Telling Time with Rocks
How do geologists know that one rock layer is older than another? How do they know that the dinosaurs went extinct 66 million years ago? They use two main methods for determining the age of rocks and fossils: relative dating and absolute dating.
Relative Dating: Which is Older?
Relative dating does not give a specific age in years. It just tells you the sequence of events—which rock layer or fossil is older or younger compared to another. Geologists use a few common-sense principles for relative dating.
The Law of Superposition: In an undisturbed sequence of sedimentary rocks, the layers at the bottom are the oldest, and the layers at the top are the youngest.
This makes sense because sedimentary rocks are formed by the deposition of sediment in layers, one on top of the other.
The Principle of Original Horizontality: Sedimentary rock layers are originally deposited in flat, horizontal layers. If you see rock layers that are tilted or folded, you know that they have been deformed by tectonic forces after they were deposited.
The Principle of Cross-Cutting Relationships: If a geologic feature (like a fault or an igneous intrusion) cuts across other rock layers, then the feature that does the cutting is younger than the layers it cuts through.
You can't cut a cake until after the cake has been baked. Similarly, a fault can't break rock layers that aren't there yet.
Index Fossils: Some fossils, called index fossils, are very useful for correlating the ages of rock layers in different places. A good index fossil is from an organism that was geographically widespread but only existed for a geologically short period of time. If you find the same index fossil in a rock layer in North America and a rock layer in Europe, you know those two layers were formed at about the same time.
Absolute Dating: How Old in Years?
Absolute dating methods provide a specific age for a rock or fossil in years (e.g., 150 million years old). The most common method is radiometric dating.
How it works: Many rocks contain tiny amounts of unstable, radioactive elements. These radioactive 'parent' atoms decay at a perfectly predictable rate into stable 'daughter' atoms.
Half-Life: The time it takes for half of the parent atoms in a sample to decay into daughter atoms is called the half-life. Each radioactive element has its own unique and constant half-life.
The Clock in the Rocks: By carefully measuring the ratio of parent atoms to daughter atoms in a rock sample and knowing the half-life, scientists can calculate how long it has been since the rock formed.
Example: Carbon-14 dating is used for dating recent organic materials (up to about 50,000 years old). Uranium-lead dating is used for dating very old rocks, as uranium has a half-life of billions of years.