Weathering rind

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A broken basalt cobble (15 x 10 cm size) showing well-developed weathering rind from Brazil, where chemical weathering is quite active. Spheroidal weathering.jpg
A broken basalt cobble (15 x 10 cm size) showing well-developed weathering rind from Brazil, where chemical weathering is quite active.
Weathering rind of a large granite glacial erratic eroding out of unconsolidated Permian till, Selwyn Rock, Inman Valley, South Australia Selwyn Rock 6.JPG
Weathering rind of a large granite glacial erratic eroding out of unconsolidated Permian till, Selwyn Rock, Inman Valley, South Australia

A weathering rind is a discolored, chemically altered, outer zone or layer of a discrete rock fragment formed by the processes of weathering. The inner boundary of a weathering rind approximately parallels the outer surface of the rock fragment in which it has developed. Rock fragments with weathering rinds normally are discrete clasts, ranging in size from pebbles to cobbles or boulders. They typically occur either lying on the surface of the ground or buried within sediments such as alluvium, colluvium, or glacial till. A weathering rind represents the alteration of the outer portion of a rock by exposure to air or near surface groundwater over a period of time. Typically, a weathering rind may be enriched with either iron or manganese (or both), and silica, and oxidized to a yellowish red to reddish color. Often a weathering rind exhibits multiple bands of differing colors. [1] [2] [3]

Contents

Although sometimes confused with weathering rinds, spheroidal weathering is a different type of chemical weathering in which spherical layers of weathered material progressively develop in situ around blocks of jointed bedrock beneath the Earth's surface, rather than in reworked and transported clasts such as cobbles and boulders. [4] [5]

Dating using weathering rinds

Weathering rinds have a long history of being used to determine the relative age of either Quaternary sediments or landforms. This is done by comparing the thickness of weathering rinds of gravel composed of similar rock types. Deposits containing gravel with thicker weathering rinds are interpreted to be older than deposits containing rocks with thinner weathering rinds. Sedimentary deposits containing gravel with weathering rinds of the same thickness are interpreted to be approximately contemporaneous in age. The use of weathering rinds in relative dating is widely used in Arctic, Antarctic, and alpine regions and in the correlation of glacial moraines and tills and fluvial sediments and terraces. [6] [7] [8]

In addition, weathering rinds have been used to determine the absolute amount of time gravel-size rock has been exposed to the weathering processes. This technique was proposed by Cernohouz and Solc [9] who first argued that the relationship between the thickness of a weathering-rind thickness and the time it took to form is expressed by a logarithmic function. This is done by determining the absolute age of sedimentary deposits containing either gravel-size rocks or artifacts using absolute dating methods such as C14 and measuring the weathering-rind thickness of rocks of similar lithology. The dates obtained from absolute dating techniques and measurements of weathering rind thicknesses are then used to construct an age versus thickness curve for dating rocks in other sedimentary deposits. This dating method has often been applied to glacial deposits in alpine regions. [6] [7] [10] [11]

Obsidian hydration

Obsidian hydration dating is a type of dating that uses the weathering rind that develops within artifacts or gravel that are composed of obsidian. When fresh obsidian is exposed to air it typically contains less than 1% water. Over time, a weathering rind, known as an obsidian hydration band and composed of hydrated glass forms, as water slowly diffuses from a broken surface, which is normally associated with manufacture of an artifact, into the obsidian. The thickness of this band can be seen, and measured, using various techniques such as a high-power microscope with 40-80 power magnification, depth profiling with SIMS (secondary ion mass spectrometry), and IR-PAS (infra red photoacoustic spectroscopy). [12] [13] [14]

The determination of absolute age from the thickness of an obsidian hydration band is complicated and problematic. First, the rate at which the hydration of glass occurs varies significantly with temperature. The rate at which the obsidian hydration band forms increases with temperature. Second, the rate of hydration and obsidian hydration band formation varies with the geochemistry of the obsidian, including the intrinsic water content, seems to affect the rate of hydration. Finally, water vapor pressure may also affect the rate of obsidian hydration. If the rate of obsidian hydration band can be controlled for the obsidian's geochemistry (e.g., the "source"), temperature (usually approximated using an "effective hydration temperature" or EHT coefficient) and other factors, it might be possible to date an artifact using the obsidian hydration technique. [12] [15]

The presence or absence of an obsidian hydration band has been used to distinguish prehistoric obsidian debitage from obsidian debitage produced by modern flintknappers. This distinction can be made because it takes about 70 years for a band to enlarge sufficiently so that it is readily detectable on a freshly flaked surface of a piece of obsidian. For example, the basis of the lack of development of obsidian hydration bands, it was concluded that modern flintknappers brought specimens of obsidian to the Poverty Point Site in Louisiana. [16]

See also

Related Research Articles

Obsidian Naturally occurring volcanic glass

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Weathering Breaking down of rocks, soil and minerals as well as artificial materials through contact with the Earths atmosphere, biota and waters

Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.

