Weathering mantles and the Age of the Earth

last edited: 07/09/2002

Ferricretes of The Broken Hill region, New South Wales
Duricrust - Britannica.com
Weathering of rocks and the formation of sediments
Piedmont saprolite
Laterite images
More on laterites
Meta-gabbro weathering in the Georgia Piedmont, USA
Weathering images
Granite weathering images
The weathered geochemical profile
Laterites of Jos Plateau, Nigeria (includes SEM photos)
Dr Daniel Mathieu's laterite research
Cosmogenic 10Be and 26Al exposure dating of Antarctic glaciation (PDF file)
Water, Energy, and Biogeochemical Budgets (WEBB) in the Luquillo Mountains, Puerto Rico
Luquillo WEBB Project Bibliography

Red brown earthy laterite above lighter coloured saprolite, typical for the lateritization of basalt (below). Campinas, Brazil. Photo: Dr W. Schellmann. See the EUROLAT page.

The chemical breakdown of crystalline igneous and metamorphic rocks under the influence of the biosphere, hydrosphere and atmosphere is an extremely slow process. In cases where such rocks are known to have been exposed to weathering for hundreds or even thousands of years within historical time, such as quarried monuments, lava flows, or alpine glacial moraines, only thin [<1cm] weathering rinds have developed during that time (e.g. Jackson and Keller, 1970; Colman and Pierce, 1981; Meierding, 1993; Dragovich, 1986; Paradise, 1993). On the other hand, deep weathering mantles formed on crystalline igneous and metamorphic lithologies cover much of the earth's surface (e.g. Migon and Lidmar-Bergstrom, 2001; Théveniaut and Freyssinet, 1999; Schwarz, 1997). Some are these are extremely thick, over 100m thick in some cases. There are also a number of weathering mantles preserved in the geologic record, from the Archaean to recent (e.g. Hill et al., 2000; Gutzmer et al., 1998; Bestland, 1996; Simon-Coinçon, 1997; Meyer, 1997; Prasada and Roscoea, 1996; Setterholm and Morey, 1989; Blank, 1978). Young-earthers need to explain how such deep and extensive weathering mantles could have developed repeatedly on an earth less than 10,000 years old.

Weathering mantle can be defined as as the "product of in situ rock weathering accumulated through time to form thick bodies of altered parent rock" (Migon and Lidmar-Bergstrom, p. 286). Although a weathering mantle will typically be capped by a soil, the soil is only a small part of the weathering mantle. The type and thickness of weathering mantle which forms is a function of weathering regime, type of parent rock, topography, and time. Common types of weathering mantle on igneous rocks include grus, a type of coarse-grained saprolite commonly developed on granitic rocks, and ferrallitic saprolites such as laterites which form from strong chemical weathering of iron-rich rocks such as basalt. Ferrallitic saprolites are characterized by extensive base leaching and high residual concentration of Fe-oxides, and/or Al and Ti. Thick weathering mantles can also develop as residuum on limestone, but more often the products of limestone weathering are simply leached away in solution or carried away in suspension, producing karst dissolution topographies.

Rates of weathering/saprolitization

The process of weathering mantle formation can be thought of in terms of the downward propagation of a weathering front over time. The weathering front refers to "the three-dimensional boundary surface that seperates altered (decomposed or disintegrated) and fresh rock" (Mignon and Bergstrom, p. 288). In most igneous and metamorphic rocks, the weathering front is fairly abrupt, only a few cm or so thick. In most sedimentary rocks, this boundary is gradational and diffuse. Here we will focus on igneous and metamorphic rocks.

Historical observations show that the chemical breakdown of many types of igneous and lithified sedimentary rock under the influence of the hydrosphere and atmosphere is an extremely slow process. For instance, sandstone and granite monuments quarried thousands of years ago still show only the most incipient signs of chemical weathering. Many formerly glaciated landscapes still display smooth glacial polish and glacial striations, showing very little evidence of post-glacial weathering. Even carvings on limestone monuments will be recognizable for at least a century.

