Relating Micrometer-Scale Weathering to Landform Morphology at Papago Park, Phoenix, Arizona

Arizona-Nevada Academy of Science

Relating Micrometer-Scale Weathering to Landform Morphology at Papago Park, Phoenix,ArizonaAuthor(s): Jennifer MillsSource: Journal of the Arizona-Nevada Academy of Science, Vol. 31, No. 2 (1998), pp. 117-126Published by: Arizona-Nevada Academy of ScienceStable URL: http://www.jstor.org/stable/40021833 .

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Relating Micrometer-scale Weathering to Landform Morphology at Papago Park, Phoenix, Arizona

Jennifer Mills1 Department of Geography, Arizona State University

Tempe, AZ 85287-0104

Abstract

Electron microscope imagery of samples from South Barnes Butte and Garden Butte in Papago Park in Phoenix, Arizona, offers a new perspective on explaining morphologic differences between these adjacent landforms. Primary minerals weather mainly along grain-boundary fractures and, to a lesser degree, internally within the granitic samples of Garden Butte and granitic boulders of South Barnes Butte. It is unknown as to whether mechanical or chemical processes led to the fracturing of these minerals.

The granitic samples do not exhibit as many void spaces or secondary minerals in comparison to the samples taken from the matrix of the breccia at South Barnes Butte. Secondary minerals are more susceptible to erosion. As such, much of the void space, or loss of mass within the matrix of the breccia of this fanglomerate, is apparently the loss of secondary minerals. The matrix of South Barnes Butte appears to be weathering and eroding at a much foster rate in comparison to adjacent granitic boulders. This differential weathering may lead to the erosion of the granitic boulders and aid in the creation of the more irregular topography at South Barnes Butte. In contrast, Garden Butte, composed mainly of granodiorite, experiences more uniform weathering producing a smoother morphology.

Introduction

One of the objectives of geomorphology is to understand the processes of landscape formation. Numerous studies have sought to explain the different appearances of weathering landforms in desert landscapes. Salt weathering, more specifically salt crystallization, is most often attributed to the formation of a variety of desert landforms such as alveoli, tafoni, shattered en echelon cobbles, and disintegrated crystalline and clastic rock fragments (Evans 1970). Salts disrupt the crystal structure within a rock by hydration expansion, thermal expansion, or growth of crystals from solution (Goudie et al. 1970, Twidale 1982, Young 1987). Many authors (Winkler 1965, Laurie 1925, Hutton 1981, Twidale 1982) postulate that salts in arid regions are deposited by wind or groundwater or reprecipitate within the parent rock itself.

Investigations of the tensional forces caused by salt crystallization in buildings and other structures have included such studies as weathering rates of granites in different climates. In studying weathering damage of Cleopatra's Needle, an Egyptian obelisk in Central Park, New York City, Winkler (1965) concluded that the majority of the damage to Cleopatra's Needle occurred in Egypt long before it reached Central Park. Salt invaded the structure through capillary action in Egypt, and expanded through hydration when the monument was brought to the humid climate of New York City.

The effects of salt weathering on rock have also been studied in laboratory experiments (Goudie et al. 1970, McGreevy and Smith 1984, Smith and McGreevy 1983, Davison 1986, Smith et al. 1987). Davison (1986), for example, conducted an investigation using laboratory simulation of air temperatures for Tunisia, Antarctica and southwest England. He used X-ray analysis to determine that an inverse relationship between salt penetration and debris production existed. The rapid evaporation of the Tunisian simulation and the rapid freezing in the Antarctic simulation caused salts to concentrate in the upper layers of rock samples. Debris production in these samples increased through the differential expansion and contraction of these salts.

Address for Correspondence: 922 North Lehmberg, Casa Grande, AZ 85222 Email: millsj@imapl .asu.edu.



1 18 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 3 1

Because salt weathering may explain many landform features in deserts, my initial hypothesis was that the different appearance between the gentle sloping Garden Butte and the rugged, tafoni- filled South Barnes Butte at Papago Park (Figure 1) reflected some interaction between salt weathering and the different rock types. Tafoni are holes or depressions, usually less than a few meters in depth and width, that commonly form on the undersides of rocks or on steep rock faces (Twidale 1982). However, neither qualitative energy dispersive X-ray analysis nor quantitative electron microprobe analysis in tandem with backscattered electron images revealed the presence of sodium, chlorine, or sulfur for salts, (e.g., NaCl, NaSO4), in South Barnes Butte or in Garden Butte (Table 1).

