Overview
This lecture discusses the driving forces behind plate tectonics: convection currents, slab pull, and ridge push, explaining how each contributes to plate movement.
Convection Currents
- Convection currents drive plate movement in the mantle by circulating hot, soft rock in a loop (convection cell).
- Heat causes mantle material to become less dense, rise, and as it cools, it sinks, creating a conveyor-belt motion.
- This circulating motion pushes magma up to form new crust (seafloor spreading) and pulls plates apart or together.
- Arthur Holmes proposed mantle convection as a key cause of continental drift and tectonic movement.
Slab Pull Theory
- Slab pull states that gravity pulls dense, subducting tectonic plates into the mantle at subduction zones.
- The weight of a descending plate (slab) drags the rest of the plate with it, causing movement.
- Dense oceanic plates sink more quickly, exerting a stronger force on the plate.
- This theory suggests plate motion is mainly driven by subduction rather than mantle convection.
Ridge Push
- Ridge push occurs at mid-ocean ridges where the lithosphere slides down the elevated, hot asthenosphere beneath.
- Gravity causes plates to slide away from the ridge toward subduction zones due to gravitational potential differences.
- The asthenosphere acts as a soft, low-density layer, allowing the lithospheric plate to slide.
- Ridge push force increases as the plate moves farther from the ridge and becomes thicker and denser.
Key Terms & Definitions
- Convection Current — circular flow of mantle material caused by heat, driving plate movement.
- Slab Pull — force from the weight of a sinking, dense plate at a subduction zone pulling the rest of the plate.
- Ridge Push — movement of plates away from mid-ocean ridges due to gravity sliding down the elevated region.
- Asthenosphere — soft, plastic-like layer of the mantle beneath the lithosphere.
- Subduction — process where one tectonic plate sinks beneath another at convergent boundaries.
Action Items / Next Steps
- Be able to describe and explain convection currents, slab pull, and ridge push as causes of plate tectonics.