4.6 Weathering Rinds: Formation Processes and Weathering Rates

4.6 Weathering Rinds: Formation Processes and Weathering Rates CT Oguchi, Geosphere Research Institute, Saitama, Japan r 2013 Elsevier Inc. All rights reserved.

4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 References

Introduction Previous Research on Weathering Rinds Temporal Changes in Rock Properties Formation Processes of Weathering Rinds A Porosity Concerned Model of Weathering Rind Development Conclusions

Glossary L*a*b* chromatic values It is a color-opponent space with dimension L* for lightness and a* and b* for the color-opponent dimensions of red-green and yellow and blue, respectively. Vickers microhardness The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd.

98 99 99 104 105 108 108

The hardness number, Vickers Pyramid Number, is determined by the load over the surface area of the indentation and not the area normal to the force. Weathering rinds The outer layer of a pebble, boulder, or other rock fragment that has formed as a result of chemical weathering.

Abstract Models of weathering rind development have been discussed by many researchers. To understand this phenomenon, considering suitable rock properties is important because weathering rates differ from rock property types. This chapter briefly reviews the modeling types such as logarithmic, power, and relaxation function of previous researches, and then, describes rock property changes by giving an example of an andesite rock type. The properties are major 10 elements, L, a, and b chromatic values, bulk density, and porosity determined by mercury porosimetry, and Vickers microhardness. This chapter also considers a formation process model of weathering rind development and a growth model of weathering rind thickness. A conceptual model of weathering-rind formation suggests that the inner white band is produced by dissolution of alkali/alkaline earth metals related to the inward and subsequent outward movement of water. The brown band is probably formed by both leaching of these metals and oxidation of irons. A porosity concerned growth model, consisting of two diffusion equations based on the two bands, was proposed. One is a normal diffusion equation showing the development of strongly weathered brown bands due to both oxidation and leaching. The other is an equation with the diffusion coefficient exponentially related to porosity of the host rock showing the growth of total weathering rinds mainly due to leaching. The important properties to consider weathering-rind developments are contents of iron and alkali/alkaline earth metals as well as porosity of rocks. Environmental conditions are also important factors to determine the degree of oxidation or dissolution of sub-bands of weathering-rinds, although these studies are expected in the near future.

4.6.1

Introduction

The meaning of the term ‘weathering rate’ is slightly different among researchers. The term has commonly been used as a synonym of the chemical denudation rate (e.g., Waylen, 1979; High and Hanna, 1970; Trudgill, 1975, 1976; Trudgill et al.,

Oguchi, C.T., 2013. Weathering rinds: formation processes and weathering rates. In: Shroder, J. (Editor in Chief), Pope, G.A. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 4, Weathering and Soils Geomorphology, pp. 98–110.

98

1981; Crabtree and Trudgill, 1985; Hirose et al., 1994, 1995). The term also represents the formation rates of weathering products that include soils and clay minerals (e.g., Alexander, 1985; Wakatsuki and Rasyidin, 1992; Garrels and Mackenzie, 1967; Yoshioka, 1975; and Suzuki and Hachinohe, 1995), dated materials such as volcanic ash (e.g., Hay, 1960; Leneuf and Aubert, 1960; Trendall, 1962; Ruxton, 1968; Haantjens and Bleeker, 1970; Menard, 1974; Nahon and Lappartient, 1977; Amit et al., 1993), and thin weathered zones of rocks such as weathering rinds, rock varnish, and hydration layers of obsidian artifacts (e.g., Friedman and Smith, 1960; Friedman and Long 1976; Katsui and Kondo, 1965). Another usage of

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Weathering Rinds: Formation Processes and Weathering Rates

the term is the changing rates of rock properties with weathering. Kimiya (1975a, b) and Crook and Gillespie (1986) investigated fluvial-terrace gravel of different ages and concluded that rock strength decreases exponentially with increasing weathering time. Oguchi et al. (1999) also pointed out that both compressive and tensile strengths declined drastically in the early stage of weathering. Such changes in rock strength are crucial to discuss the effects of weathering on geomorphic processes. However, studies on the changing rates of rock properties such as mineralogical, chemical, physical, and mechanical properties have been limited in number. Analyses of various properties of weathering products are also important because weathering progresses with simultaneous changes in several rock properties. For example, chemical changes characterstically result in the reduction of rock strength (Matsukura et al., 1983; Oguchi and Matsukura, 1999a). In order to discuss their relationships, both chemical and mechanical properties should be analyzed. Most of the previous studies on weathering, however, dealt with only changing chemical and mineralogical properties (e.g., Craig and Loughnan, 1969; Singer, 1984; Chesworth et al. 1981). So far a limited number of studies (Saito et al., 1974; Eggleton et al., 1987; Waragai, 1993; Oguchi and Matsukura, 1999a) have investigated chemical and mineralogical properties along with mechanical and physical properties.

