The boundary at the base of the new strata is the angular unconformity. The orientation of layers above and below the unconformity is different. Chronologic Age and Radiometric Dates Chronologic age specifies the age of a rock in years. Chronologic ages are based on the method of radiometric dating. Many minerals contains small quantities of radioactive elements.
Radioactive elements are so-called because they decay.
In a given population of atoms, half of the parents decay to daughters in a set time called the half life. The half-life is constant for an element. Simplistically, the ratio of parent to daughter changes through time. By measuring the ratio, we can determine the age. The date gives the time the igneous rock cooled from a melt.