America's Volcanic Past
Rocky Mountains

The Rockies:
The Rockies form 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

First Uplift:
Most of the rocks in Rocky Mountain National Park -- excluding the newer rocks of the Never Summer Mountains -- were originally shale, siltstone, and sandstone, along with some volcanic rocks deposited about 1.8 to 2 billion years ago in an ancient sea. Between 1.7 and 1.6 billion years ago, these sedimentary rocks were caught in a collision zone between sections of the Earth's crust called tectonic plates. These rocks, then in the core of an ancient Proterozoic mountain range, were recrystallized into metamorphic rocks by enormous heat and pressure resulting from the collision.

Granites found in the park probably resulted from the melting of pre-existing sedimentary or metamorphic rocks in the primordial crust shortly after the formation of the Earth. The Silver Plume granite that occurs in much of the east side of the park intruded upward into the metamorphic rocks about 300 million years after the formation of the Proterozoic mountains. The high mountains that formed here during Proterozoic time were slowly eroded and reduced to a fairly flat surface, exposing the core of metamorphic rocks and granite. This erosion occurred over a long period, from approximately 1,300 million to 500 million years ago.

Second Uplift:
About 500 million years ago, this relatively flat area became covered with shallow seas. Over the next 200 million years, several hundreds of thousands of feet of Paleozoic sedimentary rocks were deposited on the old Proterozoic surface. During the middle Pennsylvanian Period, yet another mountain range was uplifted in this area. From it the Paleozoic Period sediments were eroded. Sediments shed from these "ancestral Rocky Mountains" were deposited along the mountain flanks. The area that is now Rocky Mountain National Park was eroded again and intermittently covered by seas from the middle of the Permian Period to the end of the Cretaceous Period about 65 million years ago.

Third Uplift:
Major tectonic plates of the Earth's crust began to collide along what was then the western edge of North America about 130 million years ago. Uplift caused by this collision began to affect the area of the present Colorado Rockies about 70 million years ago. As the region began to rise, the Cretaceous sea withdrew and the thick layer of sedimentary rocks that had accumulated began to erode. Within a few million years, the sedimentary rocks of the Front Range had eroded away, and the Proterozoic igneous and metamorphic rocks again were exposed to erosion. As uplift proceeded, deep fault zones formed as enormous stresses pulled the Earth's crust apart at what is now the west side of the park. This allowed granitic magmas to rise into the present area of the Never Summer Mountains. Between 29 and 24 million years ago, the magmas reached the surface and erupted as volcanoes. The tops of the volcanoes stood several thousand feet above the present granitic masses of the Never Summers, which since have been eroded to their present size. Lava flows and extensive ash beds from the volcanoes are preserved in several areas within the park.

Addendum:
By the end of the Tertiary Period, the mountains of Rocky Mountain National Park were still fairly high but rounded. The area also was characterized by wide V-shaped stream valleys. Then, about 2 million years ago, the Earth's climate cooled and the Ice Age began. Glaciation in the park probably started about 1.6 million years ago.


Excerpts from:
U. S. National Park Service Website - Geology Fieldnotes; Rocky Mountain National Park, Colorado, April 2000

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