Lake County, Colorado
In a state known for its high mountains—Colorado boasts more than fifty “fourteeners,” or peaks exceeding 14,000 feet (4,267 m) high—Mount Elbert stands above them all. But the rocks at Mount Elbert’s summit haven’t always held such a lofty position. In fact, the nature of the rocks indicates that at one time in their history they were miles beneath the surface of the Earth. From the basement to the roof of the Rockies—such are the ups and downs of geology!
The bulk of Mount Elbert is underlain by metamorphic and granitic igneous rocks of Precambrian age. The upper part of the mountain consists mostly of gneiss containing quartz, feldspar, mica, and in some places garnet. These rocks were originally deposited as sandstone and siltstone in the Early Proterozoic Era, sometime between 1.6 and 2.5 billion years ago. The presence of a metamorphic rock type called amphibolite indicates that the original sedimentary rocks were later subjected to pressures in excess of 3,000 bars (1 bar is roughly equal to atmospheric pressure at sea level) and temperatures in the range of 450°–700°C (800°–1,300°F). These pressure and temperature conditions indicate metamorphism at considerable depth in the crust. Early in its history, the rocks that would become the gneiss of the upper mountain were intruded by magma that would crystallize to form the Denny Creek Granodiorite, which is now exposed on the lower part of the mountain’s southern and eastern slopes. Radiometric dating indicates the Denny Creek Granodiorite is about 1.6 to 1.7 billion years old.
The journey of Mount Elbert’s rocks “from the basement to the roof” took place over the course of at least two mountain-building episodes, or orogenies. The first episode took place near the end of the Paleozoic Era, around 300 million years ago, when Earth’s continental plates converged on each other to form the supercontinent Pangea. Compressional tectonic forces created folding and reverse faulting that uplifted the Ancestral Rockies in the region of what would later become the modern Rocky Mountains. The Ancestral Rockies have long since eroded away, but certain stratigraphic and structural-geologic relations attest to their past existence, as do their erosional remnants preserved in Pennsylvanian to Permian-age deposits exposed in places near Moab, Utah (Cutler Formation) and Boulder, Colorado (Fountain Formation).
Long after Pangea eventually broke apart, the North American plate underwent another episode of compressional tectonics, producing the Laramide Orogeny between roughly 75 and 40 million years ago. It was this event that created the modern Rocky Mountains, thrusting basement rocks miles above sea level. Interestingly, both the Ancestral Rockies and modern Rockies formed at a considerable distance from any known plate margin, something of a contradiction to plate tectonic theory and a puzzle that geologists are still trying to solve.
One of the geologic structures produced by compression during the Laramide Orogeny was a large arch-shaped fold called the Sawatch anticlinorium. Mount Elbert lies near the crest of this fold, although the fold isn’t obvious. Part of the reason is because after the Laramide Orogeny, tectonic forces switched to crustal extension, and the east limb of the fold was cut by normal faults. The result was the Arkansas River valley, a structural valley (graben) bounded by the Sawatch fault on the west and the Northeastern Boundary fault system on the east. Both of these faults are believed to have been active in the past 750,000 years, and the southern section of the Sawatch fault shows evidence of movement in the past 15,000 years. The graben dropped down as the result of movement along the faults, dissecting the Sawatch anticlinorium near its axial crest. The Arkansas River valley is considered to be the northernmost topographic expression of the Rio Grande Rift (for more information on the Rio Grande Rift, see the geologic page for New Mexico’s Wheeler Peak).
The last chapter in Mount Elbert’s geologic story is the shaping of the landscape by alpine glaciers during the Pleistocene Epoch, particularly during the Bull Lake glaciation (about 100,000–140,000 years ago) and Pinedale glaciation (about 10,000–40,000 years ago). Glaciers deposited moraines and eroded cirque basins on the eastern and southern slopes of the mountain, in the Box Creek and Bartlett Gulch drainages, respectively. Large ice streams flowed down the Halfmoon Creek drainage to the north of Mount Elbert, and the Lake Creek drainage to the south, depositing prominent lateral and terminal moraines in the Arkansas River valley. Twin Lakes, at the mouth of the Lake Creek drainage, occupy a natural basin within the terminal moraine complex of the Lake Creek glacier. The surface area of the lakes was enlarged by construction of the Twin Lakes Dam in 1978.
- LAT./LONG. 39.118 N / 106.445 W
- Land Status/Administration
San Isabel National Forest
- Physiographic Province
Southern Rocky Mountains
- Representative Rocks:
- Class: Metamorphic
- Type: Gneiss
- Age: Precambrian
- Landform: Massif