Geologic Time Lecture Notes

Geologic Time

Introduction to Geologic Time

Geologic time refers to the extensive span of time, documented by the Earth’s geological history, during which geological events and biological evolution have occurred. Understanding geologic time is essential for interpreting Earth's history, particularly through its rock layers and the life forms that existed at different periods.

William "Strata" Smith and the First Geological Map

In 1815, William “Strata” Smith made significant contributions to geology by creating the first published geologic map which encompassed England, Wales, and parts of Scotland. His background as a canal builder proved advantageous as he meticulously recorded the different rock layers encountered while constructing canals. This data not only benefited his work but was also crucial for others, such as mining companies seeking coal, which played a pivotal role in fueling the industrial revolution. Smith’s work is encapsulated in the phrase, "The Map That Changed the World."

Significance of Smith's Map
  • Historical Impact: The map facilitated a better understanding of the distribution of various rock types and the associated fossils.
  • Utility: It provided essential information for economic activities like mining.

Relative Geologic Time Scale

The geologic time scale, while lacking specific numerical dates, organizes time into a sequence of eons, eras, periods, and epochs, denoting only the order in which geological events transpired.

Components of the Relative Geologic Time Scale
  • Eons: The largest division of geologic time, encompassing significant geological and biological events.
  • Eras: Subdivisions of eons, characterized by notable changes in the Earth's conditions and life.
  • Periods: Subdivisions of eras, marked by distinct types of life, environmental conditions, and major geological events.
  • Epochs: Smaller divisions of periods often defining more specific time frames in evolutionary history.
Large Divisions Based on Life Forms
  1. Paleozoic Era: Referred to as the age of ancient life.
  2. Mesozoic Era: Known as the age of middle life (with 'meso' meaning middle).
  3. Cenozoic Era: Identified as the age of new life (with 'ceno' meaning new).

Development of the Geological Timescale

The relative geologic timescale was established over centuries, starting with early observations and principles derived from geological studies.

Early Principles of Relative Dating

  • In the 1600s, geologist Nicholas Steno introduced three foundational principles of relative dating:

    • Superposition: In a sequence of undisturbed sedimentary rocks, the oldest layers are at the bottom, while the youngest are at the top.
    • Original Horizontality: Layers of sediment are originally deposited in horizontal or nearly horizontal layers.
    • Lateral Continuity: Layers of sediment initially extend laterally in all directions; if they are not continuous, it indicates a disruption in deposition.

  • The 1700s saw the addition of:

    • Cross-Cutting Relationships: Anything that cuts across another geologic feature is younger than the feature it cuts through.
    • Fossil Succession: Fossils occur in a specific, recognizable order allowing for correlation of the ages of different strata.

Principles of Relative Dating

Superposition
  • Definition: In a rock sequence, the oldest rocks lie at the base with progressively younger rocks above them. This principle is crucial for understanding the geological history recorded in rock layers.
Original Horizontality
  • Definition: Sedimentary rocks are deposited in horizontal layers. Deviations from this original state suggest subsequent geological processes, such as tilting and folding.
Lateral Continuity
  • Definition: Sedimentary layers extend laterally until they thin out at the edges of the depositional basin or encounter a barrier.
Cross-Cutting Relationships
  1. Igneous Intrusion: An igneous rock body or fault that disrupts existing rock layers is younger than those layers.
    • Example: A dark dike, representing molten rock intrusion, is younger than the surrounding lighter granite.
  2. Faulting: A fault that displaces rock layers is younger than the rocks that it affects.
    • Example: Faults that tilt older rock layers like sandstone and shale are indicative of deformation events.

Fossil Correlation

Fossils, the preserved remains of organisms, provide insight into the relative ages of rock layers. Fossil assemblages, made up of specific groups of fossils from various layers, allow geologists to establish relative dating through several principles:

Faunal Succession
  • Definition: Fossils exist in a determined order throughout geological time, leading to the principle of faunal succession—specific fossil assemblages are indicative of the age of the rocks they are found within.
  • Example: Dinosaur bones are found exclusively within Mesozoic rocks, indicating that these rocks date to that specific era.
Index Fossils
  • Characteristics: Useful in determining the age of rocks and strata, index fossils are:
    • Widespread: Not limited to a single area.
    • Common: Frequently found within geological formations.
    • Easily Identified: Distinctive and recognizable fossils.
    • Short-Lived: Represent a brief time period in Earth's history.
Correlation over Distance
  • Fossils allow us to correlate rock layers that may be separated by vast distances (e.g., a layer in California may correlate to another in New York) based on shared fossil assemblages, demonstrating the utility of index fossils in stratigraphy.

Unconformities

An unconformity indicates a gap in the geological record, where geological history is missing, akin to a photo album missing certain pictures. It can signify periods of erosion or non-deposition in the sedimentary record.

Types of Unconformities
  1. Angular Unconformities: Occurs where sedimentary layers are tilted and meet at an angle.
  2. Disconformities: Parallel sedimentary layers that exhibit gaps due to periods of erosion or non-deposition.
  3. Nonconformities: Involves contact between sedimentary rock and older igneous/metamorphic rock, signaling a significant geological event.

Deformation and Its Effects on Layers

Deformation of rock layers caused by tectonic forces results in tilting, folding, and faulting.

Folding

  • Types:

    • Anticlines: Upward folds shaped like archways.
    • Synclines: Downward folds forming trough-like structures.

  • Analysis: Both phenomena occur together when the Earth's crust is compressed, leading to a wrinkling effect like crumpling paper.

Faulting
  • Faults are fractures in the Earth’s crust that result in displacements. The processes are critical in relative dating as they demonstrate the timeline of geological events.

Summary of Relative Dating Techniques

Relative dating establishes the order of events through methods like superposition, original horizontality, lateral continuity, cross-cutting relationships, and fossil correlation. These techniques allow geologists to construct a coherent picture of Earth's history based on rock and fossil evidence.

Practical Applications and Further Study

  • Understanding geologic time and relative dating principles is crucial for fields such as geology, paleontology, and archaeology, providing a framework for interpreting Earth's past and predicting geological changes.
  • Application of these principles is evident in various geological studies, mining operations, and academic research, making them essential knowledge for future scientists.