These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, 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).
Some of the radioactive elements used in dating and their decay products (their stable daughter isotopes) are uranium-238 to lead-206, uranium-235 to lead-207, thorium-232 to lead-208, samarium-147 to neodymium-143, rubidium-87 to strontium-87, and potassium-40 to argon-40.
Each radioactive member of these series has a known, constant decay rate, measured by its half-life, that is unaffected by any physical or chemical changes.
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.
Because rainfall patterns vary annually, any given set of tree ring patterns in a region will form a relatively distinct pattern, identifiable with a particular set of years.
By comparing the pattern of tree rings in trees whose lifespans partially overlap, these patterns can be extended back in time.By counting each pair of varves the age of the deposit can be determined.The absolute dating methods most widely used and accepted are based on the natural radioactivity of certain minerals found in rocks.Fission track dating is based on the fact that when uranium-238 atoms fission within a solid medium such as a mineral or a glass, they expel charged particles that leave a trail of damage (known as fission tracks) preserved in the medium.The number of tracks per unit area is a function of time and the uranium concentration.Those laid down during the fall and winter have a dark color because of the presence of dead vegetation; those deposited during the rest of the year have a light color.