thumb|right|Lichen grown in a [[Mickey Mouse shape]]
thumb|right|13 years later
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 age of exposed rock, based on a presumed specific rate of increase in radial size over time. 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, 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. (The practical limit of the technique might be 4,000 to 5,000 years. 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 millimeter per year, depending on several factors, including the size of the lichen patch.
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, in a paper concerning the European Alps.
Scope and limitations
thumb|left|The [[map lichen (Rhizocarpon geographicum), the lichen most used in lichenometry]]
Lichenometry can provide dates for glacial deposits in tundra environments, lake level changes, glacial moraines, trim lines, palaeofloods, rockfalls, seismic events associated with the rockfalls, It has also been explored as a tool in assessing the speed of glacier retreat due to climate change.
Among the potential problems of the technique are the difficulty of correctly identifying the species, the delay between exposure and colonization, the varying growth rates from region to region, growth rates not always being constant over time and depend on substrate texture and composition, the climate, and determining the lichen that is the largest. The lichenometrist Tom Bradwell has listed the following five method families as the principal ones into which most other methods can be classified:
- Largest lichen (LL): When the single largest lichen of a species is used it means that the lichen that is oldest or grows in most favorable conditions is used to date the minimum age of the exposed surface. This was the original lichenometric from which others then developed or used as reference. Despite relying upon a single lichen this technique is praised for its simplicity and allows obtaining an image of the age of rock exposure while still in the field.
- Largest five lichens (5LL): This method is a development of the LL and was developed in the 1970s to avoid reliance on one single potentially anomalous lichen. It has been proved that neither accuracy nor precision improves significantly by having more than five lichens.