The Potential for Fracture Growth in Stepped Subglacial Topography as a Quarrying Mechanism.

Understanding the rates and mechanisms of erosion by subglacial quarrying is a major unsolved problem in geomorphology. Stress enhancement due to load concentration on bedrock ledges between cavities is hypothesized to drive the growth of fractures. Prior work assumed the formation of vertically oriented tensile fractures at the downstream margins of cavities as the controlling process, but did not account for the evolution of the stress field as fractures lengthen, and in particular the dominance of the shearing mode at fracture tips. We used 2D finite element analysis and J‐integral methods to analyze stress intensity factors and fracture growth potentials at the tips of preexisting fractures in loaded bedrock steps, taking into account normal and shear components and measured rock strengths. By examining different step heights, step riser angles, rock types, prior fracture locations and orientations, and extents of ice‐rock contact zones, we identified some situations favorable for fracture growth, especially in brittle rock types. Typically, however, the growth direction will not be vertically downward but angled up‐glacier away from the step riser, a situation unfavorable for quarrying. Moreover, in many situations, the normal stress across fracture planes will be compressive. Non‐vertical step risers buttress the bedrock and also suppress fracture growth. In contrast, reducing the sizes of ice‐rock contact zones not only increases the loading magnitude, as previously recognized, but also increases intensification of tensile stress at the tips of fractures located just up‐glacier. Thus, larger cavities, and hence, fast sliding and low effective pressures, favor quarrying more strongly than previously recognized. Plain Language Summary: Glaciers carve into landscapes, in particular helping to define the terrain of high mountain ranges and high‐latitude continents. A major way glaciers achieve this is subglacial quarrying; by sliding over their substrates, glaciers unevenly concentrate their weight on bedrock bumps and ledges, causing existing bedrock cracks to grow, ultimately dislodging chunks of rock which further scrape along downstream to cause further erosion. Scientists do not yet understand how enough fracturing occurs for quarrying to happen, or how rapidly. We used software to mathematically evaluate the extent to which the uneven pressures cause bedrock flaws to pull apart or slide apart in different situations under glaciers. Factors enhancing erosion include taller bedrock steps, vertical step faces, cracks and pressures positioned closer to step edges, and rock types susceptible to shattering. Our analysis corrected some problems in previous research but also failed to identify how rock fracturing under glaciers could be extensive enough to fully dislodge rocks, an issue we continue to investigate. Key Points: Uneven distribution of a glacier's weight on stepped bedrock generates stresses large enough to grow existing fractures in some situationsHow crack growth separates blocks to allow quarrying remains unsolved; prior work does not distinguish between tensile and shear failureGrowth is favored by greater step height, vertical risers, and smaller loading zones, the latter effect being stronger than earlier thought [ABSTRACT FROM AUTHOR]