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The AAPG/Datapages Combined Publications Database
Utah Geological Association
Porphyry Weathering in a Desert Climate
The porphyry boulders and cobbles on the northern piedmont of the Henry Mountains exhibit several weathering forms. This paper discusses three of them: ashtray weathering, exhuming of feldspars and boulder sculpture. The first two have not been reported previously, whereas boulder sculpture is comprised of solution pans and tafoni, which are well documented in the literature.
Ashtrays are formed by the hollowing out of the undersides of porphyry cobbles that lie on shale of the Mancos and Morrison formations. Apparently, exchange sites on the clay minerals act as an acid to dissolve the rock. The exhuming of feldspar phenocrysts is produced by the more rapid weathering of the groundmass to leave the phenocrysts of feldspar standing in relief. Hydration of grain boundaries and physical loosening of the grains appear to be the processes operating to produce this weathering form.
Sculptured boulders contain several weathering forms, including solution pans, tafoni, exfoliation and exhumed feldspars. Solution of the rock and the growth of salt crystals are cited in the literature as the main processes in the formation of solution pans and tafoni. Regardless of the processes that formed them, these sculptured boulders add beauty and an air of surrealism to the Henry Mountains area.
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