4.6.2

Previous Research on Weathering Rinds

Weathering rinds that have developed on the surface of rocks are useful for the study of weathering because very detailed investigation can be performed on the rinds with small areal extent. Moreover, the rates of change in rock properties due to long-term weathering can be estimated using the weathering rinds of rocks in dated deposits. Table 1 is the list of the previous studies. Most of the studies have generally been confined to the estimation of the ages of Quaternary deposits. Cˇernohouz and Sˇolc (1966) first proposed that the relationship between weathering-rind thickness and formative time is expressed by a logarithmic function: d ¼ A log ð1 þ BtÞ

½1

where d is weathering rind thickness, t is time, and A and B are constants. Their research was made using basaltic rocks and they termed ‘‘weathering rind’’ as ‘‘weathering crust’’. Thus, the term weathering crust includes the meaning of a thin weathered layer made on the gravel surface. Absolute-age functions can be determined if some of the deposits with weathering rinds are dated using other methods such as 14C measurements. If the function is given, ages of landforms can be calculated by substituting measured weathering-rind thickness into the equation. This method has been applied in many examples to glacial deposits in high mountains (e.g., Birkeland, 1973; Porter, 1975; Burke and Birkeland, 1979; Anderson and Anderson, 1981; Chinn, 1981; Colman, 1981, 1982a, 1982b; Colman and Pierce, 1981; Whitehouse et al., 1986; Knuepfer, 1988; Shiraiwa and Watanabe, 1991; Koizumi and Seki, 1992; Koizumi and Aoyagi, 1993; Aoki, 1994).

99

Several studies proposed equations of weathering rind development. A logarithmic equation was proposed not only by Cˇernohouz and Sˇolc (1966) but also by Colman and Pierce (1981) (Table 1). Chinn (1981), Knuepfer (1988) and Oguchi (2004) proposed power functions. Whitehouse et al. (1988) modeled the phenomena using relaxation functions. Furthermore, Sak et al. (2004) proposed equilibrium dissolution controlled by an interface reaction. Some studies have examined not only thickness but also other properties of weathering rinds. The series of studies by Colman and Pierce (1981) and Colman (1981, 1982a) investigated weathering rinds on andesitic and basaltic stones developed on the gravels of glacial deposits at 150 sites and 17 different areas in the Western US. Colman and Pierce (1981) studied them from the viewpoint of a Quaternary age indicator, whereas Colman (1982a) focused at chemical and mineralogical alteration on weathering rinds. Kuchitsu (1991) identified the minerals of weathering rinds formed on lithic artifacts. Matsukura et al. (1994a, 1994b), Oguchi and Matsukura (1999b), and Oguchi (2001, 2004) studied mineralogical, chemical, and mechanical properties as well as colors of weathering rinds on andesite gravel from central and south Japan. Furthermore, biological effects on weathering rind development were explained by Etienne (2002), in which microerosion was considered. Dixon et al. (2002) pointed out the difference between weathering rind and rock coatings. Gordon and Dorn (1983) pointed out in situ weathering rind erosion using cosmogenic nuclides. These studies are important in the application for absolute dating using weathering rinds. Systematic studies based on concurrent investigation of several rock properties had become necessary to establish a model for the formation of weathering rinds. The reason is that the difference of rock property types sometimes shows different weathering degrees. The relationship between weathering-rind properties and weathering time remains to be determined. Arguments on the environmental influences were also important. Considering these outstanding problems, there is need to confirm the consistency between visible characteristics and several rock properties: (i) weathering rinds generally consist of sub-zones with different colors, and (ii) the zone with a certain color near the rock surface may not strictly correspond to the zone subjected to weathering. Thus, systematic investigations of weathering rinds and inner zones are important not only for improved understanding of rock weathering, but also for the re-evaluation of dating methods using weathering-rind thickness.

4.6.3

Temporal Changes in Rock Properties

This case study explains weathering rinds on andesite gravel in river terrace deposits in central Japan (Oguchi and Matsukura, 1999a; Oguchi, 2001, 2004), arriving at general observations relevant to weathering rinds in general. The rock samples were collected from coalescing alluvial fans in Nasuno-ga-hara area (361340 –371050 N, 1391500 –1411100 E, 120–560 m asl) (Figure 1). This area belongs to the humid temperate climate region and has a mean annual temperature of 11.1 1C (  0.2 1C in the coldest (January), 23.3 1C in the warmest

100

Rock type

Study area

Deposited condition

Rock properties Min.

Chem.

Phys.

Mech.

