Obsidian hydration relative dating
Since the rate of radioactive decay of any particular isotope is known, the age of a specimen can be computed from the relative proportions of the remaining radioactive material and its decay products.
By this method the age of the earth is estimated to be about 4.5 billion years old.
The varved-clay method is applied with fair accuracy on deposits up to 12,000 years old.
Streams flowing into still bodies commonly deposit layers (varves) of summer silt and winter clay through the year.
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).
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.
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.In dendrochronology, the age of wood can be determined through the counting of the number of annual rings in its cross section.Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life.