Sumdum Glacier, Powers Creek

Sumdum Glacier, Powers Creek

by | Jun 22, 2023

Sumdum Glacier starts on the south flank of Mount Sumdum with a summit elevation of 6,666 feet (2,032 m) and flows southwest to a hanging terminus at 2,100 feet (640 m) and the start of Powers Creek that flows 2 miles (3.2 km) to the eastern shore of Endicott Arm, about 89 miles (144 km) northeast of Sitka and 50 miles (81 km) southeast of Juneau, Alaska. The name was first published in 1892 by the U.S. Coast and Geodetic Survey as ‘Soundon’. The Tlingit name for the glacier reputedly refers to the booming sound of the icebergs as they break off from the glacier. Powers Creek was named in 1906 by Arthur C. Spencer and Charles W. Wright after a local prospector who was working a small gold placer along this creek. Over the last 200 years, Sumdum Glacier has retreated from Endicott Arm and now terminates on a steep slope as a hanging glacier where the principal loss of mass is from icefalls and avalanches. The glacier descends almost 5,000 feet (1524 m) in 2.8 miles (4.5 km) to an elevation where conditions are more temperate resulting in a destabilization of the interface between bedrock and ice. When the glacier slides over bumps, cavities form above the bedrock, leading to a loss of adhesion between the glacier and the bedrock, and resulting in a catastrophic failure of the ice sheet called an icefall. In addition to a loss of mass at the terminus, Sumdum Glacier also has a small accumulation zone, the area above the firn line, where the net accumulation of snow exceeds the losses from melting, evaporation, and sublimation. Because of the steep slope, the accumulated snow also has a short residence time at the higher elevations, leading to a gradual loss of ice mass, glacial thinning, and inevitable retreat upslope.

The glacially striated cliffs of Endicott Arm and Tracy Arm show the granite and partially metamorphosed rock associated with the Coast Range Batholith. The Coast Range episode of mountain building began 115 million years ago when a chain of volcanic islands riding on an oceanic plate collided with the western shoreline of the continental plate in the northeast Pacific. These islands were welded to the edge of the continent by molten rock that cooled deep beneath the Earth’s surface to form the Coast Range Batholith which is an enormous body of granitic rocks. This batholith is the largest such body of granitic rocks in North America. The western metamorphic belt forms the western margin of the batholith throughout most of southeastern Alaska. Ore-bearing outcrops occur in a northwest-trending belt of mineralized metamorphic rocks between Tracy Arm and Endicott Arm. The better-known mines and prospects are at Point Astley and Sanford Cove where gold was the principal commodity sought by the early prospectors and most of the prospects were developed around 1900 or shortly thereafter. These also yielded significant quantities of silver as a byproduct. Following the gold mining in the early 1900s, there was little activity in the area; however, a titaniferous magnetite deposit was discovered at Port Snettisham in 1918, and a zinc-copper prospect in Tracy Arm was discovered in 1916.

The Sumdum Glacier mineral belt extends for about 32 miles (51 km) along the southwest side of the Coast Range batholithic complex and contains three known important mineralized areas; the Tracy Arm zinc-copper prospect, the Sweetheart Ridge gold-copper occurrence and the Sumdum copper-zinc prospect. In 1958, the Sumdum copper-zinc prospect was discovered by the Alaska Helicopter Syndicate in rugged terrain on Mount Sumdum at elevations from 2,000 feet (610 m) to the summit at 6,666 feet (2032 m). The prospect is within a few thousand feet of the western margin of the Coast Range batholith in rocks that locally have been contact metamorphosed. The ore occurs in northwest-trending zones that are generally not traceable for more than a few hundred feet because of snow or covered by the Sumdum Glacier. However, diamond-drilling data indicate that at least two of these zones are continuous for at least 10,000 feet (3,048 m) and have a thickness of 1 to 50 feet (0.3-15 m). Pyrite and pyrrhotite are the dominant sulfide minerals; secondary minerals are chalcopyrite and sphalerite, and very minor amounts of bornite, chalcocite, malachite, azurite, and galena constitute the ore minerals. Read more here and here. Explore more of Sumdum Glacier and Powers Creek here:

About the background graphic

This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2022 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

Credit: Professor Ed Hawkins (University of Reading). Click here for more information about the #warmingstripes.

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