Denali Borough, Alaska
Denali (“The High One”) is one of the world’s truly great mountains. It’s a mountain of superlatives: highest point in the state of Alaska, highest peak in North America, greatest base-to-summit rise of any mountain in the world situated entirely above sea level. Of course, geology plays a fundamental role in the mountain’s greatness, and two geologic factors in particular: relative plate motions, and the erosional resistance of granitic rock.
In terms of plate-tectonic setting, Alaska lies at a bend in the road. South of Alaska, the western edge of the North American plate trends in a roughly northwest-southeast direction. The plate to the west—the Pacific plate—moves in a northwesterly direction relative to the North American plate, so the plate boundary is marked by strike-slip faults like the San Andreas in California and the Queen Charlotte–Fairweather fault system off the coast of British Columbia. At Alaska, the western edge of the North American plate makes an abrupt bend to the southwest; the transform plate boundary to the south transitions to a convergent boundary to the west, where the Pacific plate slides beneath the North American plate in a subduction zone at the Aleutian Trench.
A dramatic change in geology occurs where the North American–Pacific plate boundary transitions from transform (strike-slip faulting) to convergent (reverse faulting). Parallel to the trace of the Aleutian Trench is a chain of volcanoes—the Aleutian Islands—which owe their existence to magma generated by the melting of the Pacific plate as it descends into the Earth’s mantle at the subduction zone. At Alaska’s bend in the road, however, the geology is characterized by a collection of crustal blocks of varying composition that have travelled far from their places of origin. For the past 200 million years or so, small pieces of crust have been carried northward by faulting along the Pacific–North American plate boundary. Because of their composition and thickness, these “exotic terranes” were unable to subduct beneath the North American plate when they reached the bend in the plate boundary; rather, they piled up in a jumbled mass, adding crustal area to the North American plate in a process called accretion.
Alaska’s accreted terranes are bounded by faults, including the Denali fault which runs along the north side of the Alaska Range. A combination of strike-slip and reverse movement on these faults, as well as folding, characterizes the transpressional tectonic environment that effectively thickens the crust and contributes to the high average elevation of the Alaska Range.
The accreted terrane on which Denali lies is composed largely of Cretaceous-age sandstone, siltstone and shale. These marine sedimentary rocks were later intruded by magma, which altered these rocks to argillite through metamorphism due to the heat of the igneous intrusion. The magma cooled and solidified around 56 million years ago, forming the McKinley Granite. This granitic rock now forms the bulk of Denali as well as nearby Mount Hunter and Mount Huntington. Around 38 million years ago, cooling/crystallization of another body of magma formed the granodiorite of Mount Foraker. The intrusive igneous rocks of these mountains are much more resistant to weathering and erosion than are the softer sedimentary rocks that encase them. Over time, the sedimentary rocks have been eroding away at a faster rate than the igneous rocks, leaving the granitic peaks to tower over the other mountains of the Alaska Range.
Glacier movement is the main erosional force at work on Denali. High elevation, high latitude, and moisture-laden air blowing off of the Gulf of Alaska and Bering Sea combine to create huge accumulations of ice on the mountain’s flanks. Glaciers in Denali National Park and Preserve reach thicknesses of over 3,000 feet (900 m) and lengths of tens of miles; the Kahiltna Glacier—on which the lower part of the West Buttress climbing route is located—is the longest glacier in the park, at 44 miles (71 km) long. Ultimately, Denali’s fate lies in the balance between tectonic uplift related to plate motions, and the grinding away of rock by the movement of glaciers down the sides of the mountain.
A note on Denali’s summit elevation:
The traditional South Peak summit elevation of 20,320 feet (6,194 m) was established from photogrammetry in 1952. An IFSAR (Interferometric Synthetic Aperture Radar) survey in 2012 determined an elevation of 20,237 feet (6,168 m); however, while IFSAR is an extremely useful tool in certain situations, it does not provide precise spot or point elevations, especially in very steep terrain. Another elevation survey was conducted during the 2015 climbing season, using high-precision/accuracy Global Positioning Satellite (GPS) methods. The new summit elevation of 20,310 feet (6,190 m) is remarkably close to the traditional elevation, and serves as validation of the accuracy of the early surveyors and the “crude” equipment and methods available to them.
- LAT./LONG. 63.069 N / 151.007 W
- Land Status/Administration
Denali National Park and Preserve
- Physiographic Province
NA (Alaska Range)
- Representative Rocks:
- Class: Igneous
- Type: Granite
- Age: Tertiary
- Landform: Massif