7 Geologic Time

Geologic Time

• Arizona’s Grand Canyon National Park exemplifies the principles used by geologists to determine the ages of rocks.

Key Concepts

• Differences between relative time and numeric time

• The five principles of stratigraphy

• Application of relative dating principles to block diagrams

• Definitions and explanations of isotopes and mechanisms of radioactive decay

• Description of radioisotopic dating and key isotopes

• Explanation of carbon-14 formation and its use in dating

• Understanding the numeric age of the Earth and historical events

• Techniques for dating sedimentary sequences

• Definition of fossils and types of fossil preservation

• Outline of natural selection as an evolutionary mechanism

• Description of stratigraphic correlation

• List of eons, eras, and periods of the geologic time scale

• Explanation of connection between time units and rock units (chronostratigraphy vs lithostratigraphy)

Historical Overview

• Nicolas Steno (1638-1686) introduced the basic principles of stratigraphy in 1669.

• William Smith (1769-1839) observed consistent sequences in strata throughout England, producing the first national geologic map of Britain.

• Scientists developed a relative time scale based on Steno’s principles, naming units based on rock characteristics.

• The geologic time scale includes names of units and subunits, allowing for chronological ordering of Earth's history even without numeric ages.

7.1 Relative Dating

• Definition: Relative dating refers to determining if one rock or geologic event is older or younger than another without knowing their specific ages.

Principles of Relative Dating

7.1.1 Principles of Stratigraphy

• Principle of Superposition: In an undisturbed sequence of sedimentary strata, layers at the bottom are older than those above them.

• Principle of Original Horizontality: Layers of sediments are originally deposited horizontally.

• Principle of Lateral Continuity: Layers of strata are continuous until they thin out or encounter geographic barriers.

• Principle of Cross-Cutting Relationships: Geologic features like faults and intrusions that cut across rocks are younger than the rocks they affect.

• Principle of Inclusions: Rock fragments included in a formation are older than the formation itself.

• Principle of Fossil Succession: Fossils show a recognizable succession through layers, allowing correlation of strata.

7.1.2 Grand Canyon Example

• The Grand Canyon illustrates the principles of stratigraphy:

  • Layers of rock are sequenced from oldest at the base to youngest at the top per the principle of superposition.

  • Coconino Sandstone: Represents a layer below the canyon rim; continuous even across the canyon due to lateral continuity.

  • Displays cross-cutting relationships between igneous and metamorphic formations, such as metamorphic schist and granite intrusions.

  • Unconformities are present where deposition stopped or rock layers were eroded.

7.1.3 Unconformities

• Three types of unconformities:

  • Nonconformity: Sedimentary rocks on igneous or metamorphic rocks, as seen in the Grand Canyon's basal layer.

  • Disconformity: Occurs between parallel sedimentary strata; indicates periods of non-deposition or erosion.

  • Angular Unconformity: Horizontal strata overlie tilted or folded rocks that were eroded before deposition.

7.1.4 Applying Relative Dating Principles

• Example block diagram shows sequence of geological events using known properties of rock types.

  • Sequence begins with folded metamorphic gneiss, followed by fault displacements, and finally by igneous intrusions indicating relative ages.

7.2 Absolute Dating

• Canada’s Nuvvuagittuq Greenstone Belt possibly contains the oldest rocks and signs of early life.

• Relative time allows for sequencing events but lacks numeric age assignments.

• Radioactivity discovery in the late 1800s advanced numeric dating techniques through radioisotopic dating.

7.2.1 Radioactive Decay

• Definition of Isotope: Atoms of the same element with differing numbers of neutrons. Example: Hydrogen has isotopes:

  • Protium (1H): 1 proton, 0 neutrons

  • Deuterium (2H): 1 proton, 1 neutron

  • Tritium (3H): 1 proton, 2 neutrons (unstable).

• Radioactive Decay: Process where unstable isotopes transform to more stable forms.

• Half-life: Time required for half of a radioactive sample to decay into stable isotopes.

  • Example half-lives: 238U = 4.5 billion years; 14C = 5730 years.

7.2.2 Radioisotopic Dating

• Dating involves separating parent/daughter isotopes and measuring with mass spectrometers.

• Calculation of age is based on the daughter-to-parent ratio.

• Multiple parent/daughter pairs verify dating accuracy.

Key Radioactive Isotopes and Their Half-lives

• Uranium-238/Lead-206: $238U <br>ightarrow 206Pb$; half-life = 4.5 billion years

• Uranium-235/Lead-207: $235U <br>ightarrow 207Pb$; half-life = 704 million years

• Potassium-40/Argon-40: $40K <br>ightarrow 40Ar$; half-life = 1.25 billion years

• Rubidium-87/Strontium-87: $87Rb <br>ightarrow 87Sr$; half-life = 48.8 billion years

• Carbon-14/Nitrogen-14: $14C <br>ightarrow 14N$; half-life = 5,730 years

7.2.3 Carbon-14 Dating

• Carbon-14 is generated in the atmosphere; decays after an organism's death.

• Useful for dating events up to 50,000 years due to its half-life calibration.

7.2.4 Age of the Earth

• Early estimates of Earth's age (98 million years) via Lord Kelvin; later refined to 4.54 billion years using radioactive isotopes of meteorites by Clair Patterson.

7.2.5 Dating Geological Events

• Zircon Crystals: Used in uranium/lead dating; provide insights on ancient Earth.

7.3 Fossils and Evolution

• Definition of Fossils: Evidence of historical life, potentially including body parts, molds, casts, or behavioral evidence.

• Preservation probabilities are highly variable; soft-bodied organisms are less likely to be preserved.

7.3.1 Types of Fossil Preservation

• Actual Preservation: Rare, retains original materials.

• Permineralization: Groundwater minerals replace original tissues.

• Molds and Casts: Spaces left by dissolved organic materials can be filled, preserving shapes.

• Carbonization: Compression leaves a carbon silhouette.

• Trace Fossils: Indirect evidence such as footprints or burrows; studied through ichnology.

7.3.2 Evolution

• Natural Selection: Proposed by Charles Darwin; species evolve through environmental adaptations affecting survival rates.

7.4 Correlation

• Correlation: Matching sedimentary strata from different areas by age.

7.4.1 Stratigraphic Correlation

• Establishes age of strata in separated regions through stratigraphic relationships.

7.4.2 Lithostratigraphic Correlation

• Matches age based on similarities in composition and properties of strata over distances.

7.4.3 Chronostratigraphic Correlation

• Matches rocks of the same age, irrespective of their composition differences.

7.4.4 Biostratigraphic Correlation

• Uses index fossils to determine ages; best suited for narrow time intervals.

7.5 Geologic Time Scale

• The geologic time scale consists of eons, eras, periods, epochs, and ages, partitioning Earth's history.

• It flows continuously, but certain rock formations may be absent reflecting missing geological history.

Chapter Summary

• Relative dating uses principles to sequence events without numeric ages; radioactivity discovery led to absolute dating methods.

• Combined techniques contribute to accurate interpretations of the geological and evolutionary history of Earth.