Australia: The Land Where Time Began

A biography of the Australian continent 

Sandstone Landforms – Shaped by Negative Feedback between Stress and Erosion

Unique landforms (Young, Wray & Young, 2009; Turkington & Paradise, 2005) such as arches, alcoves, pedestal rocks and pillars are produced by the weathering and erosion of sandstone. It has been assumed that stresses induced by gravity are not involved in preservation (Turkington & Paradise, 2005) of landforms, instead actually increasing the rates of weathering (Gerber & Scheidegger, 1973; Gerber & Scheidegger, 1969). In this study the authors¹ have shown that increased stresses within a landform that result from vertical loading reduces the rates of weathering and erosion by the use of lab experiments and numerical modelling. The authors¹ found that when they exposed a cube of locked sand experimentally to weathering and erosion processes while exposed to sufficiently low vertical stresses the vertical sides of the cube disintegrate progressively into individual grains. The vertical stress increases until it reaches a critical value as the cross-sectional area beneath the loading decreases. The granular sediment behaves like a strong, rock-like material as this threshold is reached, which is caused by the fabric interlocking the sand grains, and the remaining pillar or pedestal landform, that is load-bearing, is resistant to further erosion. The authors¹ were able to reproduce other natural shapes, such as arches, alcoves and multiple pillars, when planar discontinuities are present, such as bedding planes or fractures. It is demonstrated by numerical modelling that under fabric interlocking resulting from negative feedback between stress and erosion, the stress field is modified by discontinuities to make a variety of shapes stable. The authors¹ concluded that the stress field is the primary control of the evolution of shape of landforms that stand alone.

Sources & Further reading

1.      Bruthans, J., et al. (2014). "Sandstone landforms shaped by negative feedback between stress and erosion." Nature Geosci 7(8): 597-601.