Overview
This lecture explains the theory of seafloor spreading, the evidence supporting it, and how it strengthens the theory of continental drift by showing how oceanic plates move.
Review of Continental Drift and Its Problems
- Alfred Wegener proposed the continental drift theory, suggesting continents move across Earth's surface.
- Early critics doubted Wegener's theory due to the lack of explanation for how plates move.
Discovery of Ocean Floor Features
- In the 1930s, sonar technology was used to map the ocean floor, revealing features like the mid-ocean ridge.
- Sonar works by sending sound waves underwater and measuring their reflection time to determine depth.
Theory of Seafloor Spreading
- Harry Hess used sonar data to develop the seafloor spreading theory.
- Seafloor spreading is the process where new oceanic crust forms at mid-ocean ridges and moves outward, pushing older crust towards subduction zones.
Evidence for Seafloor Spreading
- Youngest oceanic rocks are found near the mid-ocean ridge; oldest rocks are farther away, showing new crust forms at the ridge.
- Formation of new crust at ridges is balanced (but not always equal) with destruction at subduction zones.
- If spreading is faster than subduction, oceans widen (e.g., Atlantic Ocean); if subduction is faster, oceans shrink.
Magnetic Reversal Evidence
- Earth's magnetic poles periodically switch places, called magnetic reversal.
- As magma cools at the ridge, minerals align with the current magnetic field, creating symmetrical magnetic patterns on both sides of the ridge.
- These patterns are evidence of seafloor spreading and mid-ocean ridge movement.
Plate Movement and Mantle Convection
- Seafloor spreading demonstrates that oceans are sites of plate activity.
- New crust forms at mid-ocean ridges, old crust is destroyed at subduction zones.
- Plate movement is driven by convection currents in the Earth's mantle.
Key Terms & Definitions
- Sonar — Device that uses sound waves to detect underwater objects and map the seafloor.
- Mid-ocean Ridge — An underwater mountain range where new oceanic crust forms.
- Seafloor Spreading — Process of new oceanic crust forming at ridges and moving outward.
- Subduction Zone — Area where old oceanic crust is pushed back into the mantle and destroyed.
- Magnetic Reversal — The switching of Earth's magnetic poles, recorded in rocks.
Action Items / Next Steps
- Review diagrams of seafloor spreading and magnetic reversal.
- Read assigned textbook sections on plate tectonics and oceanic crust formation.