Plate Tectonics

Plate Tectonics

Evidence of Plate Motions

Overview
  • Textbook Reference: Pages 330-338.
  • Objectives:
    • Describe the hypothesis of continental drift.
    • Suggest evidence from land features to support continental drift.
    • Suggest evidence from fossils to support continental drift.
    • Suggest evidence from climate to support continental drift.

Key Concepts and Definitions

Continental Drift Theory
  • Definition: The hypothesis that suggests the continents were once connected as a single supercontinent (Pangaea) and have since drifted apart.
  • Key Terms:
    • Pangaea: The supercontinent that existed during the late Paleozoic and early Mesozoic eras, around 335 million years ago.
    • Meteorologist: A scientist who studies weather patterns and climate.
Evidence of Continental Drift
  • Jigsaw Puzzle Fit of Continents: The coastlines of continents like South America and Africa appear to fit together like pieces of a puzzle, suggesting they were once contiguous.

Historical Context

Alfred Wegener's Contribution
  • 1912: German meteorologist Alfred Wegener proposed the continental drift hypothesis.
  • Wegener's Theory: Stated that continents were once fused and have been drifting apart over geological time.
  • Key Figures: Marie Tharp and Bruce Hezen contributed to mapping the ocean floor which supported Wegener's ideas.

Evidence Supporting Continental Drift

Evidence from Land Features
  • Geological Alignment: Mountain ranges on coasts appear to align, indicating they were formed in the same geological period and location.
  • Coal Deposits: Found in regions that no longer support such vegetation, indicating past climatic conditions ideal for plant growth.
  • Figure 2: Illustrates structural alignment across continents, supporting evidence of continental drift.
Evidence from Fossils
  • Fossil Distribution: Organisms like Glossopteris, a fern, have been found on multiple continents, affirming they were once connected. Fossils of Mesosaurus, a freshwater reptile, present in both South America and Africa support this theory.
  • Fossil Evidence Locations: Fossils of Lystrosaurus and Cynognathus found on continents that are now separated by vast oceans further suggest a shared landmass in the past.
Evidence from Climate
  • Historical Climate Data: Wegener’s research included climate data indicating continents had different climates in the past. For example, Spitsbergen had tropical plant fossils, which indicates it was located at the equator millions of years ago, supporting the notion of continental drift.
  • Figure 3: Displays differences in climate compared to current states, supporting Wegener's hypothesis of Pangaea.

Evaluating the Hypothesis and Theory Differences

Hypothesis vs. Theory
  • Hypothesis: A proposed explanation for a phenomenon that can be tested.
  • Theory: A widely accepted explanation based on a body of evidence that has been repeatedly tested and confirmed through observation and experimentation.
  • Popper's Scientific Method: Emphasizes that a good hypothesis must be testable and capable of being falsified through evidence.

Mid-Ocean Ridges and Sea-Floor Spreading

Mid-Ocean Ridges
  • Definition: Chains of undersea mountains formed by tectonic processes.
  • Significance: Key evidence that Earth's crust is made of movable plates. The Mid-Atlantic Ridge exemplifies this.
  • Mapping Advances: Significant efforts in oceanography led to the discovery of mid-ocean ridges, providing visual proof of the dynamic nature of Earth's lithosphere.
Sea-Floor Spreading
  • Process Description: Molten rock from the mantle rises through cracks in the crust at mid-ocean ridges and solidifies, forming new ocean floor.
  • Mechanism: The ocean floor on either side of the ridge moves away from the ridge, resembling a two-way conveyor belt, gradually pushing older material away.
  • Evidence of Volcanic Activity: Rocks shaped like pillows found in ridges affirm rapid cooling of molten rock.
Subduction Zones
  • Definition: Areas where one tectonic plate sinks beneath another, recycling oceanic crust back into the mantle.
  • Importance: Subduction explains the balance of sea-floor spreading; while new oceanic crust is formed, old crust must be recycled to maintain Earth’s size.

Future Implications of Plate Tectonics

Ocean Basin Evolution
  • Pacific Ocean: Currently surrounded by subduction zones, meaning it is shrinking as crust is recycled.
  • Atlantic Ocean: Continues to widen at a rate of 2 to 5 centimeters per year due to mid-ocean ridges.
  • Future Supercontinent Formation: Speculations about continental drift indicate that as tectonic plates continue to shift, a new supercontinent may eventually form in the distant future.

Summary and Assessment

  • Lesson Review: Students are asked if they understood the concepts of continental drift, the hypothesis, and key types of evidence supporting the theory. They will articulate thoughts and interpret data relevant to the history of continental drift and its implications for Earth's geological future.