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These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, uranium-thorium, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism).
Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
Measurement of the ratio of these isotopes thus gives the age of the minerala technique for determining the age of organic materials, such as wood, based on their content of the radioisotope 14C acquired from the atmosphere when they formed part of a living plant.
The 14C decays to the nitrogen isotope 14N with a half-life of 5730 years.
The profile from one location is then compared with profiles from surrounding sites.
Stratigraphic dating assumes that the lower layers in any particular profile are older than the upper layers in that profile ("the law of superposition") and that an object cannot be older than the materials of which it is composed.
There are two basic types of dating methods, relative and absolute.
Through the investigation of many different stratigraphic contexts, a master sequence of fossil and floral assemblages may be devised for a region.As new locations are investigated, the geologist attempts to fit the new profiles into the master sequence of geological strata for that region.The depth of the strata within the master sequence provides the investigator with the relative date of any particular profile.When archaeologists have access to the historical records of civilizations that had calendars and counted and recorded the passage of years, the actual age of the archaeological material may be ascertained—provided there is some basis for correlating our modern calendar with the ancient calendar.With the decipherment of the Egyptian hieroglyphics, Egyptologists had access to such an absolute timescale, and the age, in calender years, of the Egyptian dynasties could be established.