Earth Science
Earth History
Principles of Relative Dating
Study Tip
Archaeologists use relative dating much the same way a crime scene investigator may look at clues at a scene to find out when a crime occurred.
Relative Age Dating
Used to determine the ages of materials relative to each other.
With evidence, archeologists can determine whether one rock is older than another and when a process occurred relative to those rocks.
Study Tip
Archaeologists use relative dating much the same way a crime scene investigator may look at clues at a scene to find out when a crime occurred.
Steno’s Laws
Steno’s Laws
Original Horizontality:
Sediments are deposited in fairly flat, horizontal layers.
If a sedimentary rock is found tilted, the layer was tilted after it was formed.
Lateral Continuity:
Sediments are deposited in continuous sheets that span the body of water that they are deposited in.
When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous.
(a)Original horizontality (b) Lateral continuity (c) Superposition
Superposition:
Sedimentary rocks are deposited one on top of another.
The youngest layers are found at the top, and the oldest layers towards the bottom.
More Principles of Relative Dating
William Smith (1769-1839)
Principle of Faunal Succession
Some fossil types are never found with certain other fossil types. For instance, human ancestors are never found with dinosaurs.
Fossils in a rock layer represent what had lived during the period the rock was deposited.
James Hutton (1726-1797)
Cross-Cutting Relationships
Younger rocks crosscut older rocks, and this relationship helps geologists determine the relative age of the rocks.
In this image, A must have existed first for B to crosscut it.
Earth Science
Concept Check
How are sediments deposited? How does this help determine age?
Describe Steno’s Laws (original horizontality, lateral continuity, and superposition).
Describe the theories of Smith and Hutton (Principle of Faunal Succession and Cross-Cutting Relationships).