Geologic Dating
03/11/2008
Geologic relationships can be complex, and dating rocks is especially challenging. Students in Professor Rayburn's Paleoclimatology class have been studying special techniques to successfully date rocks and other geologic materials. The focus was mainly on absolute dating, where the age in years is required. In contrast, relative dating only considers the order of events, and not the age.
Each group of students chose a specific dating technique to research, and presented their findings at a special presentation session last week. The presentations covered a wide variety of dating techniques including radiocarbon, potassium-argon, lichenometry, obsidian hydration, cosmogenic nuclides, luminescence, fission track, and amino acid racemization.
Cosmogenic nuclide dating (presented by Roy, Andy, and Jamie) requires looking at the type and amount of isotopes created on surfaces exposed to cosmic radiation. Elevation, latitude, and topographic shielding are all variables that must be considered to ensure a good date. Obsidian hydration (Kourtney, Michelle and Kelsey) looks at the amount of alteration on the surface of volcanic glass. Lichenometry (Lacy, Shannon, and Chris) is lichen dating, which is used to date surfaces back to about 500 years by lichen growth. When dating lichen, thallus size is the most important parameter. The largest thallus may be used, or multiple samples may be measured for a statistical distribution.
Students Amanda, Jennifer, and Amanda present their findings on radiocarbon dating. The change in carbon isotope ratios is used to date organic remains after they stop exchanging carbon with the atmosphere.
Jackie and Kevin talk about potassium-argon dating, one of the most important methods for dating igneous and metamorphic rocks. As argon is not normally present in minerals, it's presence indicates the decay of potassium 40, and the amount is used for dating.
The findings on Tephrochronology and fission track dating are presented by Michael, Jennifer, and Megan. Tepha layers are widespread volcanic deposits that serve as distinct, and essentially instantaneous, time lines. Fission track dating uses the count of damage zones within crystals such as zircon that are caused by the decay of uranium isotopes.
Jenai, Kimberly, and Alanna give their results on amino acid racemization dating, another method for dating organic remains. This complex method use the change in chirality, or handedness, of amino acids following the death of a sample. Many environmental variables make this a tricky method to apply accurately.
