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.
Steno’s Laws
(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.
More Principles of Relative Dating
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).