Relative dating allows us to reconstruct the sequence of geologic events in a given area, or with the help of fossils, the entire Earth. But establishing this sequence doesn't tell us much about the age (in years) of these events . For example, we know that the lava flow in the photo below is younger than the rocks below it. But how old is the lava flow? 100 years? 100,000 years? Is the nearby volcano still active? To answer these questions, geologists need to be able to assign an absolute age to geological events- in years or millions of years before present.