Chapter 10 Notes - Deep Time: How Old Is Old?

Review of Metamorphism and Geological Principles

Types of Metamorphism

  • Baked Contacts:
    • Definition: This metamorphism occurs where rocks have been heated and altered due to the proximity to an intrusion of molten rock, typically basalt or granite.
    • Effects: The heat results in the alteration of minerals in the rock, creating a baked, metamorphosed zone called an aureole.
    • Associated with regional or contact metamorphism, depending on the scale.

Law of Inclusions

  • Definition: Inclusions in igneous rocks and fragments in sedimentary rocks are older than the rocks in which they occur.
  • Explanation: When a rock contains fragments of another rock, these fragments must have been present before the rock that contains them was formed. For example:
    • If an igneous rock includes sedimentary fragments, those sedimentary rocks existed prior to the formation of the igneous rock.

Law of Fossil Succession

  • Definition: Different species of organisms appear in a definite order within strata.
  • Key Points:
    • Once a fossil species disappears from the record, it never reappears higher in sequence.
    • This principle allows geologists to correlate and date rock layers based on their fossil content.
    • Proposed by William Smith (1769-1839), considered the father of biostratigraphy.

Unconformities: Gaps in the Record

  • Definition: An unconformity is a gap of time in the geologic record that separates older rocks from younger rocks.
  • Causes of Unconformities:
    • Erosion: Removal of material which creates a gap in sequence.
    • Non-deposition: Periods when sediment is not deposited, thus leaving a time gap.
Types of Unconformities:

  1. Disconformity:

    • Definition: This type has parallel sedimentary rocks above and below the erosional surface.
    • Illustration: Refer to Figure 10.5c for a cross-sectional view of disconformity.

  2. Nonconformity:

    • Definition: This type features intrusive igneous or metamorphic rocks below the erosional surface, with sedimentary rocks situated above these older rocks.
    • Significance: Represents the largest gap in geologic time because it involves the transition between different rock types.
    • Illustration: Refer to Figure 10.5b for a cross-sectional view of nonconformity.

Visual Aids

  • Cross-sections:
    • Use Figure 10.1g for the Law of Inclusions.
    • Use Figures 10.4a, 10.4b, 10.5a, 10.5b, 10.5c, 10.5d, 10.6a, 10.6b, 10.7a, 10.8a, and Geology at a Glance for examples of different types of unconformities.
    • Ensure to illustrate disconformity and nonconformity based on provided figures for clearer understanding.

Chapter 10 Notes - Deep Time: How Old Is Old?