Decay of ignimbrite fairy chimneys of Arizona's Basin and Range Province, USA

Abstract

Fairy chimneys of ignimbrite are well studied in Cappadocia in central Anatolia, Turkey. The ignimbrite landforms of Arizona's Basin and Range, USA, in contrast, remain relatively unexplored in geomorphic scholarship, despite decades of geological research on the characteristics of the ignimbrites themselves. This research focuses on the rock‐decay processes that modify Arizona's Basin and Range fair chimneys. Dissolution of glass and groundmass, measured along joints using digital image processing of back‐scattered electron microscope imagery, reveal a 3× higher porosity than inside the rock interior. This method also measured a 3‐4× increase in dissolution near the base of the chimney compared with the rock interior, leading to notch development that promotes chimney mass wasting and subsequent ballistic impacts. Epilithic organisms studied with electron microscopy both enhance rock decay and promote case hardening. These observations are consistent with prior understanding of how fairy chimneys evolve over time: columnar joints exposed on cliff faces experience enough decay to allow erosion of joint material, this erosion separates the chimney from the rock face and notch development near the chimney base promotes mass wasting. The rock‐decay processes studied here likely operate on very different timescales. Mass wasting events leading to ballistic impacts occur at observable timescales. Varnish microlamination dating, using ultrathin sections of rock varnish formed on fairy chimneys, indicates that the case hardening of the top of fairy chimneys are much more stable than the sides; the tops last experienced surface detachment during the early Holocene, as opposed to the sides that experienced flaking throughout the late Holocene. The timescale of basal notch development is not known, with the exception that the very surface of notches erode at rates too fast to accumulate rock varnish. The timescales of epilithic organisms enhancing rock decay and dissolution of columnar joints are not known.

Rock varnish revisited

“…Each of the references linked to these sites provides details about the following sampling sites: the Sonoran Desert (Dorn, 2023), Hanaupah Canyon alluvial fan in Death Valley (Hooke and Dorn, 1992), from a Nazca Peru pukio (underground wet canal) ceiling (Clarkson and Dorn, 1991), from a morainal boulder on a Greenland outlet glacier, from the Gokyo Valley in the Khumbu region of Nepal, and from a granitic rock fracture 6 m deep from a road cut in central Arizona (Dorn and Whitley, 2023).…”

“…The remainder of the new imagery derive from sites selected to explore the different falsification tests, particularly the need to analyze samples from the different environmental settings where rock varnish is known to occur (see Supplemental File 1). Each of the references linked to these sites provides details about the following sampling sites: the Sonoran Desert (Dorn, 2023), Hanaupah Canyon alluvial fan in Death Valley (Hooke and Dorn, 1992), from a Nazca Peru pukio (underground wet canal) ceiling (Clarkson and Dorn, 1991), from a morainal boulder on a Greenland outlet glacier, from the Gokyo Valley in the Khumbu region of Nepal, and from a granitic rock fracture 6 m deep from a road cut in central Arizona (Dorn and Whitley, 2023).…”

“…However, within the orange layer, Fe is more abundant and is more commonly enhanced by budding bacteria (e.g., Figure 4(D)).

Figure 4.Fe-enrichment via budding bacteria in an orange varnish microlamination, on a fairy chimney in the Sonoran Desert (Dorn, 2023). A.

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“…Fe-enrichment via budding bacteria in an orange varnish microlamination, on a fairy chimney in the Sonoran Desert (Dorn, 2023). A.…”