Color

Rate

Equation type

Reference

Logarithmic

Cˇernohouz and Sˇolc (1966)

Basalt

Bohemia













Granite Basalt

Colorado North Cascade

Moraine Moraine

 

 

 

 

þ þ

Granite Sandstone (A)a

Sierra Nevada U.S. (Utah)

Moraine Moraine

 þ

 

 

 

þ þ

Sandstone (G)b Andesite, Basalt Andesite Basalt Sandstone (G)b Sandstone (G)b Gneiss

New Zealand Western U.S. ditto ditto New Zealand New Zealand Nepal Himalaya

Moraine Moraine ditto ditto Mne Terrace Moraine

 þ     

 þ     

      

      

þ þ þ þ þ  

L ¼ 4.64log(1 þ 0.01t) (L:thickness, t: time) 45 mm/10 000 yr  (esti. 0.3–0.7 mm/ 14 000 yr) 3 mm/10 000 yr  (Estimated 5 mm/ 6000 yr) 6 mm/9500 yr  3 mm/315 000 yr 1.6 mm/140 000 yr 8 mm/20 000 yr 7 mm/15 000 yr 4.5 mm/3300 yr

Hornfels Granodiorite Quartz porphyry Granodiorite Andesite Andesite Andesite

Japan (Kanagawa) Cen. Japan Alps North. Japan Alps Cen. Japan Alps South Japan ditto Cen. Japan

Lithic artifacts Moraine Moraine Moraine Exposure ditto River terrace

þ    þ  

    þ  

      

     þ þ

þ þ þ  þ  

4 mm/2000 yr 5 mm/20 000 yr 8 mm/50 000 yrs 7.8 mm/3500 yr   5.6 mm/660 000 yr

Andesite Andesite Basalt

ditto ditto Costa Rika

ditto ditto River terrac

þ  þ

þ  þ

þ þ 

þ þ 

þ þ 

  3 mm/1000 yr

a

Quartzarenite. Greywacke.

b

Birkeland (1973) Porter (1975)

Power function Logarithmic ditto Relaxation Power function

Power function Equilibrium dissolution controlled by an interface reaction

Burke and Birkeland (1979) Anderson and Anderson (1981) Chinn (1981) Colman (1982a, b) Colman and Pierce (1981) ditto Whitehouse et al. (1986) Knuepfer (1988) Shiraiwa and Watanabe (1991) Kuchitsu (1991) Koizumi and Seki (1992) Koizumi and Aoyagi (1993) Aoki (1994) Matsukura et al. (1994a) Matsukura et al. (1994b) Oguchi and Matsukura (1999b) Oguchi (2001) Oguchi (2001) Sak et al. (2004)

Weathering Rinds: Formation Processes and Weathering Rates

Table 1 Summary of the previous studies on weathering rinds

Weathering Rinds: Formation Processes and Weathering Rates

139° 50′

101

140° 0′ 0

400 km

Sea of Japan

R. N

aka

Pacific ocean

37° 0′ 37° 0′ Kuroiso

R. sab

i

1 2

Sekiya

3

R. ma

Ku

4 5 Sabui

6 7

R.

Nishinasuno

8

ki

Ho

Ohtawara

9 Kurobane

Nozaki

36° 50′

36° 50′

139° 50′ N

Sarado 0 1 2 3 4 5 km 140° 0′ Figure 1 Sample site locations within Nasuno-ga-hara, central Japan. 1: River Bed (0-ka surface); 2: Lower Terrace II; 3: Lower Terrace I (20ka surface); 4: Middle Terrace; 5: Upper Terrace (130-ka surface); 6: Lower Hill (290-ka surface); 7: Upper Hill (660-ka surface); 8: Mountains; 9: Sampling sites. Reproduced from Oguchi, C.T., 2004. A porosity-related diffusion model of weathering-rind development. Catena 58, 65–75.

(August) season), and has a mean annual precipitation of 1298 mm, according to the data from the nearest meteorological station (371080 N, 1401130 E, 354 m asl) (Japan Meteorological Agency, 1991). The fan deposits consist predominantly of andesite pebbles and cobbles supplied from

volcanoes in the upstream area. All fans consist of andesite gravel in loam including volcanic ash soils and represent Quaternary fluvial surfaces of different ages (Watanabe and Sagehashi, 1960; Koike, 1961; Suzuki et al., 1998). The following five surfaces suitable for rock sampling were investigated