Observation of weathering on historic basalt flows in Hawaii (Jackson and Keller, 1970) Senegal (Nahon and Lappartient, 1977), basalt clasts from the western USA (Colman and Pierce, 1981) and elsewhere also demonstrate the extremely slow rate at which chemical weathering occurs. For instance, in the Jackson and Keller study, lichen-covered basalts extruded in 1907 had mean weathering rind thickness of 0.142mm thick, while lichen-free basalts had mean weathering rinds only 0.002mm thick. According to Nieuwenhuyse (2000), andesitic lava clasts from the 2000 years old Rio Jimenez flows in tropical Costa Rica have weathering rinds <1mm thick. Click here to see an incipient, iron-enriched weathering rind. Click here to see a cross-section through a thicker weathering rind.

The measurement of solute flux rates within catchments has been used to estimate current weathering rates. Some estimates using this method include 3m/Ma for laterite development on ultramafic schists, French Guiana (Freyssinet and Farrah, 2000), saprolitization rates of 8.3, 11.4 and 9.9m/Ma for the Amazon, Orinoco and Japura basins respectively (Tardy and Roquin, 1998), and a rate of 1.3 to 3.7 m/Myr for laterites in the upper Niger Basin (Boeglin and Probst 1998). White and Blum (1995) present a compilation of rates based on solute flux/geochemical mass balance methods. The global average is reported as 6m/Ma. The average rate for tropical locations, where the fastest rates are achieved, is reported to be 22m/Ma (Thomas 1994). The fastest documented rate for any silicate rocks anywhere on earth, as reported by White et al. (1998), is 58m/Ma for the 326ha, 4.2m mean-annual precipitation (MAP), upland (600-800m asl) Rio Icacos watershed in the Luquillo Mountains, Puerto Rico.

Another method now widely used to estimate weathering and denudation rates is based on the production of the cosmogenic nuclides 10Be and 26Al within minerals exposed at the earth's surface (e.g. Monaghan et al., 1992; Brown et al., 1993, 1994, 1995; Bierman and Steig, 1996; Small et al., 1997; Braucher 1998). Although based on very different physical processes, weathering rates deduced by this method are in agreement with rates deduced from solute flux measurements in watersheds (e.g. Brown et al., 1993, 1995). Finally, I'm aware of at least one paper attempting to estimate weathering rates using paleomagnetic correlation. Théveniaut and Freyssinet (1999) estimated a rate 11.3+/-0.5m/Ma for a 54m thick bauxitic profile developed on leptynite at Mont Baduel, French Guiana, based on correlation of magnetic reversals in the profile with the geomagnetic timescale.

Weathering mantle production rates can also be estimated using radiometric dating. For example, suppose you have 10 seperate lava flows in an area, which are dated at 1, 2, 3 . . . million years. If the flows are seperated by 1 meter thick weathering mantles, then the weathering mantles are developing at a rate of 1m/Ma. As an example, Pillans (1997) presents evidence for extremely slow weathering on basalt flows in north Queensland, Australia at rates of only 0.3m/Ma. The youngest flow in the chronosequence, Toomba (13Ka), showed essentially no breakdown or soil development at all. The two oldest flows, Beckford (3.3Ma) and Wongalee (5.6Ma), showed 0.8m and 1.3m of soil development, respectively. Radiometric dating can also be applied to some minerals which form within weathering profiles, particularly the alunite-group sulfates and hollandite-group manganese oxides which can be dated via K-Ar and 40Ar/39Ar (Vasconcelos, 1999).

Some modern weathering mantles

Laterites are a form of weathering mantle covering hundreds of thousands of square km of South America, Africa, India and Australia. They are significant sources of iron ore. In a laterite profile, essentially all of the silicates, alkalis, and alkali earth metals are leached out of the parent rock, leaving behind a red saprolite enriched in iron and/or aluminum oxides and hydroxides. The degree of weathering which has taken place within the profile is often expressed in terms of molar ratios such as bases:alumina (K2O+Na2O+CaO+MgO/Al2O3), silica:alumina (SiO2/Al2O3), and silica:sequioxides (SiO2/(Al2O3+Fe2O3). In general, these ratios will all decrease upsection within a weathering profile.

