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5.4.2. Absolute dating

The absolute dating

The absolute dating  is a set of techniques to determine the specific age in millions of years, of a material or geological event.

There are numerous methods to carry out an absolute dating, which differ in the technique used and in the range of antiquity that we want to determine. Some techniques allow to calculate geologically very recent times and are not usually used in geology, although they are useful for archeology.

Some of the main absolute dating methods are:

Dendrochronology

The Dendrochronology  is the dating with the study of tree rings. Each year, the trees generate a pair of rings, one light and one dark, the thickness of which depends on the environmental conditions in which the tree has grown. If they have been harsh conditions, such as drought or poor photosynthetic yield, the rings are very fine. If conditions have been favorable, the rings are thicker.

By comparing the patterns of different trees, you can match the rings and determine the climatic conditions in which they lived.

With these studies we can compare the wood of old structures and accurately determine the age of the wood used in that construction.

Although we put it as an example of absolute dating, this technique would be difficult to apply in geology because of the little antiquity that can be determined.

Dendrocronología. Estudio de los anillos de los árboles.
Árbol talado en el patio del IES Ramón Pignatelli de Zaragoza.

Thermoluminescence

Thermoluminescence dating is an absolute dating method used in archeology that allows fired clay objects such as pottery and hearths to be dated. It can also be used to date wind, river, marine, coastal sediments, volcanic rocks, and precipitated calcium carbonate in caves.

Glacial varves

The varves glaciers are pairs of layers of small thickness that are deposited on the bottoms of lakes deicing a front glacierAnnually, two types of strata alternate:

  • A clear, silty or sandy stratum. It is deposited in spring and autumn, with the sediments from the glacier.
  • A dark layer of clay and organic matter from the lake. It is deposited in winter, when the lake freezes over.

Thus, by studying the variation of these glacial varves, more or less thick, a sequence is established that can be used to correlate them with those of another part of the region and obtain data on the climatology and advances and retreats of the glaciers in the area.

Radiometric method

The radiometric method is based on the radioactivity produced by the disintegration of nuclei of atoms of unstable chemical elements that are transformed into other stable chemical elements. They transform into isotopes or different elements at a rate that can be calculated, while releasing energy.

For example, carbon 14 turns into nitrogen 14, and uranium 238 turns into lead 206.

The time it takes for an item to move from one form to another is always the same. Therefore, if we can know the initial amount of that item and the final amount, we can calculate the time that has passed.

Each radioactive element has a half-life or half-life (T), which is the time that elapses since the initial mass of a radioactive element is reduced by half. For example, carbon 14 has a half-life T = 5730 years, so a mass of 100 grams of C14, after 5730 years, will have been reduced to 50 grams. After another 5730 years, it will have been reduced to 25 grams, etc.

Elapsed half-lives % of original nuclei not disintegrated
0  100% (no core decayed)
one 50% (half of the nuclei have disintegrated)
two 25% (half of half of the nuclei remain undisintegrated)
3 12.5%
4 6.25%
... ...

The half-lives of the most widely used radioactive isotopes are:

  • Uranium 238 - lead 206, with T = 4.510 million years.
  • Carbon 14 - nitrogen 14, with T = 5,730 years. It is used to date recent organic remains, up to 60,000 years old.
  • Potassium 40 - argon 40, with T = 1.3 billion years. It is the most used, especially because it works with igneous rocks, rocks that are very abundant on Earth and act as traps, enclosing other types of rocks.
  • Samarium 147 - neodymium 143, with T = 106,000 million years.
  • Rubidium 87 - Strontium 87, with T = 47,000 million years.