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Weathering Rinds: Formation Processes and Weathering Rates

in this paper: the present River Floodplain; the Lower Terrace I formed at 20 ka BP; the Upper Terrace formed at 130 ka BP; the Lower Hill formed at 290 ka; and the Upper Hill formed approximately 660 ka BP. These ages are based on tephra stratigraphy on the terrace deposits (Suzuki et al., 1998). It is assumed that only fresh rocks were deposited on floodplains and that weathering started soon after the terrace formation. Thus, the time between the age of each terrace and the present can be assumed to be the weathering period. The studied geomorphic surfaces and rocks are, hereafter, designated by age as 0 ka, 20 ka, 130 ka, 290 ka, and 660 ka, respectively. The weathering of the gravel in the terrace deposits may also have been affected by past changes in the weathering environment. Takahashi and Hayakawa (1995) implied that the air temperature in central Japan gently fluctuated between 0 and  7 1C during the Quaternary. This difference in temperature is small enough to cause only minor changes in chemical reaction rates based on Arrhenius’ equation. Hence, in the present study, it is assumed that weathering conditions have been approximately constant over the past several hundreds of thousands of years. Figure 2 shows rock samples with weathering rinds of different ages. All the 0-ka rocks have fresh textures through the rock surface to the interior. No brown weathered layers can be observed. The 20-ka rocks have no distinctive weathered layers, although these samples have very thin layers of

alteration that can be observed by the naked eye. The 130-ka, 290-ka, and 660-ka rock samples have both the outermost weathered layers and inner fresh or relatively fresh parts. The former layers with black or brown color under microscope correspond to ‘brown bands’ by naked-eye observation of cut rocks, whereas the latter parts with pale colors are related to ‘the interior.’ The boundary between the brown bands and the interior is sharp. Chromatic, mineralogical, chemical, physical, and mechanical properties of the weathering rinds were determined. Quantitative color measurements using a visible microspectrometer provide reproducible color data as well as information about the existence of ferric oxides and hydroxides whose identification is otherwise difficult (Nakashima et al., 1992). The color measurement yields three values of color spaces denoted conveniently by L, a, and b, which can be calculated from the basic spectral data. The L a b color system is part of a standard methodology to describe colors in a quantitative way, because they have uniform color space (Hunt, 1980). L shows lightness, in which L ¼ 0 corresponds to black, whereas L ¼ 100 corresponds to white. Both color indicators a and b show chroma. A positive value of a expresses red and a negative one indicates green, whereas a positive value of b shows yellow and a negative one

Weathering rind White band

80 Brown band

70

Rock interior

60 L*

0 ka

50 40 30 20

20 ka

a*

10 5 0

130 ka

290 ka

b*

–5 40 30 20 10 0 –10

0

2

4

6

8

10

12

14

Distance from rock surface (mm) 660 ka

Figure 2 Studied rock sample with weathering rinds.

Figure 3 Color changes with depth from the rock surface. The values of L, a, and b represents lightness, redness and yellowness, respectively. Modified from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858.

Weathering Rinds: Formation Processes and Weathering Rates represents blue. Figure 3 shows the L, a, and b values plotted against the depth from the rock surface. The sample shows that L values for the rock interior (more than 10 mm depth) increase with decreasing depth, but a and b values do not change. The values of a and b for the white band (from 10 mm to 4 mm depth) are ca. 0 and ca. 10, respectively. The L values for the white band are constant with ca. 50, showing whitish gray color. The L, a, and b values for the brown band (from 4 mm depth to the rock surface) are from 55 to 65, from 0 to 6, and from 10 to 40, respectively. Minerals were identified by X-ray diffraction analysis (XRD). The fine powdered samples were analyzed to identify both original rock-forming minerals and clay minerals. Most samples from the brown layers consist mainly of kaolin minerals, feldspar, pyroxene, and quartz. The main weathering products are kaolin minerals derived from weathering of feldspar and pyroxene. Chemical composition of bulk samples was determined by X-ray fluorescence analysis (XRF). Samples were collected from the brown layers of 130-ka, 290-ka, and 660-ka rocks as well as the interior of 0-ka, 20-ka, 130-ka, 290-ka, and 660-ka rocks. The brown layers of 0-ka and 20-ka rocks could not be collected because they were very thin. The collected samples were ground into powders fine enough for analysis. Ten major oxides were analyzed (SiO2, TiO2, Al2O3, FeO þ Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) and the relationship between chemical compositions for the brown layers and those for the interior is shown in Figure 4. The proportion of the contents of Na2O, CaO, and MgO for the brown bands to those for the interior is 0.03:1 at minimum. On the other hand, the proportion of the contents of ignition loss is 10 times higher than that of brown bands. The proportion of the FeO+Fe2O3 content is 5 times higher that of brown bands. There is little difference in total- and SiO2-values between the brown layers and the interior. Electron-probe microanalysis (EPMA) was conducted to examine element concentration of the area (qualitative mapping) through rock surface to the

