Overview
This lesson explains the mechanisms behind tectonic plate movement, detailing the shift from early theories to the currently accepted mantle convection model.
Early Theories of Plate Movement
- Continental drift theory lacked a convincing explanation for plate movement, leading to skepticism.
- Permanentism was the belief that continents and ocean basins are permanent, unchanging features.
- Wegener proposed pole-fleeing force and centrifugal force as drivers of continental drift, but these were rejected due to insufficient force.
Discoveries from Ocean Floor Mapping
- Cold War-era submarine mapping produced detailed maps of the ocean floor, revolutionizing geology.
- Harry Hess identified mid-ocean ridges as sites where new crust forms and spreads apart.
- The mid-Atlantic ridge is a prime example of mid-ocean ridge causing plate separation.
Seafloor Spreading and Subduction
- New crust forms at mid-ocean ridges and moves outward until it reaches deep ocean trenches.
- Trenches are subduction zones where denser plates sink under less dense plates.
- In oceanic-continental subduction, the oceanic plate subducts because it is denser.
- In oceanic-oceanic subduction, the older or colder plate subducts as it is denser.
Mechanisms Driving Plate Movements
- Gravity-driven mechanisms include slab pull (plate pulled into trench) and ridge push (plate pushed from ridge), but gravity is too weak to be the main cause.
- Mantle convection theory states that heat from Earth's core causes magma to rise and push plates at ridges, with cooled magma sinking at trenches.
- This convection cycle is powered by the Earth's internal heat and explains the movement of tectonic plates.
Key Terms & Definitions
- Permanentism — the belief that continents and ocean basins are permanent and unchanging.
- Pole-fleeing force — Wegener's idea that centrifugal force caused plates to drift from the poles.
- Mid-ocean ridge — an underwater mountain range where new oceanic crust is produced.
- Seafloor spreading — creation of new ocean crust at mid-ocean ridges and its outward movement.
- Trench (Subduction zone) — deep ocean regions where one tectonic plate sinks under another.
- Slab pull — the force as a subducting plate is pulled into the mantle by gravity.
- Ridge push — the force pushing plates away from a mid-ocean ridge.
- Mantle convection — movement of magma in the mantle due to heat, driving plate movement.
Action Items / Next Steps
- Review the differences between gravity-driven mechanisms and mantle convection.
- Study diagrams of mid-ocean ridges and subduction zones for better understanding.