Summary

Relative dating determines which of two fossils is older or younger than the other, but not their age in years. Radiometric dating measures the proportion of decay products of radioisotopes with known half-lives to estimate absolute age. Different isotopes with a range of half-lives cover the span of geologic time.

Radiometric Dating It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and it can be used to date a wide range of natural and man-made materials.

Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event. This method uses the orientation of the Earth’s magnetic field, which has changed through time, to determine ages for fossils and rocks.

Both absolute and relative dating have advantages and are still frequently used by geologists. Dating rocks using relative dating allows a geologist to reconstruct a series of events cheaply, often very quickly, and can be used out in the field on a rocky outcrop.

• Sediments are deposited horizontally. This is original horizontality.
• The oldest sedimentary rocks are at the bottom of the sequence. This is the law of superposition.
• Rock layers are laterally continuous.
• Rock B cuts across rock A.
• A gap in a rock sequence is an unconformity.

The Rules of Relative Dating

• The Law of Supervision.
• The principle of original Horizontality.
• The principle of cross-cutting Relationships.
• Inclusions.

What is the difference between relative and absolute age? Relative age is the age of a rock layer (or the fossils it contains) compared to other layers. Absolute age is the numeric age of a layer of rocks or fossils. Absolute age can be determined by using radiometric dating.

Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.

What is the difference between numerical dates and relative dates? Numerical dates specify an absolute age in number of years, whereas relative dates determine the order of events in relation to one another.