Physical Geology 101
Plate Motion

Plate Motivation

What force drives plate motion?
Wegener motivation model?
Problems with this model?
Hess motivation for seafloor spreading:


Convection is the transfer of heat due to circulating material.
Heat from Core is transferred Mantle.
Mantle becomes hot and rises due to lower density.
Mantle cools as heat transfers to crust. Sinks back down toward core.

Two Tier Convection

Convection in Lower Mantle, driven by heat from Outer Core.
Lower Mantle rises, transfers heat to Upper Mantle
Upper Mantle rises and transfers heat to crust.
Mantle spreads out, drags crust with it.
What is the driving force?

Plate Push

Mantle material is injected into MOR under pressure, forcing plates apart.

Plate Slide

Mid-ocean ridges are elevated areas.
Plates on either side of MOR slide down hill under influence of gravity.

Plate Pull

As plate descends into trench, it sinks faster than plate behind it.
Sinking plate pulls remaining plate with it.

Convection - Problems


Plate Push - Problems


Plate Slide - Problems


Plate Pull - Problems


Four Models - Which One?

Most geologist will agree that Convection plays a part in plate motivation.
Plate Push:
Plate Slide:
Plate Pull:

Model Confusion

Plate Pull and Plate Slide are often confused by geologists.
Many geologists will say plate pull is one of the main driving forces behind plate motivation.
But when asked to explain the mechanism - they describe what is occurring in plate slide - "gravity is pulling the plate into the trench"