Overview
The lecture explains the concept of seafloor spreading, its role in plate tectonics, and discusses four main pieces of evidence supporting the theory.
Background and Development of the Theory
- Alfred Wegener proposed continental drift, suggesting continents move over time.
- Early ocean floor mapping was crude and unreliable before sonar technology.
- Sonar mapping post-WWII revealed varied seafloor geology, not a flat plain.
Evidence Supporting Seafloor Spreading
1. Ocean Floor Geology
- The mid-ocean ridge is a jagged underwater mountain range mirroring the shape of continents.
- Active fractures and features on the ocean floor suggest movement and interaction of plates.
2. Age of Seafloor Rocks
- Rocks at the mid-ocean ridge are youngest; age increases with distance from the ridge.
- Youngest rocks form at the ridge where magma emerges and solidifies.
3. Thickness of Sediment Layers
- Sediment is thinnest near the ridge and thickens with distance, indicating older underlying rocks further away.
- Thicker sediments accumulate over older rocks due to longer exposure time to deposition.
4. Magnetic Patterns in the Seafloor
- Earth's magnetic field reverses periodically.
- As magma solidifies, minerals align with the current magnetic field, recording reversals in stripes parallel to the ridge.
- Matching magnetic patterns on both sides of the ridge provide evidence of seafloor spreading.
Key Terms & Definitions
- Continental Drift — theory that continents move slowly over Earth’s surface.
- Seafloor Spreading — the process by which new oceanic crust forms at ridges and moves outward.
- Mid-Ocean Ridge — an underwater mountain range where new ocean floor is created.
- Lithosphere — Earth's outer, rigid shell, including crust and upper mantle.
- Sediment — particles of sand, clay, and organic matter that settle on the ocean floor.
- Magnetic Reversal — periodic switch in Earth's magnetic north and south.
- Divergent Plate Boundary — region where tectonic plates move apart.
Action Items / Next Steps
- Review diagrams of mid-ocean ridges, sediment thickness, and magnetic striping for better understanding.
- Prepare for discussions or quizzes on the four main evidences of seafloor spreading.