Geologic Time: Dating Techniques and Principles

Posted on Jan 27, 2025 in Geography

Chapter 11: Geologic Time

Catastrophism vs. Uniformitarianism

Catastrophism

  • Landscape developed by catastrophes (i.e., floods)
  • Implies Earth formed over a short period
  • Archbishop James Ussher, mid-1600s, concluded Earth was only a few thousand years old

Modern Geology

  • Uniformitarianism (Fundamental principle of geology)
  • James Hutton, 1700s, proposed the idea in Theory of the Earth
  • “The present is the key to the past”
  • Biological and physical processes operating today operated in the geologic past
  • Implies Earth has formed over a vast period of geologic time (billions of years)

Principles and Laws of Relative Dating

  • Law of Superposition: Oldest rocks are deposited on the bottom
  • Principle of Original Horizontality: Sediment is deposited horizontally (inclined layers have been tilted after deposition)
  • Principle of Cross-Cutting Relationships: Younger features cut through older features

Three Types of Unconformities

  • Angular Unconformity: Tilted rocks are overlain by flat-lying rocks
  • Disconformity: Strata on either side of the unconformity are parallel
  • Nonconformity: Metamorphic or igneous rocks below the unconformity, younger sedimentary layers above

Fossils and Conditions for Preservation

What is a fossil? What conditions favor the preservation of organisms as fossils (discussed in the lab, too)?

Conditions Favoring Fossil Preservation

  1. Rapid Burial
    • Why would this be important?
    • Answer: Scavengers eat dead organisms that are not quickly buried
  2. Possession of Hard Parts (teeth, bone)
    • Soft-bodied organisms decay completely
    • i.e., fossil squid and octopus are very rare in the rock record

The fossil record is biased towards organisms with hard skeletal material that were buried quickly.

Index Fossils

  • For a fossil to be useful as an index fossil, two conditions must be met:
    1. Organisms were widespread geographically
    2. Organisms existed for a short range of geologic time
  • Trilobites are an index fossil for the Cambrian period

Radioactive Decay

  • Radioactivity: Spontaneous breaking apart (decay) of atomic nuclei
  • Radioactive Decay
    • Parent: An unstable isotope undergoing decay
    • Daughter Products: Isotopes formed from the decay of a parent

Types of Radioactive Decay

  • Alpha emission
  • Beta emission
  • Electron capture

Radiometric Dating

What kind of information does radiometric dating of an igneous rock yield? Sedimentary rock?

  • Half-life: The time for one-half of the radioactive nuclei to decay
  • Requires a closed system
  • Yields numerical dates

Difficulties in Dating the Time Scale

  • Not all rocks are datable (sedimentary ages are rarely reliable)
  • Materials are often used to bracket events and arrive at ages

Carbon-14 Dating

Describe carbon dating (C14) techniques. What type of material is useful in (C14) dating? What is the maximum age of material that can be dated using C14 techniques?

  • Half-life of only 5,730 years
  • Used to date very recent events
  • Carbon-14 produced in the upper atmosphere
  • Incorporated into carbon dioxide
  • Absorbed by living matter
  • A useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history

Isotope of Carbon (14C)

Remember from our earlier example, Carbon has 6 protons. The isotope of carbon, 14C, has a mass number = 14. How many neutrons does 14C have? 14 = 6 protons + # neutrons, so 14 – 6 = 8 neutrons. 14C has 2 additional neutrons in its nucleus and spontaneously decays.