Table 1. Quantitative electron microprobe analyses of the matrix of South Barnes Butte, using a 10 micron spot size and wavelength dispersive spectrometry. Although the microprobe only detects the X-rays of the various elements, data are conventionally presented as oxide weight percent. The relatively high value for Na in Transect 2 is due to the presence of small pieces of albite (sodium feldspar). Greater porosity is reflected in lower probe totals. Symbol: MDL = Minimum Detectability Limits.

POINT Na2O MgO A12O3 SiO2 SO3 K2O CaO TiO2 Cr2O3 MNO FeO BaO Total Porosity

MDL 0.013 0.011 0.009 0.010 0.026 0.016 0.020 0.036 0.062 0.051 0.060 0.085

Transect 1 2,63 2.02 9.90 27.34 0.10 1.10 2.17 0.17 0.04 0.06 8.68 0.06 54.27 46%

Transect 2 0.00 1.18 1.87 17.20 0.12 0.12 0.76 0.02 0.00 0.10 2.17 0.04 23.58 76%

Transect 3 0.51 0.05 0.11 4.56 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.01 5.27 95%

The focus of the study explores why these adjacent inselbergs appear so different. The Papago Park area is a popular recreation spot visited by students, bicyclists, hikers and other members of the general public. The prominent differences between South Barnes Butte and Garden Butte invariably spark the curiosity of the countless people who frequent this park. The Desert Botanical Gardens is often faced with questions concerning the underlying causes of these differences in appearance. One of the most commonly used textbooks on desert geomorphology (Cooke et al. 1993) features a photograph of the Papago Park tafoni. Yet very little research has been conducted concerning the weathering processes at work in the Papago Park area or on tafoni forms in the Sonoran Desert. Thus, the purpose of this research is to obtain a better understanding of the underlying causes of the differences in morphology between these two buttes.

I hypothesize that the granitic material within South Barnes Butte is more resistant to weathering and erosion than the matrix of the breccia, creating a differential weathering landscape of boulders that gradually protrude until they fall. In contrast, the relative uniformity of the granodiorite lithology at Garden Butte yields a relative uniformity of slope in response to weathering and erosion.



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120 Journal of the Arizona-nevada Academy of Science Vol. 3 1

Study Site

Observations made at South Barnes Butte and Garden Butte in Papago Park (Figure 1) indicate differential weathering both within South Barnes Butte and between the two buttes. South Barnes Butte is a reddish-brown, coarse-grained, poorly sorted and stratified massive arkosic breccia formed in the early Tertiary period. It consists of angular to subangular granitic clasts (one fourth of an inch to fifteen feet in diameter) protruding from a matrix of rock fragments and ferrugenous cement (P6w6 et al. 1986). Numerous tafoni are present along fractures and other locations on the butte.

In contrast, Garden Butte is composed mainly of pink, coarse-grained to porphyritic granite that is cut by veins of quartz and dikes of aplite and greenstone. It was formed sometime during the Precambrian period (P6w6 et al. 1986). The slopes are much smoother in appearance. There is also a greater presence of vegetation on Garden Butte in comparison to South Barnes Butte, hosted by the more abundant grus-dominated regolith. Grus, a product of granite disintegration, is a coarse, angular mass of rock and sand-sized mineral fragments. The lower slopes of Garden Butte have a thin cover of regolith, but the top of the butte consists of bare granite undergoing grusification.

Methods

Samples were collected from the granitic boulders and the breccia matrix surrounding these boulders at randomly chosen sites at South Barnes Butte. These sites were located on the south- facing side of South Barnes Butte just north of Elliot Ramada in Papago Park (Figure 1). Samples were also collected from the regolith and exposed rock surfaces at random locations on Garden Butte. In addition, lichen-coated samples were collected to assess the role of lichens in the weathering of Garden Butte.

