Geology for Engineers Exam Notes - Geologic Time and Dating

Rocks serve as records for geologic events and life forms throughout Earth's history.
Understanding Earth's past requires interpreting clues found in rock layers.
The geologic time scale was developed to illustrate Earth's extensive history.

Relative Dating: Placing rocks in sequence without determining their age (only their order).
Numeric Dating: Assigns actual years to events (absolute age).

Law of Superposition:
- In an undeformed sequence, each rock layer is older than the one above and younger than the one below.
- Proposed by Nicolaus Steno in the 17th century.
Principle of Original Horizontality:
- Sedimentary layers are deposited horizontally; disturbed layers indicate geological forces.
Principle of Lateral Continuity:
- Sedimentary beds are continuous until interrupted by erosion or another type of sediment.
Principle of Cross-Cutting Relationships:
- Features (like faults) that cut through rocks are younger than the rocks they disrupt.
Principle of Inclusions:
- Inclusions within a rock indicate that the rock is younger than the fragments it contains.

Represent breaks in the geological record due to erosion or non-deposition.
Types of Unconformities:
- Angular Unconformity: Older, folded rocks are overlain by younger flat-lying rocks.
- Disconformity: Represents gaps caused by erosion, with parallel layers above and below.
- Nonconformity: Found between older igneous/metamorphic and younger sedimentary rocks.

Enables the development of a global geologic time scale by matching rocks of the same age across distances.
Fossils & Correlation:
- Developed by William Smith, who used fossils to correlate strata across regions.

Permineralization: Minerals infiltrate porous material (e.g., petrified wood).
Molds and Casts: Impressions left by dissolved organisms.
Carbonization and Impressions: A thin film of carbon preserved.
Amber: Tree resin preserving fauna.
Trace Fossils: Evidence of behavior (e.g., tracks, burrows).

Fossils appear in a defined order; can be used to determine ages of rock layers.
Index Fossils: Indicators for correlating rock layers to specific time periods, widespread, and of short duration.
Environmental Indicators: Fossils can indicate historical climates and landscapes.

Radioactivity: Allows determination of Earth's age (~4.6 billion years) and provides numeric ages for geological events.
Basic Atomic Structure: Atoms consist of protons, neutrons, and electrons with isotopes being variants of elements.
Radioactive Decay: Unstable nuclei break apart, producing stable isotopes, categorized into alpha, beta emissions, and electron capture.

Half-Life: Time taken for half of a radioactive sample to decay, used to compute ages of rocks.
Radiometric Dating: Leverages parent (unstable) vs. daughter (stable) isotopes; needs a closed system for accuracy.

Uranium-238 → Lead-206; half-life = 4.5 billion years.
Potassium-40 → Argon-40; half-life = 1.3 billion years; useful for dating younger formations.

Subdivides Earth's geologic history based on relative and radioactivity-based dating.
Structured into eons, eras, periods, and epochs, with the Phanerozoic eon featuring prominent life forms.
Precambrian time accounts for ~88% of Earth's history, detailed fossil records mostly begin during the Cambrian period.

Not all rocks are suitable for radiometric dating; ages may vary across different rock types and compositions.