Lecture 8.1 Geologic time

Introduction

• James Hutton, in the 18th century, recognized the vastness of Earth’s history and the concept of geological time.

• The 19th century saw demonstrations of multiple episodes of geological processes, establishing the need for significant time spans in mountain building and erosion.

• Objectives:

  • Explain the principle of uniformitarianism and its contrast with catastrophism.

  • Differentiate numerical and relative dating.

  • Apply principles of relative dating to establish sequences of geologic events.

  • Define fossils, conditions favoring their preservation, and various fossil types.

Comprehending Geological Time

• Our short lives limit our understanding of geological time scales.

• Example comparisons:

  • 1 inch represents 1 year; 6 feet represents a human lifetime.

  • 1.6 miles represents all recorded human history.

  • 72,600 miles represents the age of Earth.

• Understanding geological time involves recognizing its immense duration and the challenges it poses to human comprehension.

Methods of Measuring Geologic Time

• Sedimentary Accumulation:

  • Attempts to estimate Earth’s age based on the accumulation of sediment over time proved unreliable due to various complications.

• Catastrophism (Early Doctrine):

  • Belief that Earth's formations were primarily the result of sudden, violent events, rather than gradual processes over time.

Uniformitarianism

• Proposed by Hutton, this principle asserts that:

  • The same processes shaping the Earth today have operated throughout its history.

  • "The present is the key to the past."

• Acceptance of uniformitarianism implies:

  • Geological processes are slow and occur over immense timescales (examples include the formation of the Grand Canyon).

The Geological Time Scale

• Relative Dating:

  • Establishes sequence of events without assigning specific ages; e.g., "You are older than your sister."

• Numerical (Absolute) Dating:

  • Calculates the actual age of geological materials (e.g., "I am 15, and my sister is 11.").

Fossils as Time Indicators

• Definition of Fossils:

  • Remains or traces of ancient organisms preserved in rocks.

  • Types of fossils include:

    • Actual remains (bones, shells)

    • Impressions (of soft body parts)

    • Casts and molds

    • Body parts mineralized

    • Behavioral evidence (footprints, burrows)

• Fossils are critical for correlating rocks of similar ages from varying locations.

Using Fossils to Determine Ages of Rocks

• Fossils show specific ranges in geological time where organisms existed.

• Each organism has defined periods of appearance and extinction, which help determine rock ages.

• Example analysis of sedimentary rocks may include:

  • Identifying overlapping ranges of fossils (e.g., pink stratum with both terrestrial and marine fossils).

• Key organisms may help constrain the age (e.g., a dinosaur’s extinction may provide a minimum age).

Case Studies in Fossil Ranges

• Fossil analysis helps bracket ages of rock units.

• Examples:

  • Pink rock unit: overlapping fossils (e.g., dinosaur, sycamore leaf) indicate a specific geological timeframe.

  • Blue rock unit: trilobites and ferns show different evolutionary timelines, allowing age bracketing.

• Conclusion:

  • The overlaps in fossil ranges provide tools for establishing the geological time scale.

Wrap-Up

• Understanding the geological time scale combines knowledge of relative and absolute dating and fossil correlation.

• The evolution of life and geological processes highlights the immense timescales involved in Earth’s history.