Absolute Age tells the actual age of a rock or fossil. Example: Many of you are 14, or the Earth is 4.7 billion years old – these are absolute ages.
Absolute age is determined by using radioactive dating.
Radioactive Dating measures the age of rock or fossil by comparing the amount of a radioactive form of an element in a rock or fossil with the amount of its decay.
Over time, things break down by releasing energy that is called radioactive decay (this is what gets measured).
While absolute age gives a measurement of how long ago a rock or fossil formed, it is not always possible to date them…so relative age is used.
Relative Age is the age of a rock or fossil compared to another rock or fossil.
It determines which events occur earlier or later than others without giving a definite date. Example: Saying you are older than your brother but younger than your sister, you are describing your relative age.
Relative age can be determined with the Law of Superposition, which says that older layers of rock are below younger layers because they formed first.
So you could determine the relative age of the rock layers in the Grand Canyon by knowing that the oldest ones are at the bottom and the youngest ones are at the top.