenies. They are sometimes collectively called the Cordilleran Orogeny because they affected the same part of North America, and also because it is difficult to determine precisely when one orogenic event ended and the next began.

The Nevadan Orogeny was characterized by a tremendous volume of igneous intrusions. Large bodies of granite called batholiths exist throughout the Cordillera. These batholiths are all Middle Jurassic through Middle Cretaceous in age. The spectacular exposure of Half-Dome at Yosemite National Park is part of the Sierra Nevada Batholith, which was intruded during the Nevadan Orogeny.

Sevier Orogeny
Throughout the Sevier Orogeny deformation occurred, as microcontinents and island arcs were colliding with the western margin of North America. These pieces of continental crust that collided with the western margin became attached to North America. The process is called accretion. These accreted areas are called suspect terranes because they have different rock types and fossils from those found in the surrounding areas. The Sevier Orogeny is characterized by low-angle thrust faults and folds caused by compression from the collisional tectonism. The collisions caused some areas of the Cordillera to be compressed or shortened by as much as 140 kilometers. These folds and thrust faults do not extend into the igneous and metamorphic "basement" rocks, but are confined to the sedimentary rocks at Earth's surface. This type of "thin-skinned" deformation began in the Late Jurassic and continued through the Late Cretaceous. The thrust faults run north-south and place older rocks on top of younger rocks in Utah, Idaho, Wyoming, Montana, and western Canada.

The Laramide Orogeny is characterized by vertical uplifts. This type of deformation is sometimes called "thick-skinned", to differentiate it from the characteristic "thin-skinned" deformation of the Sevier Orogeny. The Laramide Orogeny mainly affected the area east of the folds and faults caused by the Sevier Orogeny. Deformation caused by the Laramide Orogeny began during the Late Cretaceous Period and continued into the Cenozoic Era. Most of the deformation associated with the Laramide Orogeny occurred during the Cenozoic. The large-scale vertical uplifts of the Laramide formed much of the distinctive and beautiful scenery of the Rocky Mountains, in places such as Rocky Mountain National Park in Colorado.

LINK-UP: Find out more information about Rocky Mountain National Park.

Research the geologic history of another National Park in the Cordillera. Write a short report and present your findings to the class. Select one of the following parks: Yellowstone National Park, Grand Canyon National Park, Glacier National Park, Arches National Park, and Mesa Verde National Park.


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