Why is Mount McKinley so big?

Mount McKinley in the background as a researcher maintains a high-elevation weather station.

Mount McKinley is the crown jewel of Denali National Park and Preserve, the highest mountain in North America (at 20,310' above sea level), and a sought-after prize for sightseeing national park visitors and mountain climbing alpinists alike. Southern Alaska is a very mountainous place, so it's not surprising to find high glaciated mountains throughout many of Alaska's national parks. But few come anywhere close to the extreme height and prominence of Mount Mckinley. Why is Mount McKinley so big? Academic and government researchers teamed up to answer the question in this new paper.

The researchers begin the paper by reviewing existing theories for the generation of significant relief (differences between mountain tops and valley bottoms). Of course mountains require uplift, and tectonic forces associated with collision of the northward-flowing Pacific Plate and the relatively stationary Alaska block are responsible for the creation of most of southern Alaska's mountains. But once uplift commences, creation of a prominent peak like Mount McKinley requires a persistent contrast in erosion rate between rapidly denuding valley bottoms and more resistant summits. Mount McKinley seems to have an ideal combination of the theoretically necessary ingredients for this contrast: (1) juxtaposition of strong granitic rocks next to weaker metasedimentary rocks; (2) extremely cold high-elevation temperatures that prevent freeze-thaw activity from shattering summit rocks; and (3) strongly erosive glacier activity in the mountain valleys coupled with minimal glaciation (due to a combination of wind, low snowfall, and steep slopes) at higher elevations.

The heart of the new research is a dataset that confirms this inferred contrast between high- and low-elevation erosion rates. The researchers collected rock samples from a variety of locations on Mount McKinley and use the abundance of rare cosmogenic isotopes in those rocks to calculate long-term erosion rates. They found that high elevation erosion rates are generally slower than 50 mm per thousand years, while lower elevation erosion rates in the valley bottoms range between 140 and 900 mm/1,000 years. This roughly order-of-magnitude difference in erosion rates explains the great relief of Mount McKinley. The team also present new and existing weather data from various elevations on Mount McKinley to confirm that not only does it snow less at the highest elevations (inhibiting glacier growth), but temperatures rarely go above freezing at 14,200' and possibly never do at the summit. These data, some of which were collected by the NPS Central Alaska Network Glacier Monitoring Program, provide support for the inferred causes of Mount McKinley's contrasting erosion rates.

Note: The article below was published in 2024 and therefore refers to Mount McKinley as Denali.

Anomalously high relief on Denali, Alaska, caused by tectonic, lithologic, and climatic drivers

Abstract

We assess the growth of anomalously high relief on Denali, located in the Alaska Range, Alaska, and the tallest mountain in North America (6190 masl). Denali is 3000 m taller than most surrounding peaks. It lies inside a 19° restraining bend in the active Denali fault system that is moving at about 7 mm/yr, providing a tectonic and structural driver for ongoing rock uplift. High relief around Denali is also due, in part, to its granitic rock type and low fracture density relative to adjacent metasediments. Here we show that unique climatic conditions at high elevations also contribute to the growth of relief. We examine ¹⁰Be concentrations in 1) three new gravel samples between 3500 and 5200 m elevation from sites unaffected by recent glacial erosion, 2) previously published samples from a sidewall of the Kahiltna Glacier from 2400 to 2800 masl, 3) previously published data for samples collected from medial moraines along the length of the Kahiltna Glacier from ∼500 to 1400 masl, and 4) previously published data for alluvial samples collected along the Kahiltna River at an elevation of ∼200 masl. These samples constitute a transect extending >5000 vertical meters, and the data establish that erosion rates decrease with elevation and contribute to the growth of relief. Erosion rates for the three new high-elevation samples are calculated to 4.6 ± 0.6 mm/ka at 5200 masl, 28.6 ± 3.7 mm/ka at 4000 masl, and 38±5 mm/ka at 3500 masl. Erosion rates at intermediate elevations, on the sidewall of the Kahiltna Glacier, range between 160 and 327 mm/ka. Along the medial moraines inferred erosion rates range between 140 and 537 mm/ka, and basin-wide erosion rates calculated from sediments in the river below the glacier range between 450 and 896 mm/ka. These differences in erosion rates can create relief of 3 km within 1–10 Ma, well within the estimated period of increase in rock uplift and exhumation on Denali over the last ∼6 Ma. Meteorological data from 2130 to 5550 masl at 5 sites show temperatures rarely exceed freezing above 4000 masl elevation, indicating that frost weathering currently plays a diminished role in erosion at high elevations. The immediate implication of this temperature and erosional correlation is an increase in relief. This is the first study to directly measure a significant decrease in erosion rates at high elevations in the relative absence of frost weathering. The results highlight the combined influence of rock type, glacial erosion, and permanent sub-zero temperatures on erosion rates. In combination with active faulting, the data explain the resultant increase in relief along the southern side of the Alaska Range over the past 100 ka.

Matmon, A., P. Haeussler, and M. G. Loso. 2024. Anomalously high relief on Denali, Alaska, caused by tectonic, lithologic, and climatic drivers. Earth and Planetary Science Letters 646: 118999.