Regolith thickness instability and the formation of tors in arid environments

• Published in: Journal of Geophysical Research - Earth Surface Vol. 111; no. F3; pp. F03010 - n/a
• Main Authors: Strudley, Mark W., Murray, A. Brad, Haff, Peter K.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Geophysical Union 01-09-2006; Blackwell Publishing Ltd
• Subjects: Earth sciences; Earth, ocean, space; Emergent phenomena; Exact sciences and technology; Geomorphology; granitic rocks; hillslope; Hydrology; Modeling; Nonlinear Geophysics; pediments; Sediment transport; tors; Weathering; pediments; tors; granitic rocks; landscapes; basement; regolith; rainfall; emergence; thickness; inselbergs; joints; arid environment; weathering; exhibits; digital simulation; sediment transport; Thinning; bedrock; Feedback; fluctuations; numerical models; instability; erosion; growth
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2005JF000405

We present model results suggesting that a physical erosion–bedrock weathering feedback is responsible for the development of isolated bedrock knobs (tors/inselbergs) that often punctuate otherwise smooth pediments of homogeneous basement lithology. Tors and larger, more heavily jointed and morphologically complex exposures, inselbergs, may arise as a consequence of fluctuations in rainfall and sediment transport conditions combined with a bedrock weathering mechanism that depends on regolith thickness. Hydrogeochemical considerations and field observations in arid, granitic environments suggest that the relationship between weathering rate and regolith thickness exhibits a maximum for a finite thickness of cover. We have encapsulated this simple erosion‐weathering feedback in a numerical model simulating arid/semiarid landscape evolution that produces low‐sloping pediments punctuated by tors. Tors form during periods of higher effective moisture, resulting in local base level incision and regolith thinning on pediments, invoking a transition in which mantled surfaces lower at rates exceeding the bare bedrock weathering rate. This condition favors the emergence and growth of tors in areas covered by regolith thickness less than a threshold value. Subsequent shifts in climate or local base level that restore sediment surface lowering rates less than the bare bedrock weathering rate will lead to a progressive decrease in tor height and, ultimately, their disappearance. Thus, according to this model, tors in arid environments represent possibly transient features related to fluctuations in climate or local transport conditions rather than palimpsests of an ancient landscape derived from differential subsurface weathering followed by regolith stripping.