In archaeology

Archaeology

Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...

, palaeontology, and geomorphology

Geomorphology

Geomorphology is the scientific study of landforms and the processes that shape them...

, lichenometry is a geomorphic method of geochronologic aging that uses lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

 growth to determine the age of exposed rock

Surface exposure dating

Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault...

: lichens are presumed to increase in size radially at specific rates as they grow. Measuring the diameter of the largest lichen of a species on a rock surface can therefore be used to determine the amount of time that 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, though is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1000 years. The use of lichenometry is of increased value for dating deposited surfaces over the past 500 years as radiocarbon dating techniques are less efficient over this period. The most common lichen used for lichenometry are that of the genus Rhizocarpon

Rhizocarpon

Rhizocarpon is a lichenized genus of fungi within the Rhizocarpaceae family.-External links:* at Index Fungorum...

, for example the species Rhizocarpon geographicum, and those of the genus Xanthoria

Xanthoria

Xanthoria is a genus of lichenized fungi in the family Teloschistaceae.-External links:*...

.

Lichenometry can provide dates for glacial deposits in tundra

Tundra

In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...

 environments, lake level changes, glacial moraine

Moraine

A moraine is any glacially formed accumulation of unconsolidated glacial debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past glacial maximum. This debris may have been plucked off a valley floor as a glacier advanced or it may have...

s, trim lines, rockfalls, talus (scree

Scree

Scree, also called talus, is a term given to an accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles...

) stabilization and former extent of permafrost

Permafrost

In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...

 or very persistent snow cover ..

Among the potential problems of the technique are the difficulty of correctly identifying the species, delay between exposure and colonization, varying growth rates from region to region as well as the fact that growth rates are not always constant over time, dependence of the rate of growth upon substrate texture and composition, climate, and determining which lichen is the largest.

It was first employed by Knut Fægri

Knut Fægri

Knut Fægri was a Norwegian botanist and palaeoecologist.- Academic career :Fægri was born in Bergen, and took the dr.philos. degree in 1934 with the thesis Über die Längenvariationen einiger Gletscher des Jostedalsbre und die dadurch bedingten Pflanzensukzessionen. He was hired as a research...

 in 1933, though the first exclusively lichenometric paper was not published until 1950, by Austrian Roland Beschel , in a paper concerning the European Alps

Alps

The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....

.

Lichenometry has also been explored as a tool in assessing the speed of glacier retreat due to climate change.

Methods

Several methods exist for dating surfaces with help of lichenometry; the most simple relies on a single largest lichen while other methods use more. There are also differences in the way the lichen is measured; while some suggest that the largest diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 should be measured, other scientists prefer the diameter of the largest inscribed circle. A problem in dating lichens is the fact that several thalli can fuse together, making several minor lichens appears as a larger one of older age. 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.
• Fixed-area largest lichen (FALL): This technique was initially specially designed for dating rockfalls and talus cones with no uniform age of deposition. The largest thallus is a unit area is measured. The sample areas are usually boulders with surfaces of about 1 m².
• Size-frequency approach (SF): The analyze of size and frequency of lichens was initially done in order to study lichen populations and preexisting thalli growing on surfaces, but has afterwards also been used as an effective absolute and relative dating method.
• Lichen cover approach (LC): This method works with the premise of that the area covered by a single species will increase over time, and by measuring the total area percentage covered by a certain lichen species the age of exposure can be inferred.