Lecture 17: Relative Age and Geologic Maps

Geologic Timescale

  • Divided into time blocks: Eons, Eras, Periods, Epochs, and Ages.

  • Eons: Largest subdivision (100s to 1000s Ma).

  • Eras: Subdivisions of eons (65 to 100s Ma).

  • Periods: Subdivisions of eras (2 to 70 Ma).

  • Epochs: Subdivisions of periods.

Relative vs. Absolute Age

  • Relative age: Sequence/order of events (oldest to youngest).

  • Absolute age: Numerical age of events, often determined by radiometric dating of isotopes.

Relative Age Principles

  • Law of Original Horizontality: Sedimentary rocks originally deposited in horizontal layers.

  • Principle of Lateral Continuity: Rock layers initially continuous laterally, varying in depositional environment.

  • Law of Superposition: Older sediments on the bottom; younger ones on top.

  • Principle of Inclusions: Fragments within a rock must be older than the rock itself.

  • Principle of Cross-Cutting Relations: Features crossing another rock are younger than the rock they cross.

Types of Intrusive Igneous Rocks

  • Igneous Intrusions: Form when magma cools in pre-existing rock.

  • Batholith: Large (>100 km²) igneous intrusion, formed at depth.

  • Dike: Narrow, vertical sheet of magma.

  • Sill: Horizontal body of intruded magma between layers.

  • Stock: Moderately large magma body, often a volcano's magma chamber.

  • Laccolith: Body of magma causing uplift around it.

Exposed Intrusions on Geologic Maps

  • Intrusions appear on maps; often represented with stocks linked to a larger batholith.

Geologic Maps and Legends

  • Legend follows the Law of Superposition (oldest on bottom).

  • Geologic units labeled with colors/symbols (e.g., Pbd for Permian period formation).

Sequence of Events Interpretation

  • Consider law of superposition and cross-cutting features.

  • Identify and evaluate unconformities that represent missing time.

Types of Unconformities

  • Disconformity: Erosion/absence of layers between sedimentary rocks.

  • Angular Unconformity: Represents significant time gap with tilted/truncated beds.

  • Nonconformity: Between different rock types (igneous/metamorphic over sedimentary).

Geologic Rock Symbols

  • Common symbols include Granite, Sandstone, Limestone, Schist, Faults, and Unconformities.

  • Symbol colors may vary by map.