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The secret of the Colorado 14ers

by Jack Reed

It will hardly come as news to Trail & Timberline readers that there are some fifty-four peaks in Colorado rising to altitudes of 14,000 feet or more. The exact number depends on how one chooses to define a separate peak, and the question has been the subject of endless debate among climbers and guidebook authors.

Whatever the number, it seems remarkable that none of the fourteeners exceeds an elevation of 14,433 feet and that all of the peaks lie within a circle having a radius of only 120 miles centered in the Sawatch Range near Buena Vista.

Even more remarkable is the fact that of the hundred or so peaks over 14,000 feet high in North America, more than half are in Colorado. Many summits in Alaska, the Yukon, and central Mexico rise to elevations far above 14,000 feet, but Colorado has by far the largest area with an average elevation greater than 10,000 feet. What is the explanation of this remarkable area of high elevation nearly nine hundred miles from the edge of the North American tectonic plate?

Three kinds of mountains

The great mountains of North America can be grouped into three general categories: (1) peaks composed of rocks elevated by plate tectonic move­ments along the west­ern margin of the continent, (2) active or recently active volcanoes, and (3) peaks in areas under­lain by meta­morphic and granitic rocks that are part of the continental crust beneath most of North America.

The first group of tectonic mountains includes Mt. McKinley and its neighbors in the Alaska Range as well as Mt. St. Elias and nearby peaks in the Coast Range of Alaska and the Yukon.

The second group, the active volcanoes, includes those in the Wrangell Mountains of east-central Alaska, the Cascade volcanoes, and the great volcanoes of the trans-Mexican volcanic belt.

The third group includes all of the Colorado fourteeners and probably the fourteeners in the Sierra Nevada.

If we plot a simple graph (figure 1) showing the elevation of a given peak on the vertical axis and the number of peaks in North America that are higher than that peak on the horizontal axis, we see a remarkable distribution. The peaks of groups one and two define the high elevation part of the distribution, while the Colo­rado peaks and the peaks of the Sierra Nevada are confined entirely to the lower elevation end. This suggests that there must be some fundamental differences in the manner in which the peaks were formed.

High mountain formation

The elevation of a mountain depends on the difference between the rate at which the rocks that form it are uplifted (or in the case of volcanoes, the rate at which new volcanic material is added) and the rate at which the rocks are eroded. Rapid uplift can produce high mountains even in relatively soft and easily eroded rocks. Slow uplift can produce high mountains only if the rocks are hard and resistant to erosion. Thus, we would expect to find the highest mountains in areas of rapid uplift and erosion-resistant rocks.

The peaks of the Alaska Range lie just south of the Denali fault, along which a sliver of North America that includes much of south central Alaska is moving northwestward with respect to the rest of the continent. The highest peaks, including McKinley, lie near the prominent arc where the fault bends from northwest to southwest. Apparently the rocks at the bend are being uplifted as the northwest-moving continental sliver encounters the east-west and southwest trending segments of the fault (see figure 2).

Similarly, the Alaskan Coast Range peaks lie near the great bend of the Fairweather fault. Along this fault, parts of the Pacific ocean floor are moving northwest with respect to North America.

Rock along both the Denali and Fairweather faults probably is traveling at rates of several inches per year. Consequently, the rates of uplift of the mountain blocks may be as much as half an inch per year—geologically a very rapid rate. Most of the high peaks are carved in large bodies of hard granite that are surrounded by soft and much more easily eroded rocks.

Volcanic formation

Volcanoes typically form above zones in which ocean floor tectonic plates are moving down and under adjacent plates. This process is known as subduction. Where the down-going plate reaches a depth of sixty miles or so, it begins to melt, and the molten rock material rises through the overriding plate to erupt as a chain of volcanoes. All of the high volcanoes of North America lie above such subduction zones.

The height of a volcano depends on the rate at which lava is being erupted, and this in turn depends on the rate of tectonic plate subduction. The numerous high volcanoes of central Mexico and the Wrangell Mountains lie above zones where oceanic plates are being rapidly subducted; the lower volcanoes of the Cascades sit atop a zone of much slower subduction.