interior. Samples with thick and distinct weathering rinds (660-A1) were analyzed using polished thin sections cut normal to rock surfaces. Figure 5 shows the colored composition maps of nine elements: Si, Ti, Al, Fe, Ca, Mg, Na, K, and Mn. The maps cover the zone from the interior to the surface of the rocks. Color steps indicate the relative content of each element: the white step corresponds to larger quantities and the black step corresponds to pore space. The amounts of most elements of the sample 660-A1 are also small in the brown bands (the part between ca. 6.0-mm depth and the rock surface) and large in the interior (Figure 5). The contents of Si and Al are small within the brown bands, but they are large at the outermost zone of 2-mm thickness. The grains enriched in Si are thought to be secondary accumulated minerals. The contents of Ti are small in the brown bands and large in the interior. The contents of Fe within the groundmass of the brown bands are smaller than those of the interior, but it is large within the outermost zone of ca. 3-mm thickness. Other elements (K, Na, Mg, Ca, and Mn) for the brown bands have amounts smaller than those for the interior, and especially, Ca was much more leached out. Physical properties such as bulk density, porosity, and other physical properties are technically difficult to obtain for weathering rinds. The common method uses cored rock samples because weathering rinds are so thin. Mercury intrusion porosimetry (MIP), however, provides a method to investigate physical properties related to pores of rocks from the measurement of pore-size distribution. This MIP measurement system also yields values of total pore volume (Vt) and bulk density (rbulk). Thus, porosity (n) can be calculated by multiplying these two values. Table 2 shows bulk density and the calculated porosity. The interior of all the rocks have the large bulk density of ca. 2.4–2.5 g cm3 on average. In contrast, the bulk density for the brown bands of 130-ka, 290-ka, and 660-ka rocks are ca. 1.2–1.4 g cm3 on average. Accordingly, porosity for the brown layers is larger than that for the interior because of much larger pore volume. The

1000.00 × 10

×1

× 10

Total SiO2 AI2O3 TiO2 Fe2O3 MnO Mg0 CaO

100.00

Interior (wt.%)

× 0.1 10.00

1.00

Na2O K2O P2O5 lg. loss

0.10

0.01 0.01

0.10

1.00

103

10.00

100.00

1000.00

Brown layer (wt.%) Figure 4 Relationship between chemical composition for the brown bands and those for the interior. Reproduced from Oguchi, C.T., 2000. Rates of rock property changes with weathering: andesite gravel in fluvial terrace deposits in Nasuno-ga-hara, Japan. Science Reports of the Institute of Geoscience, University of Tsukuba, Section A 21, 59–88.

104

Weathering Rinds: Formation Processes and Weathering Rates

Weathering rind

Interior

Si

Al

Fe

Ca

Count 7000 6000

Mg

5000 4000

K

3000 2000

Mn

1000 0

Figure 5 EPMA mapping of weathering rind. Modified from Oguchi, C.T., 2000. Rates of rock property changes with weathering: andesite gravel in fluvial terrace deposits in Nasuno-ga-hara, Japan. Science Reports of the Institute of Geoscience, University of Tsukuba, Section A 21, 59–88.

interior of all the rocks have a porosity of ca. 2–5% on average, and the brown bands of 130-ka, 290-ka, and 660-ka rocks have a porosity of ca. 30–40% on average. As for mechanical properties, the Vickers Hardness Number (VHN) (Smith and Sandland, 1922) is a useful indicator. The results of the VHN measurement are shown in Figure 6. Based on the changing patterns of the VHN values (in gram-force per square micrometer, gf/mm2, units), three zones were identified as follows: 1. Brown bands have small VHN values from 10 gf mm2 to 80 gf mm2. 2. White bands have abrupt or gradual increase in VHN values with increasing depth, in which VHN values vary from ca. 100 gf mm2 to ca. 500 gf mm2. 3. Rock interiors have large and constant VHN values of ca. 500 gf mm2.

4.6.4

Formation Processes of Weathering Rinds

Figure 7 shows the changes in normalized values of the four properties with depth. The maximum value of each parameter is set to 100% and the minimum to 0%. The changes in values of CaO and FeO þ Fe2O3 are chosen because CaO has been leached from the deepest point and the pattern of change in FeO þ Fe2O3 is different from that of the other elements. In the rock interior between depths of 15 and 10 mm, the CaO content decreases slightly with decreasing depth, whereas L values slightly increase with decreasing depth. In the white band, the contents of FeO þp Feffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2O3 andffi L values increase with decreasing depth and a2 þ b2

