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Geology of The Rocky Mountains

Aspen, Colorado - home to the Rocky Mountains: draws jet-setters from around the globe and presents geologists with a tantalizing geological puzzle to deduce when and why these beautiful mountains arose.
This article appeared in Vol. 15, No. 2 - 2018

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Aspen, one of the Rockies’ most affluent resort towns, was originally founded by silver miners. Photo credit: Lon Abbott and Terri Cook.


The Rocky Mountain High in Aspen, Colorado

Aspen's Mining History

Rocky Mountain Range in Colorado, USA. Image source: ©2018 Google. Image: Landsat / Copernicus. In 1879, intrepid miners searching for gold and silver high in central Colorado’s mountains braved the dangerous climb up and over the Continental Divide. As they descended the western slopes, the men discovered the Roaring Fork Valley, a region of soaring, crimson-colored peaks surrounded by lush, green vales incised by crystal-clear streams. The area’s rich ores quickly attracted thousands of people. In just over a decade the main settlement, which was soon renamed Aspen, surged to nearly 12,000 people, and its silver production assumed global importance. Although the town’s population plummeted after the price of silver crashed during the panic of 1893, the foundations had been laid for one of the Rockies’ poshest and most celebrated resort towns.

In 1950, just three years after Aspen Mountain opened its first ski lift, the town became the first site outside Europe to host the World Alpine Championships, an event that put this surprisingly laid-back town squarely on the jet-set map. Singer-songwriter John Denver, who made Aspen famous with ‘Rocky Mountain High’ and other tunes, and ‘gonzo’ journalist Hunter S. Thompson, whose run for sheriff in 1970 cemented the town’s reputation as a countercultural mecca, were among its most famous residents.

Visiting Aspen Now

Today the town continues to draw a steady stream of movie stars, musicians, and executives. Aspen is so popular, and buildable land so scarce, that its real estate is among the most expensive in the U.S., with a median house price of $2.6 million. Fortunately, you don’t need to be a multi-millionaire to immerse yourself in the region’s spectacular scenery. Several short excursions, including the highest paved crossing of North America’s Continental Divide, bring visitors face to face with beautiful views and the chance to examine the geologic upheavals that have transpired during the last 300 million years to produce the town’s historic mining wealth and its modern ‘white gold’ – powder skiing.

Major Tectonic Events in the Rocky Mountains

The Elk Range, West of Aspen

Unlike the crystalline peaks that comprise most of the Colorado Rockies, the Maroon Bells are carved from distinctive synorogenic sedimentary rocks deposited in a trough between two Ancestral Rocky Mountain ranges. Photo credit: Lon Abbott and Terri Cook. The gorgeous Maroon Bells, a pair of 14,000-foot-high (4,260m), bell-shaped peaks south-west of Aspen, are an icon of Colorado’s alpine grandeur. The sight of the bells’ scarlet slopes mirrored in the still waters of Maroon Lake, especially during the fall when the valley’s dense groves of aspen trees glow golden yellow and the summits are dusted with snow, is among North America’s most photographed vistas. Maroon Lake is a 10 km shuttle-bus drive, bike ride, or cross-country ski jaunt up a beautiful glacial valley from the Aspen Highlands ski area.

The Maroon Bells, like many peaks in the Elk Range west of Aspen, have a distinctively different appearance compared to most Colorado peaks because they consist of layered sedimentary rocks rather than the more typical Precambrian granite and gneiss. The reasons for this difference relate to the geographies and tectonic styles during two different mountain-building episodes – the 300 Ma Ancestral Rockies Orogeny and the 70–40 Ma Laramide Orogeny, the seminal event that determined the location of today’s mountains.

The Central Colorado Trough

Transpressional faulting during the former raised two ranges — an eastern one located on the footprint of the modern Front Range, just west of the capital city of Denver, and a western one that ran from modern-day Grand Junction to Durango. Although the entire Paleozoic sedimentary section was eroded from both ranges to expose the Precambrian crystalline bedrock, these rocks were preserved in the intramontane lowland known as the Central Colorado Trough, in which Aspen lies. The synorogenic Maroon Formation was added on top, resulting in a five-km-thick sedimentary sequence in the Trough.

