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.
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.
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.
The stratigraphy may also reflect seasonal variation in the velocity of stream flow.Each decay element has an effective age range, including uranium-238 (100 million to 4.5 billion years) and potassium-40 (100,000 to 4.5 billion years).Other methods that depend on the effects of radioactive decay include fission track dating and thermoluminescence.The accelerator mass spectrometer technique reduces the amount of statistical error involved in the process of counting carbon-14 ions and therefore produces dates that have smaller standard errors than the conventional method.Paleomagnetic dating is based on changes in the orientation and intensity of the earth's magnetic field that have occurred over time.Absolute dating can be achieved through the use of historical records and through the analysis of biological and geological patterns resulting from annual climatic variations, such as tree rings (dendrochronology) and varve analysis.