Move any mountain

Eva Enkelmann tells us how climate change really can move mountains.

Studying the North American St. Elias Mountain Range is particularly ideal because this area is very active tectonically, with strong glacial erosion. A good example is the Great Alaskan Earthquake of 1964 – the world’s second largest earthquake recorded to date – that also resulted in a tsunami. Our observations, as part of a multidisciplinary research project called STEEP (St. Elias Erosion and Tectonics Project), suggest climate-driven erosion on a mountain range influences the tectonics, which can alter the motion of the rocks in that area.

Rising along the coastline from southeast Alaska through northwest Canada, the St. Elias Mountain Range consists of rugged topography with peak elevations of more than 5,500 meters. Because broad ice fields and large glaciers cover the mountains, many of these areas are hard to reach except by helicopter. To overcome the problem of getting to material covered by ice fields more than 1,000m thick, we collected the sand transported by the water at the base of each glacier and dated the single grains using thermochronology methods.

[caption id="attachment_52296" align="alignnone" width="620"] Saint Elias Mountains forms part of the Pacific Coast Range[/caption]

Thermochronology allows us to reconstruct the entire cooling history of rock as it moved from 10km deep in the crust towards the surface in a process called exhumation – the result of uplifting the surface and eroding the rocks at the top. This sampling approach revealed significant spatial variations in the exhumation rate of rocks across the mountain range, with the highest rates found around the highest mountain peaks and largest glaciers – also the area with the most concentrated tectonic stress.

My previous position at the University of Tübingen allowed us to conduct thermochronology on large cobbles, bedrock from ice-free regions close to the coast and sand material transported by glaciers to the north, located in northwestern Canada. Our goal was to obtain a high-resolution image of the spatial and temporal pattern of exhumation. Together, we revealed that the location of the most focused exhumation was not stationary, but had shifted from north to south less than two million years ago. Synthesising all thermochronology data with our STEEP collaboration efforts effectively show that the southward shift of exhumation is a tectonic response to the change in the material where the Yakutat plate is colliding with the North American plate, ultimately creating the St. Elias Mountains.

This change in the material properties is a consequence of the extremely high erosion that started in the mountains when glaciers first formed about five million years ago.

After the climate changed globally to colder conditions about 2.6 million years ago, it set the pattern for northern hemisphere glaciations. This shift intensified glaciation in the St. Elias Mountains and resulted in a chain of events. The excessive sediment deposited on top of the Yakutat plate led to a shift of the deformation from the more rigid North American Plate to the softer sediments on the Yakutat plate in the south.  During the glacial maximum the temperatures were simply too cold in the high elevation part of the glaciers, so ice at the top was not sliding and thus not eroding. However, in the lower elevation, ice and erosion reached far out into what is today the Gulf of Alaska. The result was a continuing uplift with no erosion in the high-elevated region, creating even higher mountains and shifting rock exhumation towards the south near the modern coast.

Many of us are continuing the study of the St. Elias Mountains on sediment cores recovered from the Gulf of Alaska. These core samples are precious for us because they present a continuous record of the past 10 million years in terms of sedimentation, climate conditions and also exhumation. Analysing the geology of the St. Elias Mountains with all the complications due to the massive cover by glaciers today was challenging, but also very rewarding.

I think I can speak for all of my collaborators that through this project we all learned so much more about the processes that formed the St. Elias Range and other mountain ranges around the world.

Author: Eva Enkelmann is an Assistant Professor at the Department of Geology at the University of Cincinnati.