This is an Earthcache with no physical container and log. This cache is located within ChugachState Park. To claim this cache, you must visit the site, take measurements using a ruler with metric markings, and demonstrate learning by emailing the correct answers to the following four questions to the cache owner. The midway or lower parking areas give the shortest access to QUEST 1 and 2. Parking at the upper parking gives the shortest access to QUEST 3 and 4. An Alaska State Park day use or seasonal pass will be needed for parking at the closest access points.
1. Question 1 is answered at the starting point. After reading the cache page and the informational sign before you, answer the following question. What is the predominant lichen color on the rock wall situated across the pond? From color alone, would the genus be Rhizocarpon or Xanthoria?
2. Question 2 is answered at waypoint QUEST2. What is the difference about the 16th bore line as you count bore lines from left to right? Can you postulate why this difference exists?
3. Question 3 is answered at waypoint QEST3. Describe the color, genus, and width(s) of the lichen on the VERTICAL face of the low rock outcrop at your feet. Note whether lichen appear to be more individual or more grouped. Which method(s) of lichenometry do you think would work best for measuring this rock face? Given the growth rate for this genus of lichen and assuming that the growth rate has been constant and began immediately upon exposure, how many years ago would you estimate this rock has been exposed?
4. Question 4 is answered at waypoint QEST4. Describe the color, genus, and width(s) of the lichen on the HORIZONTAL face of the rock outcrop in front of where you are standing. Note whether the lichen appear to be more individual or more merged. Which method(s) of lichenometry do you think would work best for dating the exposure of this rock surface?
Dating the age of exposed rock using Lichenometry
Lichenometry uses lichen growth to determine the age of exposed rock, because it is thought that lichens increase in diameter at specific rates as they grow. Lichen can be preserved on old rock faces for up to 10,000 years. However, lichenometry is most accurate when applied to surfaces that have been exposed for less than 1,000 years.
The most common lichen used for lichenometry are the genus Rhizocarpon and the genus Xanthoria. Growth rates for different areas and species can be obtained by measuring maximum lichen sizes on surfaces of known age, such as gravestones or historic rock buildings.
Among the potential problems of the lichenometric technique are:
1. the difficulty of correctly identifying the species,
2. delay between exposure and colonization,
3. varying growth rates from region to region as well as the fact that growth rates are not always constant over time,
4. dependence of the rate of growth upon substrate texture and composition, climate, and
5. determining which lichen is the largest.
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 should be measured, other scientists prefer the diameter of the largest inscribed circle. A problem in dating lichens is the fact that several lichen bodies (thalli) can fuse together, making several minor lichens appear as if they are a larger one of older age. The principal lichonometric methods are:
- 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.
- 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 in 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.
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Rhizocarpon geographicum
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The map lichen (Rhizocarpon geographicum) is a species of lichen, which grows on rocks in mountainous areas of low air pollution. Each lichen is a flat patch bordered by a black line of spores. These patches grow adjacent to each other, leading to the appearance of a map or a patchwork field. This species is found in most cold areas with exposed rock surfaces. The sign at McHugh Creek Picnic Area and Trailhead in Chugach State Park states that Rhizocarpon is a fluorescent green/yellow grows at the rate of 65mm in 100 years in this area.
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Xanthoria elegans
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Xanthoria elegans, commonly known as the elegant sunburst lichen, is recognized by its bright orange or red pigmentation, this species grows on rocks and man-made objects, often near bird or rodent perches. It has a circumpolar and alpine distribution. It can thrive in areas having less than 2.4 inches annual precipitation and can survive submerged in streams for much of the growing season. X. elegans has survived a 18 month exposure to solar UV radiation, cosmic rays, vacuum and varying temperatures in an experiment performed outside of the International Space Station.
It was one of the first lichens to be used for lichenometry. It is small, typically less than 5 cm (2 in) wide. It may look swollen, orange-yellow when found submerged in streams. After an initial period of one or two decades to establish growth, X. elegans grows at a rate of 0.5 mm per year for the first century.
References
Information for this cache page was found at Wikipedia, The Free Encyclopedia.