Lichens can be found in the death zone of mountains (up to 7400m a.s.l. in the Himalayan), in rainforests, deserts, temperate regions and on the coast of the sea.
The body of lichen that is visible is formed by the fungus, and is called “thallus”. Lichens can divided into three broad groups based on the shape of the thallus: the fruiticose type consists of small tubules and branches, the foliose type which have a leaf-like plant body, and the flattened crustose type. This last group comprises the most common members of the lichen family, and can be found extensively on hard surfaces, including rock outcrops, boulders, tree bark, buildings and gravestones.
The first study related to Lichenometry, the dating method that use the growth rate of lichens for dating the surface that they colonize, was carried out by the Austrian scientist Roland Beschel in 1950.
Given similar rocks and climatic conditions, the larger the lichen colony, the longer will be the time passed since the growth surface becomes exposed.
Estimating the absolute age of a material from the lichen growing on its exposed surface first requires the determination of the growth rate of the area. After measuring lichen on surfaces of known age (for example by comparing lichens on historic buildings or geomorphic features with known age) it is possible to plot a growth curve that relates lichen diameters to time.
Then is it possible to compare lichen on a surface of unknown age, within the same area, with the grow curve to determinate the surface’s age.
But because lichens colonies eventually grow together, and can no longer be measured individually, lichenometry as dating tool is used in a range less than 500 years. Under optimal circumstances lichenometry can provide a dating tool accurate to +-5 years over the last 200 years. So these organisms provide accurate dates for young glacial deposits, rockfalls and mudflows – all events that expose new rock surfaces on which lichen can grow.
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