Bedrock Fracture Characteristics as a Possible Control on the Distribution of Surge‐Type Glaciers

Abstract

Glacier surging has been studied extensively and is understood as a dynamic instability at the glacier bed. Yet an explanation for the heterogeneous distribution of surge‐type glaciers at the scale of a mountain range remains elusive. Here we investigate bedrock discontinuity properties in the basins of 16 surge‐type and nonsurge‐type glaciers in the St. Elias Mountains of Yukon, Canada. Using scaled photographs of bedrock outcrops at the margins of each glacier, we digitize traces of the bedrock discontinuities and with automated purpose‐built software, quantify discontinuity properties that are a function of length, orientation, and spacing of bedrock fractures. We obtain an unexpected result: outcrops in the basins of surge‐type glaciers are less fractured than those in the basins of nonsurge‐type glaciers. We hypothesize that the degree of bedrock fracture may control the extent and location of a clast‐rich till transition zone at the glacier bed. This zone would provide flow resistance conducive to the development of an ice reservoir and thus to surging behavior. To reconcile our observations with the global distribution of surge‐type glaciers, we speculate that surge‐type glaciers may occur in geological settings characterized by an intermediate range of bedrock fracture.