The samples collected were placed in epoxy molds in preparation for scanning electron microscopy (SEM). Each mold contained samples from a specific site. After hardening, they were polished with abrasive papers decreasing in grit size to 0.3 mm.

SEM was used to record images of secondary (SE) and backscattered electrons (BSE) at the micrometer scale. SE reveals microtopography of die mineral surface while BSE indicates variations in mineral composition. BSE is measured by the backscattered electron yield, which is a function of the atomic number of the target area on the sample (Pope et al. 1995).

The main technique used to assess weathering was BSE, supplemented by quantitative wavelength dispersive electron microprobe analyses. BSE provides the means to examine both primary and secondary minerals and fractures within the samples. Weathering of minerals may affect the differential weathering observed with the naked eye. Microphotographs of BSE images were recorded on black and white film. Darker areas on images generally represent material with a lower atomic number. Brighter areas represent material with a higher average atomic number. Microporosity from dissolution is imaged as black (low-density) areas within the BSE micrographs (Pope et al. 1995).

Transects across the specimens were analyzed by the electron microprobe. The wavelength dispersive detector utilizes X-rays given off by electrons. The wavelengths and intensities of X- rays generated from this bombardment are then defracted by an analyzing crystal onto a detector slit. The wavelengths of the diffracted X-rays make it possible to identify the elements composing the specimen. Abundance of these elements is then tabulated as oxide weight percent.



ISSUE 2, 1998 Landform Morphology at Papago Park, Arizona 121

Results

BSE images reveal substantial differences in weathering between the matrix of the South Barnes Butte breccia, the adjacent granitic clasts in South Barnes Butte, and granitic samples from Garden Butte. Figure 2 illustrates the high porosity within the matrix of South Barnes Butte. Porosity values obtained from the matrix are 95%, 76%, and 46% (Table 1) measured in randomly located microprobe transects. In Figure 2a, albite and biotite (the gray material) are surrounded by clay, a secondary weathering product. The albite and biotite are porous. Figure 2b

Figure 2. Weathered matrix of South Barnes Butte. All microphotographs are BSE images. (A) The darker grains are sodium feldspars (albite), made porous apparently by dissolution. Biotite, represented by the brighter grains, is weathered possibly by expansion and dissolution. These grains may be surrounded by illite, a decay product of biotite. (B) The biotite grain in the center is splitting. It may have lost potassium and iron because of the weathering. (C) A sodium feldspar grain is located on the left. The bright grain in the center of the microphotograph is a potassium feldspar and the gray grain to the right is biotite. Iron oxides, which appear as bright beads in the biotite, may have reprecipitated from the iron released by biotite decay. (D) This view of the sample is at a smaller magnification (2.4 mm across) and is presented to illustrate the abundant porosity seen visually as dark areas.



122 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 3 1

shows a splitting biotite grain in the center of the photograph. Figure 2c shows a splitting biotite grain to the right of a potassium feldspar, the brighter grain in the center. The bright spots appear to be iron oxides. Figure 2d is an overall view of the matrix sample with fractured minerals surrounded by porous biotite, albite and clays (the darkest area around the minerals is the epoxy mount). These images reveal high porosity and weathering within the matrix samples from South Barnes Butte.

In contrast, the granitic rocks at both South Barnes Butte and Garden Butte apparently exhibit less weathering. The granitic samples from South Barnes Butte (Figure 3) and Garden Butte (Figure 4) exhibit lower porosity and show little evidence of secondary weathering products.

Figure 3. Microphotographs of fracturing granitic clasts from South Barnes Butte. All microphotographs are BSE images. (A) Potassium feldspars (brighter grains) and sodium feldspars (darker grains) fracturing along probable crystal weaknesses. (B) Potassium and sodium feldspars are fracturing along probable crystal weaknesses. The precipitation of secondary minerals can be found in some of these fractures and may play a role in fracture growth.



