Scientists use two fundamental methods to determine the age of Earth’s ancient rocks and the life forms preserved within them. These approaches provide accurate timelines for Earth’s history, allowing for the sequencing of events and, in some cases, the pinpointing of precise ages.
Relative Dating: Understanding Sequence
Relative dating determines the chronological order of geological events and the fossils they contain without assigning a specific numerical age. This approach relies on several foundational principles developed by early geologists.
The Law of Superposition states that in an undisturbed sequence of sedimentary rock layers, the oldest layers are at the bottom, and the youngest are at the top. Sediments accumulate over time, with newer deposits settling on top of older ones.
Original Horizontality posits that most sedimentary rocks are initially deposited in flat, horizontal layers. If layers are found tilted or folded, it indicates that geological forces acted upon them after their formation.
The Principle of Cross-Cutting Relationships states that any geological feature, such as a fault or an igneous intrusion, that cuts across another feature must be younger than the feature it cuts. For example, a fault cutting through rock layers formed after those layers were in place.
The Principle of Faunal Succession uses the predictable order in which different fossil species appear and disappear through rock layers. Certain fossil species are characteristic of specific time periods, serving as “index fossils” that allow for the correlation of rock strata across different geographical locations. If the same index fossil is found in rock layers in two separate regions, those layers are considered to be of a similar relative age.
Absolute Dating: Pinpointing Specific Ages
Absolute dating assigns a specific numerical age to rocks and fossils. This method provides a precise timeline, complementing the sequential understanding gained from relative dating. The most common and reliable form is radiometric dating.
Radiometric dating relies on the predictable decay of unstable radioactive isotopes into stable daughter isotopes over time. These elements are incorporated into minerals when rocks form or into organic matter when organisms are alive. The decay rate is constant and unaffected by external environmental conditions.
The decay rate is measured by an isotope’s “half-life,” the time it takes for half of the parent isotope in a sample to decay into its daughter isotope. By measuring the ratio of remaining parent isotopes to accumulated daughter isotopes, scientists can calculate a sample’s age.
Several radiometric dating methods are used depending on the material’s age and type. Carbon-14 dating is used for relatively young organic materials, such as wood, bone, and charcoal, up to 60,000 years old. Living organisms absorb carbon-14 from the atmosphere, but upon death, absorption ceases, and carbon-14 begins to decay into nitrogen-14 with a half-life of approximately 5,730 years.
For much older rocks, Potassium-Argon (K-Ar) dating is used. This method is suitable for dating igneous and metamorphic rocks older than 100,000 years, due to the long half-life of potassium-40, which decays into argon-40. As volcanic rocks cool, trapped argon gas escapes; any argon-40 accumulating afterward results from potassium-40 decay, allowing geologists to determine the rock’s formation age.
Uranium-Lead (U-Pb) dating is a precise method for dating very ancient rocks, back billions of years. It relies on the decay of uranium isotopes (uranium-238 and uranium-235) into different lead isotopes. This technique is effective for minerals like zircon, which incorporate uranium but exclude lead during formation, so any lead found is from radioactive decay. While fossils rarely contain radioactive minerals for direct dating (except carbon-14), their age is often determined by dating igneous rock layers directly above or below the fossil-bearing sedimentary layers.