Why Is Mount McKinley So Big?

A researcher maintains a high-elevation weather station with Mount McKinley in the background. Image from the National Park Service's page.

Mount McKinley, the centerpiece of Denali National Park and Preserve, is the highest mountain in North America at 20,310 feet (6,190 meters) and a prize for sightseers and climbers alike. Southern Alaska is full of high, glaciated mountains, but few come close to McKinley's extreme height and prominence — it stands about 3,000 meters taller than most of the peaks around it. A 2024 paper by academic and government researchers, including Park Service glaciologist Michael Loso, set out to explain why. (Published before the mountain's name changed, the paper calls it Denali.)

Uplift plus uneven erosion

Mountains need uplift. Most of southern Alaska's mountains were raised by the collision of the northward-moving Pacific Plate with the relatively stationary Alaska block, and McKinley sits inside a 19° restraining bend of the active Denali fault system, moving at about 7 millimeters a year, which keeps pushing rock upward.

But building a peak as prominent as McKinley also takes a lasting contrast in erosion: valleys must wear down fast while the summit resists. McKinley appears to have every ingredient for that:

1. Hard rock beside weak rock. Strong granite with few fractures sits next to weaker metasedimentary rock.
2. Extreme cold up high. Summit temperatures are too cold for the freeze-thaw cycles that shatter rock.
3. Glaciers below, little snow above. Vigorous glaciers scour the valleys, while wind, low snowfall and steep slopes keep the heights nearly free of glaciers.

Measuring erosion

The heart of the study is data confirming that contrast. The team sampled rock and sediment across more than 5,000 vertical meters and used the abundance of a rare cosmogenic isotope, beryllium-10, to calculate long-term erosion rates:

High elevations erode at generally less than 50 millimeters per thousand years, while valley bottoms erode at roughly 140 to 900 — about an order of magnitude faster. Differences like these can build 3 kilometers of relief within 1 to 10 million years, well within the roughly 6 million years over which uplift of the mountain is thought to have increased.

The weather at the top

Weather data from five sites between 2,130 and 5,550 meters, some collected by the Park Service's Central Alaska Network glacier monitoring program, confirm that it snows less at the highest elevations, limiting glacier growth, and that temperatures rarely rise above freezing above 4,000 meters (at the 14,200-foot camp, for instance) and possibly never do at the summit. With frost weathering playing little role up high, the summit erodes slowly.

The authors say this is the first study to directly measure a large drop in erosion rates at high elevation where frost weathering is largely absent. Together with active faulting, rock type, glacial erosion and permanent below-freezing temperatures explain the growing relief along the south side of the Alaska Range over the past 100 thousand years.

Sources

• National Park Service, Denali National Park and Preserve: "Why is Mount McKinley so big?", summarizing 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. The paper's abstract is summarized, not quoted.