Lichenometry: Nature's Tiny Timekeepers
In the harsh, wind-swept landscapes of glaciers and mountains, tiny, unassuming organisms hold the key to unlocking the Earth's recent history.
When a glacier retreats or an earthquake shakes rocks loose, it exposes a fresh canvas of stone. Over time, this blank canvas begins to be painted with splotches of color—lichen. These hardy organisms grow at a slow and surprisingly constant rate. By simply measuring their size, scientists can determine how long a rock has been exposed, dating everything from ancient glacial moraines to past earthquakes1 . This technique, known as lichenometry, turns these humble life forms into precise natural clocks.
What Are Lichens?
Before delving into the dating technique, it's important to understand the clock itself. Lichens are not single organisms but a remarkable partnership between fungi and algae living in a symbiotic relationship4 . The fungus provides a protective structure, while the algae produces food through photosynthesis4 . This partnership allows them to thrive in some of the planet's most challenging environments, from arctic tundras to high mountain peaks.
Did You Know?
For lichenometry, the most important type is crustose lichens, which grow tightly attached to the rock surface like a crust4 . Among these, species of the Rhizocarpon genus, particularly Rhizocarpon geographicum, are the most valued timekeepers1 8 . They are slow-growing, long-lived, and expand outward in a roughly circular pattern, making them ideal for measurement.
The Science of Measuring Time
The core principle of lichenometry is simple: the larger the lichen, the older the rock surface it grows on9 . The technique was first developed by botanist Knut Fægri in 1933 and was refined into a formal dating method by Roland Beschel in the 1950s1 . Its most common application is in dating geological surfaces from the last few hundred to about 10,000 years, a period where other dating methods like radiocarbon dating can be less precise1 7 .
How to Date a Rock with Lichens
The process is methodical and begins with a crucial first step: calibration. Since lichen growth rates can vary by region and climate, scientists must first develop a "growth curve" by measuring lichens on surfaces of known age, such as dated historical monuments, gravestones, or rocks from documented glacial retreats5 6 .
Lichenometric Dating Methods
| Method Name | Brief Description | Key Advantage |
|---|---|---|
| Largest Lichen (LL)1 8 | Measures the single largest lichen thallus on a surface. | Simplicity; provides a quick field assessment. |
| Five Largest Lichens (5LL)1 8 | Uses the average size of the five largest lichens. | Reduces reliance on a single, potentially anomalous lichen. |
| Fixed-Area Largest Lichen (FALL)1 | Measures the largest thallus within standardized sample areas (e.g., 1 m² boulders). | Useful for non-uniform surfaces like rockfalls. |
| Size-Frequency (SF)1 8 | Measures all thalli of a species on a surface to analyze population distribution. | Provides a robust statistical population analysis. |
The Scientist's Toolkit
To the untrained eye, a lichenometrist might seem to need only a ruler. In reality, their toolkit, both physical and methodological, is specialized.
Calipers
For obtaining precise measurements of lichen thalli, often to the nearest 0.1 mm.
Lichen Identification Guide
Critical for correctly identifying the target species (e.g., Rhizocarpon geographicum agg.).
Sampling Grid or Frame
Used in the Fixed-Area (FALL) method to define the specific area being studied.
Regional Growth Curve
The essential calibrated chart that converts lichen size into an estimated age.
Knowledge of Ecesis Time
The estimated delay between rock exposure and lichen colonization; a key correction factor8 .
A Closer Look: Dating an Earthquake
To see lichenometry in action, consider a science fair project that aimed to date the 1857 Fort Tejon earthquake in California. This experiment beautifully illustrates the method's application and its real-world challenges.
The researchers hypothesized that they could find lichens of a specific size on boulders that were shaken loose during the 1857 earthquake. Their calculations, based on known growth rates for Rhizocarpon lichens, suggested that a 147-year-old lichen would be approximately 22.2 mm in diameter.
They visited three sites known to have experienced seismic shaking during that historic earthquake. At each site, they used the Fixed-Area Largest Lichen (FALL) method, measuring the longest axis of the largest crustose lichen (preferably Rhizocarpon) on each rock.
The team measured 85 lichens at one promising site. The results provided a clear, though not perfect, picture:
| Total Lichens Measured | Lichens 21-23 mm in Size | Percentage Matching 1857 Quake |
|---|---|---|
| 85 | 14 | ~17% |
The data showed that approximately 17% of the lichens measured were within the expected size range for the 1857 earthquake. This cluster of similarly sized lichens strongly suggests a single, widespread rockfall event consistent with the known seismic activity.
"The remaining 83% of lichens were likely from smaller, more recent rockfalls or represented variability in growth rates and colonization times."
Limitations and the Future of Lichenometry
Despite its utility, lichenometry is not a perfect clock. Scientists must be cautious of its limitations:
Growth Rates Are Not Universal
Lichens grow at different rates in different environments. A growth curve from Iceland cannot be applied directly to Patagonia2 8 .
The Ecesis Problem
There is an unknown delay between a rock's exposure and when a lichen first colonizes it. This "ecesis interval" can last from a few years to decades, creating uncertainty in the earliest years of a surface's exposure8 .
Species Identification
The Rhizocarpon group contains many species with slightly different growth rates, but they are notoriously difficult to tell apart in the field, leading to potential error8 .
Environmental Factors
Pollution, substrate rock type, and microclimates can all influence growth, complicating the simple relationship between size and age4 .
Because of these challenges, many geomorphologists now stress that lichenometry is most powerful as a relative dating tool—for determining which of two moraines is older, for instance—rather than for providing precise absolute ages, especially for surfaces exposed for more than a few centuries2 .
Conclusion: A Tiny Titan in Geoscience
From tracking the retreat of glaciers in a warming world to uncovering the seismic history of a mountain range, lichenometry provides a unique and accessible window into our planet's recent past. It is a powerful reminder that nature often records its history in unexpected places. The next time you see a colorful, crusty patch on a rock, take a moment to appreciate it—you may be looking at a tiny, living chronicle of Earth's story.
For further reading on this topic, the comprehensive reviews on AntarcticGlaciers.org and in scientific publications like Geografiska Annaler provide excellent in-depth resources1 8 .