values increase slightly between 10 and 7 mm depth, whereas the contents of CaO and VHN values decrease with decreasing depth and the contents of FeO þ Fe2O3 increase with decreasing depth between 7 and 4 mm depth. In the outer brown band (less than 4 mm depth), the contents of CaO and VHN values decrease with decreasing depth, whereas the contents of FeO þ Fep 2O 3 increase ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi with decreasing depth. The values of L and a2 þ b2 first increase and then decrease with decreasing depth. From these findings, rock properties of the three parts can be summarized as follows. The outer brown band is characterized by decreased alkali/ alkaline earth metal, FeO þ Fe2O3 contents, increased L, a, and b values, and very low VHN values. The white band is characterized by decreasing alkali/alkaline earth metals and VHN values toward the surface and medium L values. The rock interior is characterized by consistently high contents of alkali/alkaline earth metals and large VHN values. The rock interior also exhibits low contents of FeO þ Fe2O3 and small L, a, and b values. For the band described in this study, oxidized and dissolved zones in the upper zone (the outer brown band) underwent not only oxidation but also dissolution, and the formation of the bands needs a two-way water movement. In the rocks comprising the fluvial-terrace deposits, weathering rinds tend to occur parallel to the rock surface with nearly constant thickness. Thus, the mechanism of weathering-rind formation is related to smaller-scale water movement in and around the rocks, which is different from the large-scale movement. Gravel in terrace deposits is attacked by underground water percolating through sediments. When rainfall is abundant, water is supplied to the gravel. In this case, water movement is directed from the outside

Weathering Rinds: Formation Processes and Weathering Rates

105

Table 2 Physical properties of the interior and thickness of brown band (LI) and total weathering rind (LI þ II) Samples

Total pore volume Vt, mm3 kg 1

Bulk density rbulk, 103 kg m 3

Porosity n, %

Brown layer LI, mm

Total weathering rind LI þ II, mm

0-ka Rocks 0a 0b 0c 0d

0.018 0.009 0.011 0.016

2.40 2.44 2.55 2.55

4.30 2.19 2.79 3.98

– – – –

– – – –

20-ka Rocks 20 a 20 b 20 c 20 d 20 e

0.015 0.014 0.014 0.020 0.028

2.36 2.33 2.59 2.40 2.45

3.62 3.25 3.57 4.76 6.73

o0.10 o0.10 o0.10 o0.10 o0.10

2.88 3.03 1.88 2.00 3.00

130-ka Rocks 130 a 130 b 130 c 130 d

0.034 0.008 0.026 0.017

2.31 2.51 2.34 2.49

9.95 1.96 6.15 4.14

3.00 3.30 2.00 2.00

420.00 5.60 412.00 5.04

290-ka Rocks 290 a 290 b 290 c 290 d 290 e 290 f 290 g

0.008 0.003 0.010 0.018 0.036 0.027 0.015

2.61 2.68 2.61 2.54 2.42 2.40 2.55

2.20 0.85 2.48 4.49 8.59 6.54 3.91

2.42 1.87 3.00 3.25 3.03 3.10 3.16

2.79 4.26 6.50 410.20 421.00 415.00 8.42

660-ka Rocks 660 a

0.009

2.43

2.20

4.00

6.00

Vickers microhardness number (gf µm–2)

Brown band

Rock interior White band

600 400 200 0

0

5 10 Depth from rock surface (mm)

15

Figure 6 Vickers microhardness number versus depth from the rock surface. Modified from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858.

to the inside of the rock. In contrast, during periods of low rainfall, water supply ceases, and water movement occurs from inside to outside of a rock. These different directions of water movement are responsible for the development of the two weathering bands. Based on the analyses of the rock properties, a model to explain the mechanism of the development of the two bands is proposed (Figure 8). The growth of the white band is demonstrated as: (1) when the water from rainfall percolates into the ground and is supplied to the matrix around a cobble, water

movement toward the inside takes place; (2) alkali/alkaline earth metals are dissolved from the rock into percolating water; (3) a ‘bleached’ zone with depleted elements is formed near the rock surface; and (4) this zone gradually thickens as the dissolution ‘front’ moves into the interior of the rock. Thus, only the dissolution process formed this band. The growth of the brown band is explained as: (1) when the groundwater supply is reduced, the direction of water movement within the rock is reversed, with movement of water occurring toward the outside surface of the rock; (2) although most dissolved metals move out of the rock with the water, Fe3 þ is precipitated near the rock surface because Fe2 þ is oxidized to form Fe3 þ with low solubility in natural water (Ichikuni, 1972, p. 75); and (3) the outermost zone gradually turns brown in color due to accumulation of ferric oxide/hydroxide minerals and compounds such as Fe(OH)3, FeO(OH) (e.g., goethite and lepidochrocite), and Fe2O3 (e.g., hematite and maghemite). Both dissolution and oxidation occur in the brown band.