Sampling from bottom to top of the profile illustrates the progressive destruction and transformation of parent minerals, as well as the formation of secondary minerals such as kaolinite, gibbsite, goethite, etc. Primary texture of the parent rock, including fracture patterns, schistosity, quartz veins, etc., is often quite evident through most of the profile (e.g. the saprolite), despite the fact that the parent rock has been almost completely transformed minerallogically. This is type of isovolumetric weathering is possible because many of the secondary minerals formed by weathering (e.g. clays) are pseudomorphs of the primary minerals they replaced (e.g. feldspars). Click here to see a SEM image of a geothite pseudomorph formed from olivine, a mineral common in basalt. An idealized (and simplified) laterite profile consists of several horizons.

The bottom horizon, immediately above the weathering front, typically exhibits a coarse-grained texture (coarse saprolite, "saprock"). Corestones will typically be present also. Corestones are boulders of the parent material that are seperated from the parent body by weathering along fracture planes. In this picture you can see that corestones are being formed via preferential dissolution along preexisting joints and fractures, and in this picture you can see a cross-section through a highly weathered granite corestone. Above this horizon lies a finer grained saprolitic horizon. Quartz grains, if present, will often show extensive dissolution features (pitting, etching, perforation). Most other silicate minerals, such as feldspars, have been completely destroyed, transformed to clays. Iron content may be 2-3 times higher by volume than in the parent rock. Parent rock texture may not be as evident, but is often still quite recognizable (isovolumetric weathering). Laterites are often capped by a hardened crust up to several meters thick ('duricrust'), consisting mainly of iron (ferricrete) or aluminum (bauxite) oxides. This crust may display a distinctive nodular pisolitic texture. Such crusts commonly contain 30 to 80% Fe and/or Al oxides, even where the bedrock contains less than 2% Fe and Al. They are significant sources of iron and aluminum ore. The crust may also be enriched in titanium and manganese, which are relatively immobile. Finally, the crust or fine saprolite may be overlain by a soil.

The image below shows a cross-section through a laterite in India. Note the preservation of the primary texture (vertical lineations) of the parent rock, and the presence of saprolitized 'core stones' just above the coarse/fine saprolite boundary. Higher in the profile, former corestones are recognized by their outlines, but have been completely weathered.

Lateritic weathering section showing transition of saprolite, on which the boy stands, a mottled zone within the lower part a relict colour pattern of core boulders, followed upward by a weakly indurated brown laterite beneath a thin lateritic soil. The parent rock (granitic gneiss) is not exposed. Near Trivandrum, Kerala, India. Photo: Dr W. Schellmann. See the EUROLAT page.

Some 'fossil' weathering mantles

Migon and Lidmar-Bergstrom (2001) cite many examples of weathering mantles in central and northern Europe, ranging in age from mesozoic to recent. Some of the many examples reviewed include a 50m thick weathered horizon developed on gneiss of the Massif Central and overlain by lower Jurassic deposits (p. 295), 6 basalt flows seperated by weathered profiles up to 7m thick, a kaolinitic saprolite up to 60m thick developed on granite and gneiss and covered by Oligocene deposits on the northwest Bohemian Massif, a kaolinitic saprolite up to 123m thick developed on gneiss of the northeast Bohemian Massif. In their conclusion, Migon and Lidmar-Bergstrom (2001) state that "deep weathering was not confined to any particular period in the . . . geologic evolution of Europe, but instead it continuously was a very significant component" (p. 317).

Hill et al. (2000) describe a lateritic profile up to 30m thick sandwiched between two Paleocene basalt flows in northern Ireland. Gutzmer et al. (1998) describe a Proterozoic laterite several meters thick and covered by pisolitic duricrust developed on an early Proterozoic regional unconfiromity. Retallack and Mindszentzy (1998) describe a 3-4m thick weathering mantle developed on metamorphic gneiss and amphibolite at a Precambrian unconformity. The upper 'soil' of the profile preserves clay skins and soil microfabrics. Gneiss corestones near the base of the profile possess weathering rinds up to 7cm thick. Setterholm and Morey (1989) describe an laterally extensive, kaolinitic pre-Cretaceous (i.e. covered by late Cretaceous marine deposits) weathering profile capped by a pisolitic horizon in central and southwestern Minnesota. This profile is up to 90m thick in some places (p. H26), and is developed on a variety of Precambrian igneous and metamorphic rocks (gneiss, granite, granodiorite). Bestland et al. (1996) describes two superposed detrital laterites in the Eocene-Oligocene Clarno-John Day formations in central Oregon, each capped by thick, very mature Ultisols. These are estimated to have developed over a period of 2-4Ma, which consistent with Ar-Ar dates for associated lava flows (p. 297). Freyssinet and Farrah, 2000) estimate 17Ma for a ~50m thick latosol-capped laterite profile (~50m) developed on the Paramaca Schist in the Yaou catchment, French Guiana.