ISSUE 2, 1998 Landform Morphology at Papago Park, Arizona 123

Figure 4. Granitic samples from Garden Butte. All figures, except for Figure 4B, are BSE images of about the same scale, varying between 1 and 2 mm across. (A) Lichen-enhanced weathering. Although the organic matter in the lichen has too low of an atomic number to be seen, mineral particles are floating in the lichen. Note how the grain fractures exhibit enhanced feathering on their margins, suggesting biochemical action. (B) A microphotograph taken using secondary electrons shows a lichen on top of a sample. (C) Same area as (B) but using backscattered electrons. Note how the fractures are a mixture of void and weathered particles. (D) The granitic sample appears to have less porosity, with fractures visible in the sodium feldspar (left side of the image). Split biotite grains are also seen (lower right hand corner). (E) A split biotite grain is evident adjacent to the white arrow. The grain on the left is quartz and the brighter grain in the center is a calcium plagioclase. (F) A potassium and sodium feldspar fractured along probable crystal weaknesses. In contrast to the lichen-coated samples, the fracture margins are not as feathered and there is little porosity.




A lichen, which has too low of an atomic number to be seen in backscatter, was detected at the top of the image in Figure 4a. Mineral particles (possibly dust) appear to be "floating" in the lichen and grain fractures. The lichens exhibit enhanced feathering on their margins. In Figure 4b, secondary electron microscopy shows lichens on top of a sample; Figure 4c is the same area, but in BSE. The fractures in this sample show a mixture of void space and weathered particles.

The BSE image of the granitic sample shown in Figure 4e reveals that most of the pore spaces develop at the boundaries between the albite, anorthite, and biotite grains. The fractured surfaces are not feathered and there are no signs of etching. The granitic samples contain little microporosity in comparison to the albite from South Barnes Butte shown in Figure 2a. There is no clear evidence of clay minerals in these samples.

Field observations are consistent with the findings from electron microscope imagery. While the matrix at South Barnes Butte can be easily flaked away with bare hands, the granitic clasts require a tool, such as a rock hammer, to remove pieces.

Discussion

The key issue in this paper is the relationship of weathering to the contrasting topography of South Barnes Butte and Garden Butte. There are clear differences in weathering between the materials of the granodiorite clasts and the matrix of the breccia at South Barnes Butte. The matrix within the breccia, consisting primarily of silt and clay-sized products that show the characteristic structure of clays seen using secondary electrons (Bohor and Hughes 1971), has developed greater porosity than adjacent granitic boulders. In other words, the matrix has lost much more mass. Also, as stated in the results, the matrix can be easily flaked away with the bare hands whereas the granitic clasts require a steel tool to remove pieces. One possible interpretation may be that more intensive and extensive weathering leads to greater matrix erosion, the net result being granitic clasts protruding from a fine matrix.

A related consequence of the differential weathering is the formation of tafoni in South Barnes Butte, a feature that adds to the rough appearance. A comparable phenomenon may be the formation of tafoni in the High Sinai, Egypt. Schattner (1961) noted that many tafoni in the Sinai study site formed along joints and cracks. However, tafoni of various sizes were also located at a far distance from these fracture planes. Schattner followed the "core" hypothesis proposed by Soloman in 1926 (Schattner 1961) which held that "cores, " or "nuclei," of granite detach because they are surrounded by progressively less resistant zones of rock. The lack of resistance found in these zones stems from weaknesses created during the formation of the rock's crystalline structures. Schattner (1961) noted that many of the tafoni appeared to be initiated by the detachment of these granitic "cores. "

I suggest that the tafoni-dominated surface of South Barnes Butte may form in a similar way to the summit region of the High Sinai. The "core" theory could be extended to South Barnes Butte. As mentioned previously, granitic clasts protruding from surrounding breccia are common at South Barnes Butte. BSE images reveal that the matrix of South Barnes Butte's breccia is very weak in comparison to its granitic boulders. To extend the "core" theory, the matrix could be analogous to the weaker material surrounding the granitic boulders, or "nuclei." Eventually the stress induced by the mass of the protruding granitic boulders, or "nuclei," would cause the matrix to break and release the granitic clasts. The granitic boulders would topple to the ground leaving behind the depressions in which they formerly rested. With the "core" model, the abundance of tafoni found on South Barnes Butte would be a predictor of an abundance of strong boulders surrounded by a weakened matrix.

