Geological Principles and Dating Methods

Uniformitarianism and Earth Science

• Uniformitarianism: The present is the key to the past.
• Earth science is grounded in modern observation.
• Geological record serves as a "memory bank" of Earth processes.
• Understanding rock types and their relationships is crucial.

Learning Outcomes

• Apply geological principles to determine the relative age of features in an outcrop.
• Explain the principle of radiometric dating and its use in determining Earth's age (4.56 Ga).
• Use terms from the geological timescale to indicate the absolute age of rocks, minerals, and events.
• Read the geological record to identify past Earth system interactions.
• Reconstruct key events in the geological history of Zealandia.

Key Principles

• Relative age determination involves observing relationships between rock units.
• Absolute age determination relies on geochronology.
• The geologic timescale provides a shorthand for communicating about time.

Relationships Between Rock Units

• Superposition: In undisturbed sedimentary rock sequences, the oldest layers are at the bottom, and the youngest are at the top.
• Original Horizontality: Sediments are originally deposited in horizontal layers. Tectonic forces can later deform these layers.
  • If sedimentary rocks are found to be tilted, it indicates deformation after the original deposition and lithification. Uplift and erosion may also occur.
• Inclusion: Clasts within a rock unit are older than the rock unit itself.
  • For example, if clasts from a lower rock unit are included in an overlying sedimentary layer, the lower unit is older.
  • Magma can intrude existing rock, picking up inclusions, which then solidify within the igneous rock.
• Faults: Fractures in rock caused by tectonic stress.
  • Reverse Fault: Tectonic stress causes one block to be pushed up and over the other block.
  • Normal Fault: Tectonic stress causes one block to slide down relative to the other block.

Unconformities

• Gaps in the geologic record where layers of rock are missing due to erosion or periods of non-deposition.
• Disconformity: An erosional surface between parallel layers of sedimentary rock.
  • Sediment deposition stops, time passes, and new sediment may be deposited, but the original sediment erodes away. Sediment deposition then resumes.
• Angular Unconformity: An erosional surface between tilted or folded rock layers and overlying horizontal layers.
  • Rocks are tilted by tectonic stress, uplifted to the surface, and eroded. New sediment is then deposited on top of the erosional surface.
• Nonconformity: An erosional surface between metamorphic or igneous rocks and overlying sedimentary rocks.
  • Metamorphic and/or igneous rocks form deep within the Earth, are uplifted to the surface, eroded, and then covered by new sediment. The contact is a non-conformity.

Absolute Dating

• Radioactive Decay: Provides an absolute chronometer.
• Henri Becquerel (1852-1908): Discovered radioactivity in 1895, revealing a new internal heat source for the Earth.
• Ernest Rutherford (1871-1937)
• First radiometric dating of a uranium mineral occurred in 1905.

Geologic Time

• "a" = annum = years ago
• "1 Ka" = 1 kilo-annum = 1 thousand years ago
• "1 Ma" = 1 mega-annum = 1 million years ago
• "1 Ga" = 1 giga-annum = 1 billion years ago = 1000 million years ago
• Age of the Earth: 4.543 Ga.
  • Oldest mineral: A 4.4 billion-year-old zircon crystal from the Jack Hills of Western Australia, dated using ^{238}U.

Chronostratigraphic Tools for Correlation

• Faunal Succession: Fossils succeed each other vertically in a specific, reliable order over long distances.
• Biostratigraphy: Microscopic phyto- and zooplankton evolve rapidly, making instances of evolution and extinction useful as chronostratigraphic benchmarks.
• Magnetostratigraphy: Magnetic minerals in oceanic crust and other rocks align with Earth’s magnetic polarity, which flips periodically.

Early Estimates of Geologic Time

• Archbishop Ussher: Used biblical calculations to estimate Earth's age.
• Hindu Calendar: Estimates the age of the universe to be ~2 Ga.
• Hutton (1785): Proposed 'No vestige of a beginning, no prospect of an end.'
• Lyell (1867): Used rates of evolution to estimate 20 Ma for an evolutionary cycle.
• DeGeer (1884): Studied glacial varves to create yearly records extending tens of thousands of years.
• Haughton (1878): Estimated the total thickness of fossiliferous sediments and their rate of deposition to calculate age.
• Joly (1899): Used salt content in the oceans to estimate age.
• Kelvin (1862, 1897): Used conductive cooling models to estimate the solidification of Earth's crust.

Principals of Radiometric Dating

• The number of atoms of an unstable isotope decreases with time, which is expressed with the below equations.

• - \frac{dN}{dt} \propto N

• - \frac{dN}{dt} = \lambda N

• T_{\frac{1}{2}} = \frac{ln 2}{\lambda}

  • Where:
    • N - Number of atoms of an unstable isotope
    • \lambda - Radioactive decay constant (fraction of atoms that decay in unit time, e.g., yr-1)
    • T_{\frac{1}{2}} - Half-life (the time for half the atoms to decay)

• Radioactive parent isotopes decay to stable daughter isotopes.
  *Radiogenic isotopes are present in measurable concentrations for 5 to 10 half-lives.

Layer-Counting for Age Calculation

• Where sediments accumulate annually, layer-counting can be used to calculate age.