Accuracy of dating
Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older. The discovery of means for absolute dating in the early 1900s was a huge advance. The methods are all based on radioactive decay:
Fossils may be dated by calculating the rate of decay of certain elements.
- Certain naturally occurring elements are radioactive, and they decay, or break down, at predictable rates.
- Chemists measure the half-life of such elements, i.e., the time it takes for half of the radioactive parent element to break down to the stable daughter element. Sometimes, one isotope, or naturally occurring form, of an element decays into another, more stable form of the same element.
- By comparing the proportions of parent to daughter element in a rock sample, and knowing the half-life, the age can be calculated.
Scientists can use different chemicals for absolute dating:
- The best-known absolute dating technique is carbon-14 dating, which archaeologists prefer to use. However, the half-life of carbon-14 is only 5730 years, so the method cannot be used for materials older than about 70,000 years.
- Radiometric dating involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years. Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age.
The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old. Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates. A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age. Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other.
There is only a 1% chance of error with current dating technology.
Every few years, new geologic time scales are published, providing the latest dates for major time lines. Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the 1960s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old. Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. It is accurate to within a few thousand years. With modern, extremely precise, methods, error bars are often only 1% or so.
Conclusion: The strict rules of the scientific method ensure the accuracy of fossil dating.
Conclusion
The fossil record is fundamental to an understanding of evolution. Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Fossils can also show us how major crises, such as mass extinctions, happened, and how life recovered after them. If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe. Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. Repeated, and tough, regimes of testing have confirmed the broad accuracy of the fossils and their dating, so we can read the history of life from the rocks with confidence.