The Laramide Orogeny

The next major tectonic event to affect Colorado, during the latest Cretaceous and early Tertiary, was the Laramide Orogeny. Most of the discrete ranges built during this event rose in approximately the same location as the earlier Ancestral ranges, so the Precambrian crystalline rocks underlay a notably thin sedimentary section that was quickly eroded, thereby producing Colorado’s quintessential crystalline peaks. But in the Aspen area, the Laramide rise of the Elk Range inverted the Central Colorado Trough, whose layered rocks, most prominently the deep red Maroon Formation, comprise most of the range’s spectacular peaks, including the Maroon Bells.

Highest Point of the Sawatch Range

The drive over Independence Pass, the Continental Divide’s highest paved crossing, offers tantalizing scenery and the chance to reconstruct key events that shaped the southern Rockies. Photo credit: Lon Abbott and Terri Cook. The drive east of Aspen up to Independence Pass, the crest of the Sawatch Range, the largest and tallest of Colorado’s Laramide ranges, ascends the highest paved crossing of North America’s Continental Divide. Deep snow closes the pass each winter, but its snaking switchbacks, narrow pavement, and spectacular scenery make it a great summer driving or cycling adventure.

A trip over Independence Pass also provides an opportunity to track some of the key tectonic events that shaped today’s southern Rockies. By the Jurassic, erosion had erased the last vestiges of the Ancestral Rockies topography, and subduction had commenced along the west coast of North America. That plate compression initiated the Sevier Orogeny, producing a series of fold-and-thrust belts that stretch from the Canadian Rockies in the north to western Arizona in the south. Colorado, by contrast, was a lowland traversed by sluggish, meandering rivers. After a Cretaceous rise in eustatic sea level flooded that lowland, Colorado became part of the Western Interior Seaway, in which more than two kilometers of marine shale accumulated. Those marine rocks host many of Colorado’s most important oil and gas systems, including in the Denver Basin and the Piceance Basin west of Aspen.

The Roaring Fork River tumbles over Precambrian granite at The Grottos along the drive up Independence Pass, which climbs over the Laramide-age Sawatch anticline. Photo credit: Lon Abbott and Terri Cook. One puzzling attribute of the Laramide Orogeny is the fact that deformation in Colorado occurred 1,000 km from the active tectonic boundary in California. Geologists explain this unusually large gap between trench and mountains by invoking nearly flat subduction of the Farallon Plate, which produced a compressional stress field unusually far to the east.

The Sawatch Range consists of a large anticlinal arch bounded on the west by the Castle Creek reverse fault that lies just west of Aspen. The Independence Pass road ascends the dome’s western flank, with the drive affording sweeping alpine vistas of the range’s Precambrian granites. The pass stands a lofty 12,095 ft (3,687m) above sea level. After you catch your breath, it’s worth strolling along the gentle trail that traverses the wildflower-filled tundra south of the pass.

After a kilometer the granite gives way to welded tuff as you cross the ring fracture of the 34 Ma Grizzly Peak Caldera, one of the numerous rhyolitic and andesitic volcanoes that erupted across the Colorado Rockies between 37–24 million years ago. The cause of this so-called ‘Ignimbrite Flare-up’ in the aftermath of the Laramide Orogeny is yet another tectonic puzzle.

