How old is the Earth?
Scientists estimate that the Earth is about 4.6 billion years old. That is a hard number to imagine. If all of Earth's history were squeezed into one single day, humans would appear only in the last few seconds before midnight. To work out this great age, scientists study clues left in the rocks. They cannot travel back in time, so instead they act like detectives, reading evidence that has been kept in the ground for millions of years.
Reading the rock record
Rocks often form in flat layers called strata. In undisturbed ground, the oldest layer is at the bottom and the youngest is at the top, because new layers settle on top of older ones. By studying the order of layers, scientists can tell which events happened first, almost like reading the pages of a very old book from bottom to top.
Example
In a cliff, a fossil found in a lower layer of rock is older than a fossil found in a layer above it.
Fossils are the preserved remains or traces of living things from long ago, found inside sedimentary rock. They act like a calendar, showing which plants and animals lived during each time. Scientists also use radioactive dating, which measures how much of certain elements inside a rock have slowly broken down, to find its age more exactly.
Remember
- The Earth is about 4.6 billion years old.
- In undisturbed rock, older layers lie below younger ones.
- Fossils and radioactive dating help estimate age.