Date of Graduation
8-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Geosciences (PhD)
Degree Level
Graduate
Department
Geosciences
Advisor/Mentor
Covington, Matthew D.
Committee Member
Gulley, Jason D.
Second Committee Member
Shaw, John B.
Third Committee Member
Marshall, Jill A.
Fourth Committee Member
Tullis, Jason A.
Keywords
Debris cover; Glaciers; Himalaya; Landscape evolution; Topographic depression
Abstract
Rock debris covers many glaciers in Earth’s highest mountains and the debris impacts glacier response to changes in climate. Interactions between debris, drainage, glacier dynamics, and climate produce contrasts in melt rates that cause debris-covered glaciers to develop complex, hummocky topography. Contrasts in melt rate produce thousands of hills–hummocks–and depressions. Ponds and ice cliffs often form in the depressions. Although ponds and ice cliffs occupy only a small fraction of the surface, they account for nearly half of glacier melt. Understanding the processes that control the development of hummocky topography is key to predicting the evolution of debris-covered glaciers. In this dissertation, I incorporate field observations on Himalayan and Alaskan debris-covered glaciers, quantitative topographic analyses, and mathematical models to improve understanding of the development of hummocky topography. First, I examine the distribution and geometry of depressions on the Ngozumpa Glacier, Nepal. The depressions exhibited a power law distribution of areas and followed power law perimeter-area and depth-area scaling relationships, suggestive of positive feedback growth. With a simple geometric model, I showed that positive feedback growth produces similar power law distributions. Debris deposition in depressions is a negative growth feedback. I infer that meltwater drainage and debris removal englacial sink points explains the positive feedback growth. Later, I develop a process-based topographic evolution model to simulate the growth of depressions. The model includes stream incision and debris removal at an internal sink point, in addition to hillslope transport and sub-debris melt. The model shows that debris removal is essential for depression growth, and growth rates increase with incision rates. A range of reasonable parameter values produce positive feedback growth and geometric scaling similar to depressions on the Ngozumpa Glacier. These findings highlight the importance of englacial drainage to the evolution of debris-covered glaciers. To better understand how differential melt and hillslope transport influence the development of topographic relief, I develop a model to simulate the growth of debris-covered cones of ice. I explore the production of topographic relief from differential melt caused by contrasts in debris thickness. I show that cone heights scale with the square root of debris volume and I derive a characteristic length, composed of the ratio of the debris diffusion rate and the bare ice melt rate, that predicts cone heights. These results help explain how increasing melt rates produced higher topographic relief on some Himalayan glaciers. Together, these investigations provide new, quantitative insights to controls on the development of hummocky topography. The models presented here can simulate the topographic evolution of the slow-moving termini of debris-covered glaciers. Field measurements are needed to better constrain channel incision rates and debris diffusion rates. Future development should incorporate ice cliffs, ponds, and glacier dynamics. With long time series, high-resolution debris thickness and elevation data, the model provides a framework for predicting the evolution of hummocky topography on debris-covered glaciers.
Citation
Strickland, R. M. (2024). Controls on the Development of Hummocky Topography on Debris-Covered Glaciers. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5500