4.6.5

A Porosity Concerned Model of Weathering Rind Development

The original definition of the weathering rind is ‘an outer crust or layer on a rock fragment formed by weathering’ (Gary et al., 1972) or ‘a hard and thin weathered layer with Si, Fe, or Mn

106

Weathering Rinds: Formation Processes and Weathering Rates

Outer brown band

Inner white band

Rock interior

CaO

100 (%)

FeO + Fe2O3

0 100 (%)

0

L*

100 (%)

(a*2 + b*2)1/2

0 100 (%)

0

VHN

100 (%)

0 0

5

10

15

Depth from rock surface (mm) Figure 7 Changes in rock properties with depth from the rock surface. Modified from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858.

enrichment at surfaces of rock blocks’ (Maruyama, 1981). In recent years, the definition of the term ‘weathering rind’ includes the formative process of oxidation with color alteration: ‘oxidation phenomena which stain the parent rock red–yellow when exposed to air or near-surface groundwater for some time’ (Anderson and Anderson, 1981; Goudie et al., 1985). Caine (1983) also called the weathering rinds ‘oxidation–hydration rinds’. However, the term ‘weathering rind’ has been used more frequently to represent a colored weathered zone near the rock surface identified not by chemical analyses but by eye observations (e.g., Chinn, 1981; Watanabe, 1990; and Aoki, 1994). As noted before, the identification of such colored weathering rinds is generally difficult because some rocks have more than one colored zone near the surface. Andesite rocks investigated in the present study also have both brown and white bands. Based on the measurements of rock properties, weathering rinds with two zones, oxidation zone and dissolution zone, can be redefined. Both the thicknesses of these zones are used to construct a growth model of weathering rinds for the 0-ka, 20-ka, 320-ka,

450-ka, and 830-ka rocks. The relationships between these thicknesses and weathering period are shown in Figure 9. In general, the thickness of a weathered zone has often been approximated using a diffusion equation (e.g., Friedman and Long, 1976). L ¼ ðD  tÞ1=2 ¼ D1=2  t 1=2

½2

where L (mm) is the thickness of the weathered zone, t (year) is the weathering period, and D (mm2/yr) is a diffusion coefficient. Using eqn [1], the diffusion coefficients for the brown bands (DI) and the total weathering rinds (DI þ II) can be calculated from the thickness data and the weathering time. According to Drever (1997, p. 357), an effective diffusion coefficient (Deff ) is related to an apparent diffusion coefficient and rock porosity (n): Deff ¼ D=n

‘D ¼ nDeff

½3

In this study, the relationships between porosity (n) and the diffusion coefficients DI and DI þ II cannot be expressed as

Weathering Rinds: Formation Processes and Weathering Rates

H2O H2O

Fe2+

‘Brown’ band

‘White’ band Ca2+

H2O

H2O

H2O

H2O

Mg2+

Ca2+ Rock surface

H2O

Fe3+

Fe2+

H2O Na+

Fe3+ precipitation Fe(OH)3, FeO(OH), Fe2O3

Na+

Fe2+ H2O

H2O Ca2+

Mg2+

H2O

H2O Rock surface

1. Alkali/alkaline earth metals are dissolved due to the effects of predominantly inward moisture movement.

107

Rock surface 3. Precipitation of oxidized Fe3+ in the outermost layers gives rise to the formation of the ‘brown’ band.

2. Mobilised elements in the substrate are drawn out by the predominant outward movement of moisture creating the bleached ‘white’ band.

4

Dissolution zone

25 20 15

Oxidation zone

10 5 0 0

200

400

600

800

1000

Time (ka) Figure 9 Relationships between weathering time and thickness of the oxidation zone and dissolution zone. Modified from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858.

Diffusion coefficient (mm2 1000 yr–1)

Oxidation and dissolution zones (mm)

Figure 8 Formation processes of white and brown bands. Reproduced from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858.

Total weathering rind (dissolution zone)

3

y = 0.0431e0.4287x R 2 = 0.7527 2

1 Brown band (oxidation zone) 0 0

2

4

6

8

10

12

Porosity (n %)

the linear eqn (Figure 10). The DI values are low and almost constant (DI ¼ 0.0283) irrespective of the n values, whereas the DI þ II values increase exponentially with increasing n values. The relationship between DI þ II and n is better expressed (see Figure 11) as: DIþII ¼ 0:0431expð0:4287nÞ

ðR2 ¼ 0:7527Þ

½4

Combining eqns [1] and [3], the following equation is obtained: LIþII ¼ ðDtÞ1=2 ¼ f0:0431expð0:4287nÞ  tg1=2

½5

In the same way, LI ¼ ðDtÞ1=2 ¼ ð0:0283  tÞ1=2

½6

Figure 10 Relationships between porosity and diffusion coefficient for brown band (LI) and the weathering rind (LI þ II) (Oguchi, 2001). Open circles (J) show total weathering rinds and solid circles (K) show outer brown rinds. The data plots with arrows show the minimum diffusion coefficients. Modified from Oguchi, C.T., 2001. Formation of weathering rinds on andesite. Earth Surface Processes and Landforms 26, 847–858, and Oguchi, C.T., 2004. A porosity-related diffusion model of weathering-rind development. Catena 58, 65–75.

