How do we determine the age of rock layers? Question: How do we determine the age of rock layers?

Absolute Dating Also known as Radiometric Dating Radiometric dating is based on the fact that certain naturally occurring radioactive isotopes decay, or transform into a different element, at known rates. Rock age can then be determined by measuring the ratio of the radioactive materials remaining to the "decayed" element. Nearly exact ages can be determined.

Relative Dating Relative Dating places geologic events in their order of occurrence by comparing one event to another. Exact ages of the geologic events CANNOT be determined using this method. Ages are described as being older or younger than another layer of rock.

Rules of Relative Dating Law of Superposition states that in a sequence of undeformed rocks, the oldest rock layers are on the bottom and the youngest are on top.

2. Principle of Original Horizontality states that layers of rock that are NOT horizontal have been deformed by movements of Earth’s crust. Examples: Folding, Faulting and Tilting

FOLDING FAULTING Rock layers are “folded” due to compression stress. Faulting is when very large sections of rock break apart and move in different directions. Faulting can be the result of compression, tension or shearing forces. TILTING Rock layers are tilted sideways as result of stress.

3. Principle of Cross-cutting Relations states that geologic features such as faults and igneous intrusions are younger than the rock layers they cut through.

4. Principle of Faunal Succession states that groups of fossil plants and animals occur in the geologic record in a specific order. A period of geologic time can be determined by its respective fossils.

Index Fossils are used to determine the relative ages of rock layers from different parts of the world. Index fossils are fossils of plants and/or animals that lived for only a short time in history. Because they existed for only a short period of time, index fossils help scientists determine the age of rock layers that have been changed through a force such as folding, faulting, erosion and others.

Steps to Determining the Relative Age of Rock Layers are: Work from oldest to youngest (bottom to top). Pay attention to any folds, faults or intrusions.

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