Relative Dating

Relative Dating Definition: the placing of events in their proper chronological order Does not indicate exactly how long ago something occurred (absolute dating), only that it preceded or followed another event Several logical doctrines, principles, and laws have been formulated in the application of relative dating….

Doctrine of Uniformitarianism Developed by James Hutton, Scottish physician and geologist (1770s) Natural processes occurring today have been occurring since the Earth formed. Only the rate, intensity, and scale with which they occur have changed. “The present is the key to the past”

Uniformitarianism v. Catastrophism Uniformitarianism implies that change is generally constant and gradual. However, some changes in earth’s history appear to have occurred because of brief, catalysmic events. Catastrophism--theory that earth’s present is the result of previous catastrophic events Initially, uniformitarianists argued bitterly with catastrophists. Now, both sides are recognized as important in earth’s history.

Principle of Original Horizontality Sedimentary rocks are deposited in horizontal planes (beds) Deformations of this horizontality must have occurred after the sedimentary rock formed.

Law of Superposition Older rocks are below younger rocks.

Principle of Cross-cutting relationships Cross-cutting rocks are younger than the layers they cut across Intrusion—a narrow layer of igneous rock that cuts across preexisting rock layers

Inclusions Inclusions—rock fragments from one layer that are present in an adjacent layer If a rock layer contains pieces of adjacent strata, the adjacent strata must have been there first. Layer A is older than layer B

Other ways of determining relative age Because earth is constantly changing (weathering, erosion, earthquakes, volcanoes, etc.), it’s difficult to find an undisturbed sequence of rock layers. Unconformity: when an eroding surface is buried by the deposition of younger rocks above it. This results in a gap (unconformity) in the rock record. There are several types of unconformities…

Types of Unconformities Disconformities When horizontal sedimentary rocks overlie horizontal sedimentary rocks

Types of Unconformities Nonconformities When sedimentary rocks overlie nonsedimentary rocks. Suggests that the nonsedimentary rocks were uplifted, subjected to weathering and erosion, then subjected to deposition by sedimentary rocks.

Types of Unconformities Angular Unconformities When originally horizontal sedimentary rocks are uplifted and tilted by crustal movements, exposing them to weathering and erosion. When deposition resumes, new horizontal layers are laid down on top The layers beneath remain in tact but at an angle