The Ghostly Town of Ashcroft, Castle Creek

The ghost town of Ashcroft, which in its heyday boasted 2,000 residents, today consists of nine historic buildings, including a post office, an assay office, and two of the 20 original saloons. Photo credit: Lon Abbott and Terri Cook. The ghost town of Ashcroft, located along the Castle Creek Road south of Aspen, is one of the best places to explore the Aspen area’s rich mining legacy. The town was built during a two-week period in 1880 by a group of intrepid prospectors led by ‘Crazy’ Charles Culver and W.F. Coxhead. By 1883, the camp had a population of close to 2,000 and hosted a school, sawmills, and 20 saloons. Although Ashcroft’s initial silver production was fantastic, the deposits were shallow, and big strikes in Aspen soon lured away most of the inhabitants. By the turn of the 20th century, only a handful of single men remained, and they reportedly spent most of their days in the local bar rather than working their claims. By 1939, all of the original inhabitants had passed away. Today, visitors can stroll past more than a dozen false-fronted buildings, labeled with interpretive signs, to learn more about the silver camp’s history.

The rich silver lodes in Ashcroft, Aspen, and other nearby camps all formed when magma bodies and accompanying mineralized fluids intruded the crust during the Laramide Orogeny. They are part of a belt of Laramide-age plutons called the Colorado Mineral Belt. Although such magmatic belts commonly form during orogenies, they are usually oriented perpendicular to the subduction direction. Paradoxically, the Colorado Mineral Belt is oriented north-east – south-west, parallel to the direction of plate subduction. This unusual alignment presents yet another puzzle associated with the Laramide Orogeny that geologists continue to grapple with.

World’s Largest Hot Springs: Glenwood Hot Springs Resort

The Aspen area’s dramatic scenery is enhanced by the stark contrast between the Maroon Formation’s vivid red sedimentary rocks and numerous light-gray middle Cenozoic intrusions, one of which is Mount Sopris, the peak on the left skyline. Photo credit: Lon Abbott and Terri Cook. As puzzling as many Laramide characteristics are, these attributes form just part of the riddle that scientists have yet to solve to explain why Colorado even hosts mountains today. Several lines of evidence indicate the Colorado Rockies cannot be explained solely as the result of the Laramide Orogeny. There’s no better locale to contemplate this puzzle than the world’s largest hot springs pool at the historic Glenwood Hot Springs Resort, located adjacent to the Colorado River in Glenwood Springs, 65 km from Aspen.

The pool’s water is derived from the Yampah Spring, which issues 8,500 liters per minute at a toasty 50°C. The mineral-rich water is then diluted to 34°C in the main pool. According to both heat flow and seismic data, these and many other hot springs scattered throughout the Colorado Rockies are the product of unusually warm mantle underlying this region. It is this warm, buoyant mantle that supports the state’s high topography, not the typical crustal root that sustains most other compressional mountain ranges. Seismic data reveal the absence of a root beneath the Colorado Rockies, and structural studies document 10–20% crustal shortening during the Laramide. Isostatic calculations indicate that mountains produced by such modest crustal shortening and thickening would, on average, be only half as high as today’s average 3,000m-high range.

Although most geologists agree that the spring that feeds the world’s largest hot springs pool in Glenwood, Colo. is the product of the region’s unusually hot mantle, they still debate when the mantle heated up. Photo credit: Lon Abbott and Terri Cook. Although scientists generally agree that Colorado’s mountains are supported by warm, buoyant mantle, they lack consensus regarding when that mantle heated up and the modern mountains rose. Mount Sopris, the summit that dominates the vista on the return drive to Aspen, illustrates the crux of this debate. It is one of several impressive Elk Range peaks that consist of light gray, 34-million-year-old granite. Those plutons likely formed in magma chambers that fed Ignimbrite Flare-up volcanoes similar to the Sawatch Range’s Grizzly Peak Caldera. Many scientists argue that mid-Tertiary mantle heating is what produced the huge volume of magma that fed the Flare-up, as well as the rise of the modern Colorado Rockies due to associated thermal expansion. But other scientists who note that a vigorous cycle of erosion commenced across the region sometime between 5–10 million years ago argue that mountain uplift in response to mantle heating at that time is likely what triggered that erosion.

Solving the Puzzle...

Xenolith in welded tuff, going for scale. Photo credit: Lon Abbott and Terri Cook. Contemplation of these tectonic puzzles provides geologists with an added incentive to relax in and enjoy Aspen’s captivating alpine scenery.

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