Figure 10 shows the development of the total weathering rind and brown band thicknesses based on the obtained diffusion equations. In the case of dense andesite (n ¼ 0), the thickness of the total weathering rind is almost equal to that of the brown band. Total weathering-rind thickness on porous andesite, however, is much larger than that of the brown band. It is revealed that not only weathering time but also rock porosity controls the total weathering-rind thickness.

108

Weathering Rinds: Formation Processes and Weathering Rates

Total weathering rind (dissolution zone)

Thicknesses of oxidation zone and total weathering rind (mm)

20

n = 5.0 % n = 4.0 %

15

n = 3.0 % n = 2.0 %

10

n = 1.0 % n=0% 5 Outer brown band (oxidation zone) 0 0

200

600

400

800

1000

Time (ka) Figure 11 Development of the thicknesses of the outer brown band (dotted curve) and weathering rind (solid curves) with different rock porosity values (n) based on eqns [5] and [6]. Modified from Oguchi, C.T., 2004. A porosity-related diffusion model of weathering-rind development. Catena 58, 65–75.

4.6.6

Conclusions

In order to elucidate mechanisms of weathering-rind formation, the relationships between several rock properties were examined specifically for weathering subbands, the brown band and white band compared to the rock interior. The interior of the rock is composed of fresh rock-forming minerals and is characterized by high contents of alkali/alkaline earth metals, low L, a, and b color indicator values and high Vickers microhardness numbers. The white band is depleted in Ca, especially, relative to the rock interior of the rock. The brown band is very brittle and contains clay minerals, which are absent from the rock interior and white band. The brown band also has much higher L, a, and b color indicator values than the inner two parts, a higher content of FeO þ Fe2O3, but a lower alkali/alkaline earth metal content. A conceptual model of weathering-rind formation suggests that the inner white band is produced by dissolution of highsolubility alkali/alkaline earth metals related to the inward and subsequent outward movement of water. The brown band is probably formed by: (1) dissolution of alkali/alkaline earth metals and (2) oxidation of Fe2 þ to Fe3 þ , which has low solubility and therefore forms mineral precipitates close to the rock surface. This provides valuable information on the linkages between weathering processes and rock properties. Based on property analysis, a porosity-concerned growth model for weathering rinds was also proposed. The model consists of two diffusion equations based on the two types of weathering-rinds identified. One is a normal diffusion equation showing the development of strongly weathered brown bands due to both oxidation and leaching. The other is an equation with the diffusion coefficient exponentially related to porosity of the host rock showing the growth of total weathering rinds mainly due to leaching. A significant implication of this result is that studies which use weathering-rind thickness as a dating tool under a humid temperate environment should also to take rindcharacterization and original rock porosity into consideration.

Weathering-rind developments are supposed to be different from rock types. The important properties to consider them are contents of iron as well as alkali and alkaline earth metals and porosity of rocks. If the rock is a mafic type, the rinds are clearly distinguished from the original rocks as its color turns to brown due to oxidation of iron. Dissolution of calcium will occur as well, because these rocks are rich in alkaline earth metals. If the rock is a felsic type, the oxidation zone is expected to be unclear, unless iron stains are input from outside of the rock or small amounts of ferric minerals are decomposed within the rock. Porosity will control thickness of the dissolution zones in most cases. Degree of oxidation or dissolution should be determined by temperature and precipitation as well. Therefore, it is also important to consider rock properties and climate conditions for weathering-rind studies.

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Biographical Sketch Chiaki T. Oguchi was born in Yokohama, Japan. She obtained her Bachelor Degree in Physical Geography in 1991 from Meiji University, Tokyo, Japan. She received her Master Degree in Science from the Institute of Geoscience, University of Tsukuba, Japan in 1993. She was awarded Doctorate Degree in Science from the Institute of Geoscience, University of Tsukuba, Japan in 1998. Her research title was ’Rates and Mechanisms of Development of Weathering Rind on Andesite in the Dated Fluvial Terraces’ under the advice of Prof. Dr. Yukinori Matsukura. She was appointed as Assistant Professor at the Institute of Tsukuba, University of Tsukuba, Japan in 1998. She joined as a Research Fellow at Japan Science and Technology Corporation-Japan Society for the Promotion of Science, Japan International Research Center for Agricultural Sciences in 2001. She joined as Associate Professor, Geosphere Research Institute, Saitama University, Japan in 2004.