Roundness (geology)
Roundness is the degree of smoothing due to abrasion of sedimentary particles. It is expressed as the ratio of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere.
Measure of roundness
Rounding, roundness or angularity are terms used to describe the shape of the corners on a particle (or
- Very angular: corners sharp and jagged
- Angular
- Sub-angular
- Sub-rounded
- Rounded
- Well-rounded: corners completely rounded
This six-fold category characterisation is used in the Shepard and Young comparison chart and the Powers chart but the Krumbein chart has nine categories.
Rounding of sediment particles can indicate the distance and time involved [
Speed of rounding will depend on composition,
Angularity (A) and roundness (R) are but two parameters of the complexity of a clast's generalised form (F). A defining expression is given by:
F=f(Sh, A, R, Sp, T) where f denotes a functional relationship between these terms and where Sh denotes the shape, Sp the sphericity and T the micro-scale surface texture.[2]
An example of this practical use has been applied to the roundness of the grains in the Gulf of Mexico in order to observe the distance from the source rocks.[3]
Abrasion
Abrasion occurs in natural environments such as
Recent studies have demonstrated that aeolian processes are more efficient in the rounding of sedimentary grains.[4][5] Experimental studies have shown that the angularity of sand-sized detrital quartz can remain virtually unchanged after hundreds of kilometers of fluvial transport.[6]
Paleogeographic value of determining the degree of roundness of clastic material
Roundness is an important indicator of the genetic affiliation of a clastic rock. The degree of roundness points to the range and mode of transport of clastic material, and can also serve as a search criterion in mineral exploration, especially for placer deposits.
Alluvial debris in major rivers tend to exhibit a high degree of roundness. Alluvium from small rivers is less rounded. Deposits of ephemeral streams exhibit little rounding with angular clasts.
Clast rounding in non-sedimentary environments
See also
References
- ISBN 9780914696148.
- ^ Whalley, W.B. Surface textures. (2003) In, Encyclopedia of Sediments and Sedimentary rocks, Ed. G.V. Middleton, Kluwer, p.712-717
- .
- S2CID 125572789.
- S2CID 129174293.
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- ^ hdl:1877/etd7323.
- ISBN 978-0-385-18101-3
- ^ ISBN 0-7167-1456-6
- hdl:1877/etd10951.
- Mountain Encyclopedia. - Moscow: Soviet Encyclopedia, 1987. - Vol. 3 - S. 553.
- Geological dictionary . - M. : Nedra, 1978. - T. 2. - S. 29.
- Kulik, NA, Postnov AV Geology, petrography and mineralogy in archaeological research . - Methods of Earth and Man in archaeological research : Comprehensive Training Manual. - Novosibirsk: Novosibirsk State University, Institute of Archaeology and Ethnography SB RAS, 2010. - S. 39–96.
- Alexei Rudoy. "Окатанность обломочных горных пород". Knol. Retrieved 2011-01-30.[permanent dead link]