Thus, all of the North American peaks that fall on the upper limb of the elevation curve lie in areas where uplift or addition of material is so rapid that they far exceed the rate of erosion, even though erosion in these areas is also rapid.

Colorado geology

The geologic setting of the Colorado peaks is quite different. These mountains lie in a region that was affected by a period of mountain-building called the Laramide orogeny that occurred between 75 and 65 million years ago. During this orogeny [an orogeny is a period of mountain formation caused by folding the earth’s crust—ed.] the ancient metamorphic rocks and granite that form the crust beneath the continent rose in a series of uplifts in New Mexico, Colorado, and Wyoming.

“Basement” rocks in the uplifts rose tens of thousands of feet higher than similar undisturbed rocks beneath the High Plains to the east. As the uplifts rose, they also eroded. In many of these uplifts, thin layers of younger sedimentary rocks that once lay on the basement were stripped away leaving the basement rocks exposed at the surface (as in the Front Range, Mosquito and Tenmile Ranges, and the southern parts of the Sangre de Cristos).

Where thick and erosion-resistant sedimentary rocks had been deposited on the basement rocks, the sedimentary rocks formed the cores of the Laramide uplifts (as in the Elk Mountains and the northern part of the Sange de Cristos).

By the end of the Laramide era the uplifts had been eroded to a landscape of low mountains with summit elevations only of 3,000 to 6,000 feet and broad valleys whose floors stood only a thousand feet or so above sea level. Later, between about forty and twenty million years ago, great volumes of volcanic rocks were erupted and filled many of the low areas in the post-Laramide surface. These are the rocks that now form large parts of the San Juan Mountains.

The broad area of high elevation that includes much of Colorado, Wyoming, and New Mexico roughly coincides with the region affected by the Laramide orogeny. The post-Laramide land surface has risen by as much as 8,000 or 9,000 feet over the last twenty million years; much of the uplift may have taken place in the last five to ten million years. The modern landscape was shaped during this late uplift: erosion cut all of the major canyons and sculpted all of the present ranges. The elevations of the highest peaks would thus be controlled by the height of the highest parts of the post-Laramide surface and by the amount of post-Laramide uplift.

But why are there no fourteeners in Wyoming and New Mexico?

Colorado is unique

Perhaps the answer lies in two geologic features that are unique to Colorado. The Colorado mineral belt is a relatively narrow northeast-southwest belt marked by a number of intrusions of granite-like igneous rocks from the Laramide orogeny and later widespread volcanic eruptions. The belt is so named because almost all of the major mining districts in Colorado lie within it. It is thought to be underlain by large bodies of igneous rocks that are relatively light and possibly still warm. Such bodies tend to rise, elevating the land surface above them.

The Rio Grande rift is the other significant feature. It is a linear belt of relatively narrow fault-bounded basins produced by separations in the earth’s crust. The narrowest and most active part of the rift lies in southern and central Colorado along the San Louis and upper Arkansas valleys. The rift is still actively opening, advancing northward like a tear made when you pull apart a piece of cloth. When rifts of this sort develop in a continent, they act as conduits for heat from within the earth. The rocks on the flanks are heated and expand. Because of the expansion, the rocks become lighter and tend to rise, forming mountains along the rift margins.

If we examine a map showing the locations of the Colorado fourteeners (Figure 3), we see that all but two are in or near the Colorado mineral belt or on one of the flanks of the Rio Grand rift. The highest and most numerous peaks lie near the intersection of these features.

The exceptions are Pikes Peak and Longs Peak, both of which are carved in large masses of resistant granite that probably stood as high points on the post-Laramide land surface. Thus the Colorado 14ers seem to be the result of late local uplift related to the mineral belt and the Rio Grande rift superimposed on broad uplift of the entire Rocky Mountain region. They form a mountain group quite distinct from the mountains of the present margin of the North American plate.

Jack Reed is a forty-year member of the Colorado Mountain Club and a geologist emeritus at the U.S. Geological Survey, where he is working on the compilation of the new geologic map of North America. This article is condensed from his 1994 presidential address to the Colorado Scientific Society.