CSDMS 2016 annual meeting poster RachelGlade

Presentation provided during SEN - CSDMS annual meeting 2016

Blocks control hillslope evolution in layered landscapes

Rachel Glade, CU Boulder Boulder Colorado, United States. rachel.glade@colorado.edu

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Rocky hillslopes dotted with large blocks and covered by a thin, non-uniform soil are common in both steep landscapes and arid environments, as well as on other planets. While the evolution of soil-mantled, convex hillslopes in uniform lithology can be well-modeled, the influence of lithology and geologic structure on hillslope form and evolution has yet to be properly addressed. Landscapes developed in layered rocks feature landforms such as mesas and hogbacks that exhibit steep, linear-to-concave up ramps scattered with blocks derived from the resistant rock layers. Beyond the ramp, no blocks are to be found. This morphology serves as a strong target for numerical modeling. Our hybrid continuum-discrete numerical model shows that interactions between resistant blocks and underlying easily weathered rock explain the form and evolution of a hogback, a tilted feature that exemplifies this class of landforms. Our model consists of a dipping hard rock layer sandwiched between less resistant layers. The hard layer releases resistant blocks that then armor the underlying rock from weathering. Fine sediment transport is treated with a traditional soil depth-dependent continuum hillslope flux law, while movement of individual resistant blocks is treated discretely. Blocks interfere with the flow of soil, damming it upslope, and developing a wake of thinning soil downslope into which the block eventually moves. We find that feedbacks between block release, weathering of blocks and soft rock, and sporadic downslope movement of blocks are necessary to capture the essence of these landscapes. Insights from our numerical model lead to a simple analytical solution that predicts the steady state hillslope form and slope angle from block size, spacing, rate of weathering, and the efficiency of soil transport. Our results illuminate previously unrecognized hillslope feedbacks, improving our understanding of the detailed geomorphology of rocky hillslopes and the large-scale evolution of landscapes developed in layered rock.

Hogback evolution through time in our numerical model, plotted every 400,000 years. The adjoining slope reaches steady state, parallel retreat by 1.6 Myr in which slope and release rate remain constant. Slopes are concave, with a slight convexity at the top. Blocks decrease in size as they weather and move downslope, and do not persist beyond the base of the ramp.

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