Conclusion

The rates at which rocks are altered by chemical weathering has been documented under many natural and laboratory settings (e.g. Colman and Dethier, 1986; White and Brantley, 1995). Even at the fastest documented chemical weathering rates, thick weathering mantles on crystalline igneous and metamorphic rock would take on the order of 10^5 - 10^6 years to form. Since these features are present at many levels in the geologic column, the earth is at least many millions of years old. Young-earthers should explain how such deep and extensive weathering features could have developed repeatedly on an earth less than 10,000 years old.

References

Bestland, Erick A., Gregory J. Retallack, Andrea E. Rice, Andrea Mindszenty, 1996. Late Eocene detrital laterites in central Oregon: Mass balance geochemistry, depositional setting, and landscape evolution. GSA Bulletin, v. 108, pp. 285–302.

Blank, H. R., 1978. Fossil laterite on bedrock in Brooklyn, New York. Geology 6, pp. 21-24

Boeglin, J.L., and Probst, J. 1998. Physical and chemical weathering rates and CO₂ consumption in a tropical lateritic environment: the upper Niger basin. Chemical Geology, Vol. 148 pp. 137-156.

Bierman, P.R. and Steig, E., 1996. Estimating rates of denudation using cosmogenic isotope abundances in sediment. Earth Surface Processes and Landforms, 21(2): 125-139.

Braucher, R., D.L. Bourlès, F. Colin, E.T. Brown, B. Boulangé, 1998. Brazilian laterite dynamics using in-situ-produced 10Be. Earth and Planetary Science Letters 163, pp. 197-205.

Brown, E.T., Stallard, R.F., Larsen, M.C., Raiseck, G.M., and Yiou, F. 1993, Denudation rates based on accumulation of in situ produced 10Be compared with watershed mass balance results in the Luquillo Experimental Forest, Puerto Rico [abs] EOS, Transactions: American Geophysical Union, vol. 74, no. 43, p. 295.

Brown, E.T., Bourlès, D.L., Colin, F., Sanfo, Z., Raisbeck, G.M. and Yiou, F., 1994. The development of iron crust lateritic systems in Burkina Faso, West Africa examined with in situ-produced cosmogenic nuclides. Earth Planetary Science Letters, 124: 19-33.

Brown, E.T., Stallard, R.F., Larsen, M.C., Raisbeck, G.M. and Yiou, F., 1995. Denudation rates determined from the accumulation of in situ produced 10Be in the Luquillo Experimental Forest, Puerto Rico. Earth Planetary Science Letters, 129: 193-202.

Colman, S. and Dethier, D.P. (Eds), 1986. Rates of Chemical Weathering of Rocks and Minerals. Academic Press, Orlando, Florida 603 pp.

Colman, S.M., and Pierce, K.L., 1981. Weathering rinds on andesitic and basaltic stones as a Quaternary age indicator, Western United States. US Geol. Surv. Prof. Pap. 1210.

Dragovich, D., 1986. Weathering rates of marble in urban environments, eastern Australia: Zeitschrift fur Geomorphologie v. 30, pp. 203-214.

Freyssinet, P., and Farah, A.S., 2000. Geochemical mass balance and weathering rates of ultramafic schists in Amazonia. Chemical Geology, v. 170, pp. 133-151.

Fritz, B., Tardy, Y., 1976. Sequences des mineraeux secondaires dans l' alteration des granites et roches basiques; modeles thermodynamiques. Bull. Soc. Geol. Fr. 18, p. 7-12.

Gutzmer, Jens, Nicolas J. Beukes, 1998. Earliest laterites and possible evidence for terrestrial vegetation in the Early Proterozoic. Geology: Vol. 26, No. 3, pp. 263–266.