Chert A hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of crystals of quartz (silica) that are very small (microcrystalline or cryptocrystalline). Quartz is the mineral form of silicon dioxide (SiO2). Chert is often of biological origin (organic) but may also occur inorganically as a chemical precipitate or a diagenetic replacement (e.g., petrified wood). Geologists use chert as a generic name for any type of microcrystalline or cryptocrystalline quartz.

Geochronology Science of determining the age of rocks, sediments and fossils

Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.

Conglomerate (geology) A coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts

Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Geoarchaeology Archaeological sub-discipline

Not to be confused with Archaeogeography.

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Obsidian hydration dating (OHD) is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian.

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Plucking (glaciation) glacial quarrying

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Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

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A rock veneer is a geomorphic formation in which rock fragments (clasts) of gravel or cobble size form a thin cover over a surface or hillslope. Rock veneers are typically one or two clasts thick and may partially or fully cover the ground surface. Veneers typically form in semiarid and arid regions where chemical weathering rates and the potential for mass wasting are low. Other names for a rock veneer are rock-fragment cover (RFC), stone pavement, desert pavement, stony mantle, hammada and reg.

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Geology of Moldova regional geology of Moldova

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Geology of Latvia

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References

  1. ↑ Colman, SM, and KL Pierce (2001) Weathering rinds on andesitic and basaltic stones as a Quaternary age indicator, Western United States. Professional Paper no. 1210. United States Geological Survey, Reston, Virginia.
  2. ↑ Neuendorf, KKE, JP Mehl, Jr., and JA Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN   0-922152-76-4
  3. ↑ Oguchi, CT (2001) "Formation of weathering rinds on andesite." Earth Surface Processes and Landforms . 26(8):847–858.
  4. ↑ Fairbridge, RW (1968) Spheroidal Weathering. in RW Fairbridge, ed., pp. 1041-1044, The Encyclopedia of Geomorphology, Encyclopedia of Earth Sciences, vol. III. Reinhold Book Corporation, New York, New York.
  5. ↑ Ollier, CD (1971). Causes of spheroidal weathering. Earth-Science Reviews 7:127-141.
  6. 1 2 Goudie, AS, 2004, Rind, Weathering. in AS Goudie, ed., pp. 853-855, Encyclopedia of Geomorphology, vol. 2 J-Z Routledge, London-New York. ISBN   0-415-32738-5
  7. 1 2 Wagner, GA (1998) Age Determination of Young Rocks and Artifacts: Physical and Chemical Clocks in Quaternary Geology and Archaeology. Springer Verlag, New York, New York. 466 pp. ISBN   9783540634362
  8. ↑ Anderson, LW, and DS Anderson (1981) Weathering Rinds on Quartzarenite Clasts as a Relative-Age Indicator and the Glacial Chronology of Mount Timpanogos, Wasatch Range. Arctic and Alpine Research. 13(1):25-31.
  9. ↑ Cernohouz, J, and I Solc (1966) Use of sandstone wanes and weathered basaltic crust in absolute chronology. Nature 212:806–807.
  10. ↑ Chinn, T (1981) Use of rock weathering-rind thickness for Holocene absolute age-dating in New Zealand. Arctic and Alpine Research 13(1):33–45.
  11. ↑ Knuepfer, RLK (1988) Estimating ages of late Quaternary stream terraces from analysis of weathering rinds and soils. Geological Society of America Bulletin. 100(1):1224–1,236.
  12. 1 2 Walker, M (2005) Quaternary Dating Methods. John Wiley & Sons Ltd, Chichester, England ISBN   978-0-470-86926-0
  13. ↑ Stevenson, C, I. Liritzis, and M. Diakostamation (2002) Investigations towards the hydration dating of Αegean obsidian. Mediterranean Archaeology & Archaeometry. 2(1):93–109.
  14. ↑ Stevenson, C, and SW Novak (2011) Obsidian hydration dating by infrared spectroscopy: method and calibration. Journal of Archaeological Science. 3 (7):1716-1726.
  15. ↑ Anovitz, LM, M Elam, L. Riciputi, and D Cole (1999) The failure of obsidian hydration dating: sources, implications, and new directions. Journal of Archaeological Science. 26(7):735–752.
  16. ↑ Boulanger, MT, MD Glascock, MS Shackley, C Skinner, and JJ Thatcher (2014) Likely Source Attribution for a Possible Paleoindian Obsidian Tool from Northwest Louisiana. Bulletin of the Louisiana Archaeological Society. no. 37:89-107.
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