Are the mountains rising?

At the human scale, the St. Elias Mountains appear old, but their size, their steep and unstable slopes and abrupt edges belie their geologic youth. The mountain front as seen west of Haines Junction, is at least 1600 m (4000 feet) high, but less than 16 million years ago it was a region of low hills. It has risen against the Denali Fault, a strand which appears to be inactive.

As mountains rise they capture more precipitation from maritime weather systems. With their height and northern latitude precipitation comes in the form of snow. which compacts into ice. Slowly moving glacial ice is one of nature’s most effective rock-removing agents, and much of the sediment produced is carried by rivers out of the mountains and eventually to the Pacific Ocean. The submarine delta of the Copper River, which drains the St. Elias Mountains in eastern Alaska, contains sediment equivalent to the wearing down of the entire land surface of its watershed by 51 m in the last 10,000 years (Sheaf et al., 2003). Thus erosion by glaciers can wear the mountains down almost as fast as they are rising. But there are exceptions. Mount Steele (elevation 5071 m / 16, 400,) in the northern St. Elias Mountains is underlain by granite that cooled 9 million years ago. Since most granites cool about 3 km beneath the earth’s surface, this ancient surface must have been raised at least 8 km since that time – an average of 1 mm/year.

To determine how fast mountains rose in the past (called paleo-uplift calculations) geologists combine the rate at which rocks cool to form crystals, the increase in temperature with depth (called the geothermal gradient) and the speed at which the mountains are eroded. A first approximation is gained by determining the age of crystal formation in igneous rock now exposed at high elevations, compared with the same rock type that crystallized some time later, and is now exposed at low elevations.
A sequence of four cross sections of an uplifting and eroding mass.
Paleo-geothermal gradient
Photo Government of Yukon



A laboratory technique called fission track dating (http://en.wikipedia.org/wiki/Fission_track_dating) involves microscope analysis of tiny apatite crystals to determine the age of rocks collected at various elevations up the mountain.
The data points are derived from the length of fission tracks in the mineral apatite, found in granitic rock at various elevations on Mount Logan.
The data points are derived from the length of fission tracks in the mineral apatite, found in granitic rock at various elevations on Mount Logan.
Photo Government of Yukon



The uplift may be a consequence of strain buildup along the transform boundary, plate convergence, thermal expansion of the upper mantle, or a combination of all of these. Whether the mountain belt is arching, tilting landward, rising on thrust faults, or deforming internally remains unclear. It is also possible that recent uplift partly reflects rebound due to the reduction in mass of the ice since the end of the last Ice Age.

In 1992, an expedition sponsored by the Royal Canadian Geographic Society, the Geological Survey of Canada and the Canadian Parks Service made the first step toward determining the present-day rise of Mount Logan. The climbers visited accessible bedrock to install brass survey markers whose position was determined by high-precision GPS (Global Positioning System) satellite receivers. These sites will be visited every decade or so for re-measurement. Changes in elevation or lateral position may indicate uplift or tectonic movement.

A GPS antenna temporarily placed upon the summit of Mount Logan's highest peak to determine its elevation in June 1992.
A GPS antenna
Photo Michael Schmidt