lichenometry

Lichenometry is a geochronological dating method used in geomorphology and archaeology that estimates the age of exposed rock surfaces by measuring the size of lichens, typically crustose forms, growing on them. The approach relies on the approximate constancy of lichen growth rates for a given climate and substrate, so the maximum observed lichen size on a surface can be related to time since exposure using species-specific growth curves.

Practically, researchers identify the lichen species with a reliable growth record (most often Rhizocarpon geographicum) and measure the diameter of the oldest thallus on the surface. Growth-rate curves are built from surfaces of known age and then used to convert observed sizes into ages. Measurements are usually taken on bedrock faces near glacial features to minimize microclimatic variation.

Applications include dating moraines and other glacial landforms, rockfalls and rock avalanches, shoreline retreat, and some archaeological rock features where alternative dating methods are unavailable. The method is most effective in cold, moist environments on resistant rock surfaces; growth rates vary with species, rock type, elevation, aspect, and moisture.

Limitations include potential changes in growth rates over time, post-exposure colonization, and local microclimate effects that can bias ages. Lichen growth often saturates at larger sizes, reducing precision for older features. Calibration requires independent age controls, and estimates are generally considered approximate, with uncertainty depending on environmental variability and the quality of calibration data.

History: Lichenometry was developed in the early 20th century and has since become a standard tool in alpine and arctic geomorphology, with ongoing regional calibrations and refinements.