Dating Geological Events with Lichens

In the mid-twentieth century, field researchers investigating alpine geomorphology realised that certain slow-growing organisms could serve as natural chronometers. The technique that emerged, termed lichenometry, relies on measuring the physical growth of crustose lichens to determine how long exposed rock surfaces have been open to the air. Unlike plants that require established soil, these composite organisms—formed by a symbiotic union between fungi and photosynthetic algae or cyanobacteria—can colonise bare mineral substrates almost immediately after fresh stone is unveiled by retreating glaciers, rockfalls, or human construction. Because some species expand at remarkably consistent and sluggish rates, the diameter of an individual lichen thallus can provide a reliable minimum age for the underlying surface, offering scientists a practical means of dating events from recent centuries and millennia.

Among the various lichen taxa used in dating, crustose varieties of the genus Rhizocarpon, particularly the yellow-green Rhizocarpon geographicum, are widely favoured. These organisms adhere tightly to stone, forming a flat, crust-like body that resists detachment by severe weather, wind, or grazing animals. Their exceptionally slow metabolic rate enables them to survive in harsh alpine and polar environments where other living forms perish. Crucially for researchers, Rhizocarpon lichens can persist for astonishingly long periods; in subpolar regions, individual colonies have been estimated to survive for several thousand years. Because they grow outwards radially from a central point of origin, measuring their maximum diameter yields a direct proxy for the duration of uninterrupted growth on that particular stone face.

To convert a physical measurement into a calendar date, scientists must construct a regional calibration curve. This involves measuring the size of lichen thalli growing on structures or geological deposits of known historical age. Old church walls, gravestones, dated bridge abutments, and documented landslides frequently serve as baseline calibration points. By plotting lichen diameters against verified exposure dates, researchers establish the local rate of radial expansion per decade or century. Once this curve is established for a specific geographic zone, investigators can apply it to undated geological or archaeological features in the surrounding area, assuming that the regional microclimate and climatic conditions influencing growth have remained broadly comparable over time.

Lichenometry has proved especially valuable in glacial studies and palaeoseismology. In alpine valleys across the world, glaciers have repeatedly expanded and retreated, leaving behind distinct ridges of rocky debris known as moraines. While conventional radiocarbon dating requires preserved wood, peat, or bone—materials that are rarely preserved within coarse glacial gravel—lichens often flourish across the stable boulders of newly formed moraines. By measuring the largest lichens on successive ridges, geomorphologists have successfully reconstructed the timelines of past glacial advances, particularly during the Little Ice Age. Similarly, the method has been used to establish the frequency of prehistoric earthquakes by dating rock avalanches triggered by seismic shocks.

Despite its widespread utility, lichenometry is subject to various environmental and biological complications. Lichen growth is not uniform across all settings; moisture levels, seasonal temperature variations, solar radiation, and even the mineral composition of the host rock can alter growth speeds considerably. A thallus exposed to frequent fog on a north-facing slope may expand much faster than one situated in a dry, sun-baked location nearby. Furthermore, lichen development does not follow a perfectly linear trajectory across an organism’s entire lifespan. Instead, it typically features an initial colonisation lag, followed by an accelerated juvenile phase, an extended period of steady linear growth, and eventually a decelerating senescent phase. Failing to account for these distinct growth intervals can introduce significant dating errors.

Another persistent dilemma relates to sampling strategy. Early practitioners often searched exclusively for the single largest thallus on a given surface, presuming it represented the earliest coloniser after exposure. However, subsequent investigations revealed that unusually large specimens might result from two separate lichen colonies fusing together, an anomaly that produces an artificially inflated age estimate. In response, modern researchers increasingly employ statistical methods that sample dozens or hundreds of lichens across a landform, calculating size distributions rather than relying on an isolated extreme. This population-based approach helps ensure that anomalous individuals do not distort the final age determination.

Today, lichenometry is rarely used in total isolation. Instead, geoscientists routinely combine it with other analytical techniques, such as cosmogenic isotope dating and historical aerial photography, to cross-validate results. While laboratory-based isotope methods can provide precise chronologies for older geological formations, lichenometry remains an invaluable field tool due to its simplicity, cost-effectiveness, and non-destructive nature. Moreover, because it requires little specialised equipment beyond callipers and a camera, it is particularly suited to remote expeditions. It allows researchers to quickly survey extensive landscapes without needing to extract and transport bulky stone samples for destructive laboratory processing, ensuring that fragile historical and natural surfaces remain undamaged.