Final answer:
The absolute age of a rock is expressed in years and is determined through radiometric dating, which measures the ratio of parent to daughter isotopes and calculates age based on known isotope half-lives.
Step-by-step explanation:
The absolute age of a rock body is expressed as the number of years since the rock formed. It is usually determined using radiometric dating techniques. For this, geologists measure the ratio of parent isotopes to daughter products within the rock and calculate the age based on known half-lives of the isotopes. For example, the uranium-lead dating method involves measuring the ratio of uranium (the parent isotope) to lead (the daughter product). By knowing the half-life of uranium, which is 4.47 billion years, geologists can determine the age of a zircon crystal, and thereby estimate the age of the rock in which it is found. This process gives a numerical age, which provides a precise figure compared to relative dating methods.