Earth’s Mechanism: Mantle Convection & Plate Motion – Quick Review

Lithosphere & Plates

• Lithosphere = crust + uppermost mantle; broken into rigid sections (plates).
• Plates “float” on the soft, plastic-like $\text{asthenosphere}$.

Plate Tectonic Theory

• Earth’s surface is fragmented into slow-moving plates.
• Interactions occur at three main boundaries:
– Convergent ➜ subduction, mountains, volcanoes, trenches, earthquakes.
– Divergent ➜ seafloor spreading, rift valleys, mid-ocean ridges.
– Transform ➜ lateral slip faults, earthquakes.

Mantle Convection (Core Mechanism)

• Heat source: radioactive decay in the core and lower mantle.
• Sequence:

1. Core heats lower mantle $\Rightarrow$ hot, less-dense magma rises.
2. Near surface it cools, contracts, becomes denser and sinks.
3. Continuous rise–sink cycle forms convection cells.
   • Result: horizontal drag on the base of plates, initiating and sustaining motion.

Additional Driving Forces

• Ridge Push (gravitational sliding): newly formed, higher-elevation lithosphere at mid-ocean ridges slides downslope, pushing older crust away.
• Slab Pull: cold, dense, subducting plate edge sinks and pulls the trailing plate into the trench.
• Mantle Convection provides the overall energy; ridge push and slab pull are surface expressions of that energy.

Heat-Transfer Modes

• Convection – movement of heated fluid/magma (main mantle process).
• Conduction – molecule-to-molecule transfer (e.g., through solid rock).
• Radiation – emission of energy as waves (minor inside Earth).

Key Definitions & Values

• Density: mass per unit volume; warmer material $\Downarrow$ density.
• Subduction Zone: region where one plate descends beneath another.
• Divergent Boundary: plates move apart; magma upwells.
• Convection Current: circulating flow driven by $\Delta T$ and $\Delta \rho$ (temperature & density differences).