Hill, I.G., Worden, R.H., Meighan, I.G., 2000. Geochemical evolution of a paleolaterite: the Interbasaltic Formation, Northern Ireland. Chemical Geology 166, p. 65-84.

Jackson, T.A., and Keller, W.D., 1970. A comparative study of the role of lichens and 'inorganic' processes in the chemical weathering of recent Hawaiian lava flows. Am. J. Sci. 269, pp. 446-466.

Meierding, T.C., 1993. Inscription legibility method for estimating rock weathering rates. Geomorphology, v. 6, pp. 273-286.

Meyer, R., 1997. Paleoalterites and Paleosols: Imprints of Terrestrial Processes in Sedimentary Rocks, A.A. Balkema, Rotterdam.

Migon, P., and Lidmar-Bergstrom, K.L. 2001. Weathering mantles and their significance for geomorphological evolution of central and northern Europe since the Mesozoic. Earth-Science Reviews, v. 56, 285-324.

Monaghan, M.C., McKean, J., Dietrich, W. and Klein, J., 1992. 10Be chronology of bedrock-to-soil conversion rates. Earth and Planetary Science Letters, 111: 483-492.

Nahon, D., and Lappartient, J.R., 1977. Time factor and geochemistry in ironcrust genesis. Catena 4, pp. 249-254.

Paradise, T.R., 1993. Analysis of weathering-constrained erosion of sandstone in the Roman Theater of Petra, Jordan, [PhD dissert.]: Tempe, Department of Geography, Arizona State University, 202p.

Pillans, B., 1997. Soil development at a snail's pace: evidence from a 6Ma soil chronosequence on basalt in north Queensland, Australia. Geoderma 80, pp. 117-128.

Prasada, N., and Roscoea, S.M., 1996. Evidence of anoxic to oxic atmospheric change during 2.45-2.22 Ga from lower and upper sub-Huronian paleosols, Canada. Catena 27 (2), pp. 105-121.

Schwarz, T., 1997. Lateritic bauxite in central Germany and implications for Miocene paleoclimate. Palaeogeography Palaeoclimatology Palaeoecology 129, pp. 37-50.

Setterholm, D. R., and Morey, G. B., 1989. An extensive pre-Cretaceous weathering profile in east-central and southwestern Minnesota. U.S.G.S. Bulletin. no. 1989-H, pp. H1-H29.

Simon-Coinçon R., Thiry M. and Schmitt J.M., 1997. Variety and relationships of weathering features along the early Tertiary palaeosurface in the southwestern French Massif Central and the nearby Aquitain Basin. Palaeogeography Palaeoclimatology Palaeoecology, 129, 51-79.

Small, E.E., Anderson, R.S., Repka, J.L., and Finkel, R. 1997. Erosion rates of alpine bedrock summit surfaces deduced from in situ 10Be and 26Al. Earth and Planetary Science Letters, 150, pp. 413-425.

Tardy, Y., and Roquin, C., 1998. Derive des continents, laterites et paleoclimats tropicaux. Editions BRGM, Quebec, Canada.

Thomas, M.F., 1994. Geomorphology in the Tropics: a Study of Weathering and Denudation in Low Latitudes. Wiley, Chichester, England.

Vasconcelos, P.M., 1999. K-Ar and 40Ar/39Ar geochronology of weathering processes. Annual Review of Earth and Planetary Science 27, pp. 183-229.

White, A.B., and Blum, A.E., 1995. Effects of climate on chemical weathering rates in watersheds. Geochimica Cosmochimica Acta 59, pp. 1729-1747.

White, A.B., Blum, A.E., Schulz, M.S., Davison, V.V., Stonestrom, M.L., Murphy, S.F., Eberl, D., 1998. Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: I. Long-term versus short-term weather fluxes. Geochimica et Cosmochimica Acta 62, 2, pp. 209-226.

White, A.F., and Brantley, S.L., (eds.), 1995. Chemical weathering rates of silicate minerals. Reviews in Mineralogy 31, 583 p.

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Zeese, R., Schwertmann, U., Tietz, G.F. and Jux, U., 1994. Mineralogy and stratigraphy of three deep lateritic profiles of the Jos plateau (Central Nigeria). Catena 21, pp. 195-214.