Principal Investigators:          Dr. Mary C. Bourke

                                                Dr. Heather Viles, University of Oxford, Oxford Rock Breakdown Laboratory

Collaborators:                         Dr. Nathan Bridges, NASA-Jet Propulsion Laboratory

                                                Mr. James Holmlund, Western mapping Company, Tucson

Funding: NASA Planetary Geology and Geophysics (2005-2008)

Project Summary

Specific processes on Earth can be shown to produce a particular size and shape of morphological response on rocks. For example, rocks subject to fluvial transport tend to be well rounded (Howard, 1998), whilst those freshly collapsed from bedrock walls are angular. Rocks in aggressive salt weathering environments tend to develop cavernous forms (Heslop, 2003) whilst those subject to the effects of insolation often exfoliate, exhibiting curvilinear sheets of debitage (Ollier, 1978). The situation is complicated somewhat by the lithological imprint; some rocks respond more rapidly and in different styles to breakdown processes.  Over time, the inheritance of both initial lithology and past processes may also complicate the relationship between current process and morphological response. Here we use the term rock breakdown to include weathering and geomorphological processes that act upon an initial rock shape. 

Weathering studies have been largely divorced in recent years from work on erosive processes such as aeolian abrasion. While these studies have enabled specific form–process associations to be better understood, there has, as yet, been no effort to investigate the combined effects of processes operating on a rock surface over time. It is therefore often incorrectly assumed that the links between process and form are well understood. This assumption had led to geomorphologists using form as a proxy for process (present and past). This approach has been applied, not only to rocks on Earth but also to other planetary surfaces (e.g. Mars, Basilevsky et al., 1999).

There is therefore a need to establish feature persistence, that is, the exact nature and strength of the different morphological signatures resulting from specific processes, on different rock types, and how they combine with one another.  A first step towards constraining the links between process, inheritance, and morphology is to identify pristine features produced by different process regimes.  We are developing an atlas of geomorphic signatures on boulders. These atlases are intended as a useful and instructive photographic guide to those in the planetary geomorphology community who are tasked with analyzing clasts captured by Lander and Rover instruments on planetary surfaces.

Figure 1 Percussion feature on flood-transported boulder, Oak Creek Canyon, Arizona.

We apply innovative techniques and analysis routines to topographic datasets obtained by high resolution laser scanning on basalt boulders in field and experimental conditions (Figs 2 & 3). The data will be used to characterize and discriminate rock breakdown caused by fluvial transport, aeolian abrasion and weathering in Mars analog environments. We aim to provide clear quantitative descriptors of diagnostic signatures of specific breakdown forms in pristine and weathered samples. Our approach will be the first to consider the relative efficacy of weathering and erosion together in the persistence of signatures of erosion, transport and weathering on boulder surfaces.

Figure 2 James Holmlund collecting laser scan data sets at night at Tinajas Altas mountains of Southwestern Arizona. 

The overall scan data was collected at a resolution of 1.5 cm.  A second set of high-resolution data was collected at an approximate resolution of 0.7 mm.

Figure 3 Scan data set output collected on a granite boulder-like outcrop in the Tinajas Altas mountains of Southwestern Arizona. 

References

Basilevsky, A. T., Markiewicz, W. J., Thomas, N., and Keller, H. U. (1999). Morphologies of rock within and near the Rock Garden at the Mars Pathfinder landing site. Journal of Geophysical Research 104, 8617-8636.

Heslop, E. E. M. (2003). "Clast breakdown in the Atacama Desert, Chile:An intergrated field and laboratory approach." Unpublished D.Phil. thesis, University of Oxford.

Howard, A. D. (1998). Long Profile Development of Bedrock Channels: Interaction of Weathering, Mass Wasting, Bed Erosion, and Sediment Transport. In "Rivers Over Rock: Fluvial Processes in Bedrock Channels." (K. J. Tinkler, and E. E. Wohl, Eds.), pp. 297-319. American Geophysical Union, Washington, DC.

Ollier, C. D. (1978). Inselbergs of the Namib Desert - processes and history. Zeitschrift fur Geomorphologie 31, 161-176.

Conference presentations:

Bourke, M.C., Brearley, J.A., Haas, R., and Viles, H.A., (2005), The surface features of 'pristine' flood-transported boulders: LPSC XXXVI, abs. 2253.

Viles, H.A., Brearley, J.A., Bourke, M.C., and Holmlund, J., (2005), What processes have shaped basalt boulders on Earth and Mars? Studies of feature persistence using facet mapping and fractal analysis: LPSC XXXVI, abs. 2237

Heslop, E.A., Viles, H.A. and Bourke, M.C., 2004. Understanding rock breakdown on Earth and Mars: Geomorphological concepts and facet mapping methods, LPSC XXXV, abst. 1445

Theses:

Brearley, J. A. (2005). Does form follow process on terrestrial and Martian basalt boulders? A facet mapping and fractal analysis approach. Unpublished Undergraduate thesis, University of Oxford.

Mace, D. (2005). Basalt rock breakdown: fractal and geometric morphometric analysis. Unplublished MSc thesis, University of Oxford.