Relative Dating and Geologic Principles

Relative Dating

  • Relative dating is putting rocks into order based on their positions.
  • Geologic time provides a frame of reference for understanding rocks, fossils, geologic structures, landscapes, tectonic events, and changes over the last 4.6 billion years.

Relative Age Dates vs. Numerical Age Dates

  • Relative age dates (relative dating): Placing rocks and events in their proper sequence of formation (e.g., this happened before that).
    • No numbers associated, just an order of events.
  • Numerical age dates (numerical dating): Specifying the actual number of years that have passed since an event occurred (e.g., dinosaurs went extinct 65 million years ago).
  • Relative age dating can be done without special equipment, while numerical dating requires specialized dating techniques.

Principle of Uniformitarianism

  • The principle of uniformitarianism, summarized as "the present is the key to the past," suggests that processes occurring today can be inferred to have occurred similarly in the geologic past.
    • Example: Observing ripple marks forming on a beach today allows us to infer that similar ripple marks in ancient rock formations indicate a past beach environment.

Sir Charles Lyell & Principles of Geology

  • Sir Charles Lyell wrote Principles of Geology (1830-1833), which established relative ages of Earth's materials and fossils.
  • Lyell's principles provide a method to put rocks in order of events without assigning numerical ages.

Physical Principles of Geology

  • These principles are used to determine the order of events in geological history.
Principle of Superposition
  • Oldest rocks are on the bottom, and youngest rocks are on top.
  • Sediment deposition fills a basin, with the oldest layers forming first at the bottom.
Principle of Original Horizontality
  • Layers are typically laid down horizontally or flat due to gravity.
  • If layers are tilted or folded (e.g., anticlines and synclines), it indicates that these deformations happened after the initial deposition.
Principle of Lateral Continuity
  • Rocks are usually deposited over a large distance, creating laterally continuous units.
  • Erosion can disrupt this continuity, but the separated units are still considered the same.
Principle of Cross-Cutting Relations
  • If an igneous intrusion (dike) or a fault cuts through existing rock layers, the intrusion or fault is younger than the layers it cuts.
Principle of Inclusions
  • Inclusions are rock fragments within another rock sequence.
  • The inclusions are always older than the enclosing material.
    • Doesn't matter where it is in the section, the inclusions will always be older.

Unconformities

  • An unconformity is a time gap in the rock record due to nondeposition or erosion.
Angular Unconformity
  • Represents a large gap in time where tilted or folded rock layers are eroded, and new horizontal layers are deposited on top.
  • The beds below are at an angle to the beds above.
Nonconformity
  • Igneous or metamorphic rocks are capped by sedimentary rocks.
  • Indicates a period of uplift and erosion, where rocks formed below the Earth's surface are exposed at the surface before sedimentary deposition.
Disconformity
  • Parallel strata bound a surface of nondeposition or erosion within sedimentary rock layers.
  • Represents an interruption in sedimentation, such as an erosional surface or a paleosol (fossil soil) horizon.
  • Can be a result of erosion or nondeposition.

Grand Canyon

  • The Grand Canyon illustrates all three types of unconformities.
    • Nonconformity: igneous rock in contact with sedimentary rock
    • Angular unconformity: tilted rocks at an angle to horizontal rocks
    • Disconformities: missing gaps in time between sedimentary rocks