The scarcity of accessible rock, combined with the remoteness of the mountain, its high altitude and severe weather conditions, limit geological field work. Rock is exposed in largely inaccessible cliffs beneath which the glacier has large cracks (crevasses), or are overhung with ice (seracs). On the high plateau of Mount Logan and adjacent ridges, small areas of rock poke through the snow and ice (nunataks) and many of these have been visited by geologists. Rock samples have also been retrieved by mountaineers and analyzed in university laboratories. These studies, combined with regional geological maps and research throughout the St. Elias Mountains, help to understand this geologically active mountain belt. In this section you will find information about the local geology of Mount Logan, and the implications of its active plate tectonic setting for earthquakes and the rising mountains.

The Mount Logan we see today is surrounded by an icefield up to 300 m thick, but beneath it is mostly granitic rock. Geologists call this rock type ‘quartz diorite’ because its composition is about 20% quartz, 65% white and pink feldspar and 15% brown biotite mica. Only the upper part of this enormous granitic body is exposed; because it is more than 60 km long and 20 km wide, it is called a batholith. It cooled from a molten state about 153 million years ago (Jurassic period), after intruding into sedimentary and volcanic rocks that formed about 270 million years ago (early Triassic period). Only small patches of the sedimentary and volcanic rocks remain: engulfed in the quartz diorite they indicate to geologists what environments used to be there before the intrusion.
The nunatak near Northwest Col contains three rock types.
Nunatak near Northwest Col
Photo Government of Yukon

These patches of sedimentary and volcanic rocks have been changed by the heat of the rising magma to schist, a dark striped rock type. The process is contact metamorphism.

Rock ridges that buttress the south face of Mount Logan and King Peak are mostly dark-coloured shale, sandstone and siltstone of the Chugach Formation. They were deposited in a deep marine environment between 70 and 100 million years ago (Cretaceous period). These sedimentary rocks are juxtaposed with the older Mount Logan batholith along the Border Ranges Fault. Later, both rock types were intruded by granite. This smaller body, called the King Peak pluton, is lighter coloured granite containing 35% quartz, 50% feldspar of two types and 15% black biotite, although there is compositional variation. This granite cooled from a molten state 51 million years ago. The sharp contact of these two intrusive rock types is visible in cliffs north of King Trench.
Exposed rock along King Trench.
Exposed rock along King Trench
Photo Charlie Roots