aiming for the top

Mount Logan: Exploration
Geology and Geophysics

by: R. Campbell, L.Currie, G. Holdsworth, R. Parrish, M. Schmidt and C. Yorath

The St. Elias Mountains and the Icefield Ranges are dominated by vast, permanent icefield as if still locked in a miniature iceage. These icefields flow between high and rugged mountains seeking outlets through which to discharge their mobile mass of ice. The highest mountain by far, Mount Logan (5959) is mostly covered by snow and ice, with rock outcrops occurring close to its many subsidiary peaks, on many ridges and on some steep avalanche prone faces (see photo). The scarcity of accessible rocks, combined with the remoteness of the mountain, the high altitude and unpredictable and hazardous weather conditions have limited geological field work carried out there. However, in the last two decades, a basic understanding of the geology has emerged from studying the rocks (petrology) retrieved by mountaineers (some of whom were professional geologists) and by studying geochemical and geophysical data. In addition, information from aerial photographs and satellite imagery has been useful.

The mountain belts in this region have been generated by compressive and shearing forces produced by the movements between the Pacific Plate and the North American Plate two of the 20-odd plates that make up the earth's crust. Relative to North America, the Pacific plate is moving generally north-westward, while the continental plate is moving toward and over the marine plate. The regional geology indicates that these tectonic movements have been occurring continuously for about the last 160 million years (Ma) and discontinuously for as much as 600 Ma.

Within the Western part of the North American plate geologists have recognized and delineated five belts (or zones) which are in the order of 200 km wide and thousands of km's long, running sub-parallel to the western coastline. Within these belts smaller divisions called Terranes have been defined on the basis of geological components which indicate a specific environment and location of formation. Faults separate these units which are typically 10's to 100's of km in width and length. They may have been displaced great distances over millions of years. A major fault line separating two terranes traverses the lower part of the south face and ridges of Mt. Logan. In the accompanying photo this can be identified as the boundary between the dark metamorphosed sedimentary rocks in the foreground (Valdez group within the Chugach Terrane) and the lighter (grey) granite rocks in the background which belong to a batholith imbedded in the Wrangellia-Alexander Terrane (containing rocks 130-430 Ma old). The fault, inactive for the last ~50 Ma, is called the Border Ranges fault which can be traced from far to the SE in Alaska, arcing northwestward through to Kodak Island. High pressures and temperatures within parts of Wrangle had already caused melting of some of the deeper rocks at least 150 ma ago during an earlier episode of activity. When the Border Ranges Fault was active, the Chugach terrane rocks on the south side were forced to pile up or slide obliquely under the Wrangellia terrane on the north side, causing it to be squeezed as well as subsequently to be lifted up. The once molten rocks deep in Wrangellia began to rise, cool and solidify, defining a giant granitic batholith, the top of which became exposed as covering strata was continually stripped off by erosion. Thus the birth of what was to become he future Mt. Logan, had begun.

Later, other faults formed successively to the south and west, so the uplift of the batholith was maintained as more and more sedimentary sequences (formed in the Pacific ocean) were plastered onto and forced under the edge of the growing continental plate by compressive and shear forces. Mount Logan, as we see it today, is composed mainly of the upper part of a rather complex batholith which is still rising. A permanent snow and ice cover may have developed on the upper mountain several million years ago. During the Pleistocene (Ice Age) the ice cover expanded: icefields and large piedmont glaciers formed, many of which can still be seen today in reduced form, both areally and in thickness. The snow and ice cover on the top of Mt. Logan may be thousands of years old where it is in contact with the rock 200-300m below the surface.