In recent decades scientists have demonstrated the extreme degree to which the Earth’s crust is shaped by a close collaboration between tectonic and climatic forces. The tight coupling between Earth’s internal and surficial processes is most evident where glaciers are the dominant form of erosion. Glacial erosion can be extremely rapid in certain settings and is thus capable of removing rock as fast as mountains can be tectonically built. The onset of glacial conditions in the last few million years has had a dramatic effect on the dynamics of plate boundaries. Yet fundamental questions remain unanswered with regard to how glaciers erode mountains. Specifically, we asked whether there are limits to the glacial conditions that favor rapid erosion, such as limits to the extent of glacier coverage or type of ice flow. Characterizing how glaciers erode is particularly important for predicting the geomorphic response to future climate change.

We addressed the conditions that control glacial erosion by examining how rates of erosion changed with time in three tectonic settings: the active mountains of south central Alaska, a rejuvenated Atlantic margin in northwest Scotland, and an inactive tectonic margin in New Hampshire. Glacial histories, ice coverage, and nature of ice flow varied drastically among locations, from rapidly flowing, wet-based alpine glaciers to slow, frozen-based ice sheets. In each location, we measured rates of erosion spanning different timescales to test whether erosion rates increased as climate cooled and glaciers became more active in the last few million years. Our tools included long-term cooling history of the rocks, short term history of rock exposure to cosmic radiation at the surface, rates of erosion of specific landscape elements (glacier beds, ridges, and basin-wide erosion rates), and the chronology of the glaciers themselves.

What we have discovered is largely consistent with previous findings, yet illustrates important considerations for future efforts. First, erosion rates appear to accelerate in the late Cenozoic in the active mountains of Alaska, a result that is consistent with other studies showing how onset of glacial erosion can increase the rates of downwearing and deformation in plate collision zones. At the same time, our work suggests that regional tectonic events, particularly the dynamics of plate subduction, have also contributed to the increase in rock uplift over the past few million years. Our results also provide the first regional quantification of glacial history in the region. Second, we find that onset of Quaternary glaciation does not seem to be the dominant signal with regard to long-term erosion rates in Scotland or northeastern North America. This result is partially consistent with earlier work that identified several off-margin, earlier Cenozoic tectonic and magmatic events that have shaped both regions. A new twist that we have discovered, however, is that methods currently in use for estimating short term erosion rates using accumulation of nuclides produced by cosmic radiation are likely flawed. In an effort to measure these rates in our study areas, we have collected supporting data that shows some assumptions of the methodology are invalid, in turn leading to a new model of how to use this technique in glacially eroded areas.

Our work thus reinforces the idea that glaciers can increase rates of erosion and rock uplift in tectonically active mountain belts. It also suggests that there are specific limits to the conditions under which glaciers will have this effect, most likely related to the basal ice temperature, hydrologic conditions, and original topography. Cold-based ice sheets in passive tectonic regions are not likely to yield dramatic increases in erosion and rock uplift rate. The most important lesson we have learned, however, is that quantifying changes in la...

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