how is relative dating used to evaluate geologic time.
http://en.wikipedia.org/wiki/Relative_dating
Relative dating is a method used to determine the age of rocks and fossils in comparison to other objects or events in the geologic record. It relies on the principles of stratigraphy, which is the study of rock layers (strata) and their order of formation.
To use relative dating, geologists make use of several key principles:
1. Principle of Original Horizontality: This principle states that most sediments are originally deposited in horizontal layers. Any deformation or tilting of rock layers occurred after their formation.
2. Principle of Superposition: This principle states that in an undisturbed sequence of sedimentary rocks, the youngest rocks are found at the top, while the oldest rocks are found at the bottom.
3. Principle of Cross-Cutting Relationships: This principle states that if a geologic feature, such as a fault or igneous intrusion, cuts across another layer of rock, it must be younger than the rock it cuts through.
4. Principle of Inclusions: This principle states that any rock fragments contained within another rock must be older than the rock that contains them.
By observing these principles, geologists can determine the relative order of events in the geologic record. For example, a layer of rock that contains dinosaur fossils would be older than a layer that contains mammal fossils, based on the Principle of Superposition.
In addition to these principles, relative dating can also be aided by the use of index fossils. These are fossils that are widespread and existed for a relatively short period of time. By identifying and comparing the presence of index fossils in different rock layers, geologists can correlate the ages of those layers over vast distances.
Overall, relative dating allows geologists to create a relative timeline of Earth's history, indicating the order and relative ages of different events and formations. It helps us understand the sequence of events that shaped the planet and the life forms that lived during different periods of geologic time.