Relative Dating

• Relative dating is accomplished by placing events in logical sequential order.
• Why is geological time important?

  To understand the physical and biological history of our planet.
• There was no reliable method for absolute dating before radiometric dating, therefore relative dating methods were used.
• Do relative dating tell us the exact age of fossils?

  No, but they can be used to estimate the age of fossils
• What are the fundamental principles of relative dating?

  Superposition, cross-cutting, original horizontally, lateral continuity, inclusions, and fossil succession.
• Superposition

  The oldest layer is deposited on the bottom, the youngest is on the top
• Original horizontally

  Sediment and some volcanic units is originally deposited horizontally and later tilted/folded
• Lateral continuity


  Sediment extends laterally in all directions until it thinks and pinches out or terminates against the edge of a depositional basin.
• Cross-cutting

  James Hutton recognized that an igneous intrusion must be younger than the rock it intrudes.
• Lava vs sill cross-cutting

  Sills bake the top and bottom of rock while lava flow bake the bottom only.
• Inclusion

  Inclusions are older than the host rock
• Angular Unconformity

  Tilted/folded sedimentary rock overlain by flat-lying sedimentary rock.

  An erosional surface on tilted/folded rocks, over which younger rocks were deposited.
• Nonconformity

  Igneous/Metamorphic rock overlain by flat-lying sedimentary rock.

  An erosional surface cut into igneous/metamorphic rocks and overlain by younger sediment rocks.
• Disconformity: separates younger from older strata of sedimentary rocks that are parallel to each other
• Correlating rock units

  Correlation — demonstration of equivalency of rock units from one area to another.
• Time equivalence is usually demonstrated by the occurrence of similar fossils in strata.