Garden Butte, in contrast, shows few signs of differential weathering as revealed by field observations and BSE images. In comparison to South Barnes Butte, the breakdown of the granite



ISSUE 2, 1998 LANDFORM MORPHOLOGY AT PAPAGO PARK, ARIZONA 125

is apparently the result of grusification. Biotite weathering creates tensional stresses in the granitic rock, leading to fractures that separate sand-sized minerals. Detachment of the minerals occurs along these fractures, creating grus (Gerrard 1994). This process occurs more uniformly on Garden Butte because it is composed mainly of granite. The uniformity of this process appears to produce a smoother slope form.

A case could be made by some that the presence of lichen cover at Garden Butte accelerates weathering and might produce the more abundant soil cover. Lichens are known to attack rock surfaces with oxalic acid or with water acidified by respired carbon dioxide (Viles and Pentecost 1994). The greater porosity in Figures 4a-c indicates that dissolution may be enhanced by biochemical activity. However, the vast majority of rocks at Garden Butte lack lichens. Non- lichen coated rocks in Garden Butte exhibit lower porosity; the grains show no signs of etching, and the fractures do not exhibit feathering. For instance, Figure 4d, an overall view of a granitic sample, shows fracturing of sodium feldspar (albite). The angular fractures within the albite show little feathering at the margins. Also, these grains exhibit no signs of etching.

It is unlikely that some of the fractures and/or porosity were introduced during the collection of the samples. The samples were not pounded or bashed during removal. Samples were removed by hand in most cases. In addition, some of the fractures appear to have the reprecipitation of weathered solutions (e.g. , bright line in fracture, upper right quadrant of Figure 3b) - an event that would occur naturally, not during sample preparation.

Conclusions

The contrasting forms of South Barnes Butte and Garden Butte in Papago Park are a case study in differential weathering. Garden Butte is composed of a relatively uniform granodiorite that weathers and erodes into grus in a relatively uniform fashion. In contrast, South Barnes Butte is a fanglomerate with granodiorite clasts in a sandstone and mudstone matrix that has undergone differential weathering and erosion. The different forms apparently are the result of different weathering processes acting on different rock types of varying scales.

A counter-intuitive conclusion results in linking weathering at the micron scale to weathering at the meter scale. The more vegetated Garden Butte does not appear to be the more thoroughly weathered landform. Rather, the less vegetated South Barnes Butte exemplifies greater weathering at both the micron and meter scales. The key limitation to stability at South Barnes Butte appears to be the abundance of pores in the matrix, which could lead to a faster rate of erosion of the bigger boulders, in turn creating the irregular tafoni-filled landscape.

Definitive conclusions about the cause of differences in appearance between South Barnes Butte and Garden Butte are difficult to make. There may be multiple factors acting in tandem to account for the differences. However, the combination of field observations and BSE images suggest that one possible explanation could be differential versus more uniform weathering. The development of high porosity in the silt and clay-sized matrix samples versus minimal porosity and angular fracturing within granitic samples provides the basis for this possible explanation.

Acknowledgments

I thank James Clark for assistance in scanning electron microscopy, Ron Dorn for discussions, and Ramon Arrowsmith and an anonymous reviewer for comments. This research was supported in part by the donors of the Petroleum Research Fund of the American Chemical Society through Grant 29151-AC2.




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GERRARD, J. 1994. Weathering of granitic rocks: environment and clay formation. Pp. 3-20. In: D. A. Robinson and R. B. G. Williams (eds.), Rock Weathering and Landform Evolution. John Wiley and Sons, New York.

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SMITH, B. J. and J. P. MCGREEVY. 1983. A simulation study of salt weathering in hot deserts. Geografiska Annaler 65: 127-133.

SMITH, B. J., J. P. MCGREEVY, and W. B. WHALLEY. 1987. Silt production by weathering of a sandstone under hot arid conditions: an experimental study. Journal of Arid Environments 12:199-214.

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VILES, H. and A. PENTECOST. 1994. Problems in assessing the weathering action of lichens with an example of epiliths on sandstone. Pp. 99-116. In: D. A. Robinson and R. B. G. Williams (eds.), Rock Weathering and Landform Evolution. John Wiley and Sons, New York.

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Relating Micrometer-Scale Weathering to Landform Morphology at Papago Park, Phoenix, Arizona