Geologists tell time based on relative and absolute dating techniques. Relative age dating is the process of determining the sequence of events. For example, if you look at someone who is wearing shoes and socks, you know that they must have put on their socks BEFORE they put on their shoes. Absolute age dating is the process of determining when something happens in exact time units such as hours, days, or years ago. In the example of the person with shoes and socks, you might find out that that person put on their shoes and socks on at 7:30 in the morning. Absolute and relative age dating are equally important and a full understanding of geologic time requires a full understanding of both techniques.

There are a variety of methods that geologists use to determine relative and absolute ages. Most people have heard of radio carbon dating. This is one way to determine an absolute age for relatively young (less than 80,000 years) objects that were once alive, such as wood or bones. There are many other elements that can be radiometrically dated accurately back to billions of years old. There are also many other techniques for determining absolute age that don't require radiometric techniques but most of these techniques are beyond the scope of this class and we will concentrate on relative age dating techniques.