Overview
This lecture discusses the three main forces responsible for plate tectonics: convection currents, slab pull, and ridge push, and explains how each contributes to plate movement.
Convection Current
- Convection currents move heat within Earth’s mantle, driving the movement of tectonic plates.
- Convection involves hot, less dense mantle rising and cooler, denser material sinking, forming convection cells.
- This motion acts like a conveyor belt, pushing magma upward to form new crust and causing seafloor spreading.
- Sinking cold magma pulls plates together, leading to subduction.
- The theory of mantle convection was proposed by Arthur Holmes, who theorized that mantle motion drives continental drift.
Slab Pull Theory
- Slab pull states that gravity acting on subducting (sinking) oceanic plates drives plate movement.
- As a dense tectonic plate subducts into the mantle, it pulls the rest of the plate along with it.
- The heavier (denser) the slab, the stronger the pulling force acting on the plate.
- Slab pull theory suggests that mantle convection is a result, not a cause, of plate movement.
Ridge Push
- Ridge push occurs at mid-ocean ridges where the lithosphere slides down the higher, hotter ridge due to gravity.
- The elevated position of oceanic ridges causes plates to slide away from the ridge toward subduction zones.
- The difference in gravitational potential energy between the spreading center and the subduction zone drives this movement.
- As the plate moves away from the ridge, it becomes thicker and denser, increasing the ridge push force toward the subduction zone.
Key Terms & Definitions
- Convection Current — A circular flow in the mantle driven by the rise of hot material and sinking of cool material.
- Slab Pull — The force caused by the sinking of a cold, dense plate into the mantle at a subduction zone.
- Ridge Push — The sliding of a plate away from a raised mid-ocean ridge due to gravity.
- Asthenosphere — The soft, plastic layer of the mantle beneath the lithosphere that enables plate movement.
- Subduction — The process where one tectonic plate sinks beneath another into the mantle.
Action Items / Next Steps
- Be able to describe convection current, slab pull, and ridge push.
- Prepare to explain the causes of plate tectonics in class discussion.