Case Hardening Vignettes from the Western USA: Convergence of Form as a Result of Divergent Hardening Processes

Abstract

The rock weathering literature contains the hypothesis that case hardening exemplifies equifinality, where the same end state can be reached by many potential processes in an open system. We present analytical data from six different sites in the western USA to assess the hypothesis of equifinality. Case hardening can be produced on: (1) sandstone in Petrified Forest National Park, Arizona, from the addition of silica glaze, rock varnish and heavy-metal skins; (2) sandstone in Whoopup Canyon, Wyoming, from silica glaze that formed originally inside subsurface joints combined with externally applied iron film, silica glaze, and rock varnish; (3) welded tuff in Death Valley, California, from the accumulation of rock varnish and heavy metal skins of Mn and Fe; (4) sandstone in Sedona, Arizona, from the protective effects of rock varnish accretion and heavy metal skins of Mn and Fe; (5) basalt on the Big Island, Hawai'i, from the accumulation of silica glaze inside vesicles; and (6) sandstone at Point Reyes, California, from a lithobiont mat of fungi and lichen. Each developed the general form of a case-hardened shell, protecting the surface from erosion. In accordance with the hypothesis of equifinality, the processes that led to similar appearance differ.

Insights into the formation of rock varnish in prevailing dusty regions

“…In contrast to other sites studied where cementation of the whole-rock and, thus, case hardening are significant (e.g. different sites from western USA, Dorn et al, 2012), the CPP rock coating does not seem to significantly reinforce or indurate the poor welded ignimbrite. In fact, there is a sharp and rough contact between the CPP rock coating and the underlying ignimbrite without evident interaction between them.…”

“…different sites from western USA, Dorn et al, 2012), the CPP rock coating does not seem to significantly reinforce or indurate the poor welded ignimbrite. different sites from western USA, Dorn et al, 2012), the CPP rock coating does not seem to significantly reinforce or indurate the poor welded ignimbrite.…”

“…As a result of case hardening effect, a differential resistance to weathering and erosion is created; indurated surfaces erode more slowly than unprotected rocks. Accordingly, desert varnish can strongly control landform surfaces or earth topography through case hardening (Conca and Rossman, 1982;Dorn, 2004;Dorn et al, 2012).…”

Defining tafoni

“…Pope et al (1995: 38) appropriately portrayed the complexity of decay processes in the terms of ''synergy'' by saying, ''variability in weathering [rock decay] involves synergistic interactions of biological, chemical, and physical factors.'' For tafoni, the mere existence of so many different supported formation theories supports the concept of polygeneitywhere a single form can be the result of a multitude of processes (Dorn et al, 2012).…”

Protecting Stone Heritage in the Painted Desert: Employing the Rock Art Stability Index in the Petrified Forest National Park, Arizona

“…A reliable and replicable system with minimal training time, completing a RASI assessment for a rock art site (or individual rock art panel) results in a score of instability severity, providing managers with a snapshot of the current state, strength, and potential longevity of rock art panels, or rock faces, with as fine a detailed assessment as necessary for any given project. Using these data, site managers can create specific priorities by integrating RASI results for individual sites in a geographic information system (GIS) database (cf., [14,15]), and those sites in greatest danger can then be mapped across scales-whether it be across a state, province, country, within a specific site, or the rock art panel itself.…”