How to chronologically date stones?

Submitted by hoofdletter t3_yepxg7 in askscience

Yesterday I listened to a historian who talked about the Goths. At the beginning he talked about that they don't know much about the beginning of the Goths but that they expect that they lived in what now is Poland. Why they expect that was due to signs of a similar culture found there. He showed 1 example of such signs, it was a ring of stones (like Stonehenge but way smaller and not stacked) in a forest. The stones were around 0,5m tall and probably artificially rounded on the top. Afterwards I asked how they know how old those stones are, but he couldn't answer my question.

So that's my question to you. How do they know when those stones were placed there? Because you can't just use the age of the stones, they are way older. Can they find that date from the chipping done in the stones? Or maybe the change in the soil? Or is there something else that is more easy to date found nearby?

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bcustead t1_itzl8j6 wrote

You're right that the stones don't give us a lot of direct information, but we can learn some things from it. For example, the sorts of tools used on it tells us about the technological level of the builders and the style of construction and source of the stones can give us clues about who built it. These can help narrow down the site's age.

Our most accurate method to determine the age of such sites is radiocarbon dating, but radiocarbon dating requires organic material. So we date the other archaeological artifacts that are found at such sites such as remains from its construction or evidence of the site's use. From these we can make inferences about the age of the stone construction.

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hoofdletter OP t1_itzpiex wrote

That's a very nice explanation! Thank you!

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ParatusLetum t1_iu07rhh wrote

Piggybacking on that I seem to recall that charcoal remains of fires can be useful. We figure the tree that was used might be in an average age range for that tree narrows the window down considerably. Often times these sites were used for gatherings or ceremonies so they would use fire to cook and for warmth etc.

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hoofdletter OP t1_iu09mzd wrote

That's also a very interesting way to investigate that.

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AfricaByTotoAoe t1_iu2zfpy wrote

The age of a stone is tricky to determine, and often paleoscientists need to use multiple techniques to determine a rocks age. Dating will often be a multi step process involving determining the age of both the rock itself, and everything around it from soil sediments to artifacts found at the site.

In geocience, radiometric dating techniques often form the baisis for age measurements of rock, and work by measureing the relative abundance of radioactive isotopes of elements compared to the stable isotopes they decay to. If the isotope's half life is known, and the isotope it decays to is rare, scientists can determine how long it has been since a rocks formation from how much of the radioactive isotope has decayed to the more stable form. Techniques like Fission track dating of the damage trails left by the decay of unstable Thorium isotopes, U-Pb isotope dating of zircon crystals or glass or Potassium/Argon isotope dating are all examples of this technique and can determine the age of formation of a rock. (Lowe, D., 2011. Tephrochronology and it's application a review. In Quaternary Geochronology. V6 3-4 pp 423, Ch 9, has a good summary of these techniques)

The ages determined by these methods won't determine the age of a stone structure however, only the ages of the stones in the structure, which will only give an estimate of maximum age. This can be millions of years off depending on the type of rock. Electron spin resonance is a radiometric technique that can give an idea of when a surface was exposed if the stone is worked (Grun, R., 1997. Electron Spin Resonance Dating. In Chronometric Dating In Archeology vol 2), but these analytical techniques tend to be used in tandem with relative, and age equivalent dating techniques in archeological studies. These work by determining the ages of things around the structure (such as soil built up around the base of a stone, artifacts found on site, etc) to determine ages the structure can't be younger than.

Determining the age of organic matter in the soil built up around a stone since it was emplaced, or of human remains or artifacts found at the site can give a good estimate for the age of a stone. This can be done reliably using Radiometric dating of C14. Another good example of age equivalent dating is tephrochronology, dating using volcanic deposits. As volcanic material is emplaced in a wide radius over the course of a few weeks at most, if you radiometrically date the age of volcanic ash overlaying a stone structure you can use that to get a date the rocks cant have been emplaced after. The same can be done for ash layers under the stone get estimates of maximum age (Lowe 2011). If buildup of sediments around a stone is consistant or seasonal, you can estimate how long a stone has been in one place by how much sediment has built up around it, though this can be unreliable over very long timescales. A combination of deposition modeling, and of relative dating of prior and recent structures and deposits can then be used to establish a range of dates between which the stone must have been emplaced, and to estimate a precise date within that range for when the rock was emplaced.

Hope this answers your question!

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hoofdletter OP t1_iu3l587 wrote

That more than answered my question! Thank you for the extensive reply! That was a nice insight in the combination of techniques used!

Thank you so much kind stranger!

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MegavirusOfDoom t1_iuerft5 wrote

geochronology Dating methods

1.1 Radiometric dating

1.2 Fission-track dating

1.3 Cosmogenic nuclide geochronology

1.4 Luminescence dating

1.5 Incremental dating

1.6 Paleomagnetic dating

1.7 Magnetostratigraphy

1.8 Chemostratigraphy

1.9 Correlation of marker horizons

https://en.wikipedia.org/wiki/Geochronology Scientists have developed a bunch of methods, for example quartz when hidden from sunlight develops some chemical changes over time, so we can tell when a quartz was last exposed to sunshine. They also can use pollen in the soil and find what era of history that tree belongs to. They can also use carbon like charcoal and do carbon dating. Generally they need to define strata where all the rocks were laid together and use multiple chemical dating methods.

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hoofdletter OP t1_iuexiln wrote

Wow, that's a lot of methods! Thanks for the list and the Wikipedia link!

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