Reading Earth's History in the Rocks
Geologists act as detectives of Earth's past. To piece together the planet's history, they need to determine the age of rocks and fossils. They use two main approaches: relative dating and absolute dating.
Relative Dating: Establishing the Sequence
Relative dating determines the age of a rock or geologic feature relative to other rocks or features. It doesn't provide an age in years, but it establishes a sequence of events from oldest to youngest. This is done using several principles of stratigraphy (the study of rock layers).
Law of Superposition: In an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top.
Principle of Original Horizontality: Sedimentary layers are originally deposited in flat, horizontal sheets. Tilted or folded layers indicate that a deformation event occurred after the layers were deposited.
Principle of Cross-Cutting Relationships: A geologic feature (like a fault or an igneous intrusion) that cuts across another feature is younger than the feature it cuts.
Index Fossils: The fossils of organisms that were widespread but lived for only a very short, specific period of geologic time. Finding an index fossil in a rock layer allows a geologist to precisely correlate its age with other rock layers around the world.
Absolute Dating: Determining the Numerical Age
Absolute dating provides a numerical age for a rock or fossil in years. The most powerful and common method is radiometric dating.
Radiometric Dating and Half-Life
The Principle: Some elements in rocks are radioactive, meaning their atoms are unstable. These unstable 'parent' isotopes spontaneously decay into stable 'daughter' isotopes at a constant, predictable rate.
Half-Life: The half-life of a radioactive isotope is the time it takes for half of the parent isotopes in a sample to decay into daughter isotopes. This rate is unaffected by temperature, pressure, or chemical conditions.
The Geologic Clock: By measuring the ratio of parent isotopes to daughter isotopes in a mineral and knowing the half-life, scientists can calculate the time that has passed since the mineral crystallized (i.e., the age of the rock).
Common Isotope Pairs:
Carbon-14 (decays to Nitrogen-14): Has a short half-life of ~5,730 years. Used to date recent organic materials (wood, bones) up to about 50,000 years old.
Uranium-238 (decays to Lead-206): Has a very long half-life of ~4.5 billion years. Used to date very old rocks.