Editing Lichenometry (section)

{{Short description|Geomorphic method of geochronologic aging}}
[[File:Lichen forming a Hidden Mickey.jpg|thumb|right|Lichen grown in a [[Mickey Mouse]] shape]]
[[File:Lichen Mickey 13 years later.jpg|thumb|right|13 years later]]
[[File:Lichen_17_years_later.jpg|thumb|right|17 years later]]
In [[archaeology]], [[palaeontology]], and [[geomorphology]], '''lichenometry''' is a geomorphic method of [[geochronologic]] dating that uses [[lichen]] growth to determine the [[Surface exposure dating|age of exposed rock]], based on a presumed specific rate of increase in radial size over time.<ref>Holocene climatic and glacial history of the central Sierra Nevada, California, R.R. Curry, pp. 1–47, 1969, Geological Society of America Special Paper, 123, S.A. Schumm and W.C. Bradley, eds., 1969</ref><ref name=LSN>Lichens in relation to management issues in the Sierra Nevada national parks, McCune, B., J. Grenon, and E. Martin, L. Mutch, Sierra Nevada Network, Cooperative agreement CA9088A0008. Oregon State University, Corvallis, Oregon, and Sequoia and Kings Canyon National Parks, Three Rivers, California, [http://openjournals.wsu.edu/index.php/pnwfungi/article/view/1031]</ref>{{rp|9}} Measuring the diameter of the largest lichen of a species on a rock surface can therefore be used to determine the length of time the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years,<ref name=Benedict>{{cite journal|journal=[[American Antiquity]]|volume=74|issue=1|date=January 2009|pages=143–172|title=A Review of Lichenometric Dating and Its Applications to Archaeology |last=Benedict |first=James B.|doi=10.1017/S0002731600047545|s2cid=83108496 }}</ref> providing the maximum age limit of the technique, but it is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1,000 years.<ref>Sowers, J.M., Noller, J.S., and Lettis, W.R., eds., 1997, Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology. U.S. Nuclear Regulatory Commission, NUREG/CR 5562.</ref> (The practical limit of the technique might be 4,000 to 5,000 years.<ref name=Benedict/>)

Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period.<ref name="innes">{{cite journal |first=John L. |last=Innes |title=Lichenometry | journal=Progress in Physical Geography | volume=9 | issue=187 | url=http://ppg.sagepub.com/cgi/reprint/9/2/187 }}</ref> The lichens most commonly used for lichenometry are those of the genera ''[[Rhizocarpon]]'' (such as the species ''[[Rhizocarpon geographicum]]'') and ''[[Xanthoria]]''. The measured growth rates of ''R. geographicum'' tends to fall within the range of 0.9–0.3&nbsp;millimeter per year, depending on several factors, including the size of the lichen patch.<ref>{{Cite journal|last=Armstrong|first=R. A.|title=Growth Curve of the Lichen ''Rhizocarpon Geographicum''|date=August 1983|journal=New Phytologist|language=en|volume=94|issue=4|pages=619–622|doi=10.1111/j.1469-8137.1983.tb04870.x|issn=0028-646X|doi-access=free}}</ref>

The technique was first employed by [[Knut Fægri]] in 1933, though the first exclusively lichenometric paper was not published until 1950, by Austrian [[Roland Beschel]],<ref>{{cite journal |last=Beschel |first=R. |author-link=Roland Beschel |year=1950 |title=Flechten als Altersmasstab rezenter Moränen |journal=Zeitschrift für Gletscherkunde und Glazialgeologie |volume=1 |pages=152–161 }}</ref> in a paper concerning the European [[Alps]].<ref>{{cite web|url=http://mc2.vicnet.net.au/home/date/web/lich.html |title=Lichenometry |access-date=2009-02-25 |url-status=dead |archive-url=https://web.archive.org/web/20080823052446/http://mc2.vicnet.net.au/home/date/web/lich.html |archive-date=2008-08-23 }}</ref>