The Alaska Range is a 600-mile arc of mountains from the Alaska–Canada border to the Alaska Peninsula. It's highest in the middle, a region of towering peaks and huge glaciers inside Denali National Park and Preserve — a place so geologically active and complex that scientists are still piecing together its past. It holds rocks carried from thousands of miles away, fossils of sea creatures pushed up from the ocean floor, new igneous rock, and some of the oldest rocks in Alaska. Its height and northern latitude make it a land of eternal winter, where deep snow compresses into glaciers that grind at peaks still rising.
It would be one of the world's great geologic showcases even without North America's highest mountain.
Mount McKinley by the numbers
Mount McKinley's north face from Wonder Lake. NPS
| Summit | 20,310 feet — the highest on the continent |
| Base | about 2,000 feet above sea level |
| Base to summit | over three and a half vertical miles — compared with Everest, which rises from a 14,000-foot plain to 29,028 feet |
| Wickersham Wall (north face) | one of the world's highest continuous mountain faces, 14,000 feet from the Peters Glacier to the North Peak |
| Permanent snow and ice | over 75% of the mountain |
| Glaciers | up to 45 miles long and 3,700 feet thick |
| Recorded extremes | winds over 150 mph; −93 °F |
NPS
The land of eternal winter
Many consider it the world's coldest mountain. Everest is at 28° north — the same latitude as Florida's Walt Disney World — while Mount McKinley is at 63° north, 2,400 miles farther north, and that makes an enormous difference.
The mountain is so massive it makes its own weather, much as a big boulder in a river churns up rapids. Storms from the Gulf of Alaska and the Bering Sea slam into it, and sunshine can turn to a blizzard of fierce wind, intense cold and heavy snow in no time. Climbers must read the warning signs to decide when to climb, when to retreat, and when to dig in.
Rocks 101
USGS
| Rock type | How it forms | Examples |
|---|---|---|
| Igneous | molten rock solidifies — plutonic when magma cools slowly deep underground, volcanic when lava cools fast at the surface | granite (plutonic); basalt, rhyolite, andesite (volcanic) |
| Sedimentary | sediments laid down by water or wind — usually by rivers in lakes and seas — are buried and compressed into layers; their fossils reveal ancient environments | sandstone, limestone, shale, chert |
| Metamorphic | existing rock changed by intense heat and pressure without fully melting; minerals recrystallize, sometimes in wavy layers called foliation | around Denali: schist, slate, quartzite, marble |
Why is it so high?
USGS
Plate tectonics. Earth's crust is broken into plates floating on the mantle, which is mostly solid but moves slowly. Heated from the core, mantle rock rises; cooler, denser rock sinks and warms again — a convection current that shifts the plates. The Pacific Plate is moving north about as fast as your fingernails grow. Denser oceanic plates sink beneath continental ones — subduction — and the Pacific Plate is diving beneath the North American Plate under mainland Alaska, bringing along chunks of land from far away called terranes. The strain between the plates builds and releases in earthquakes.
NASA
Uplift and faults. Like the hood of a car buckling in a crash, subduction pushes up the Alaska Range and coastal ranges. Two faults matter most: the Denali Fault and the Hines Creek Fault. Land south of them moves west relative to the north by about 1 centimeter a year, and a big bend in the Denali Fault just north of Mount McKinley makes the rocks bunch up — right where the mountain stands. Those forces are still at work: the mountain rises about half a millimeter a year — a kilometer in the next two million years, a blink in geologic time.
Granite. The mountain is mostly granite, formed in a batholith — a great mass of magma within the crust (its parts, defined by chemistry, are plutons). Granite is usually less dense than the rock around it, so over millions of years the pluton floated up, like a cork released underwater — slowly. Erosion helped expose it, but granite resists erosion, so the rock rises faster than it wears away.
Subduction, uplift and slow erosion together explain the mountain's height.
Weathering, erosion and ice
Even as the range rises, weathering breaks rock into pieces and erosion carries them off by water, wind, glaciers and gravity. At Denali, ice does most of the work.
NPS
Up high, snow and ice built up year after year with little melting, until the ice was thick enough to compress under its own weight and flow down valleys as glaciers. In past ice ages — most recently about 10,000 years ago — glaciers covered the range and much of Alaska, and south-central Alaska has been buried in ice many times. Today's glaciers are shrinking remnants, but their marks are everywhere.
- A glacier can flow from several feet a year to several feet a day, grinding its bed into fine silt — glacial flour — that turns meltwater streams milky (glacial milk) and settles downstream as outwash.
- It plucks larger rocks and drops them as moraines — ridges and hills of unsorted debris — or as erratics, boulders unlike the rock around them.
- Rock-laden ice carves jagged ridges and U-shaped valleys; stranded ice blocks melt into kettle lakes; and the long lakes of the upper Susitna Valley, south of Denali, all point the way the ice moved.
A glacial erratic on a ridge in Denali. NPS
Sources
Based on "The Alaska Range and Mount McKinley: Geology and Orogeny," Denali National Park and Preserve, National Park Service, prepared with support from the Geological Society of America and GeoCorps America; a work of the United States government in the public domain; rewritten in hubnx's own words. The article says the mountain is a mile taller base-to-summit than Everest, which its own figures do not bear out, so only the figures are given.
In these publicationsDenali National Park & Preserve
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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