9-4 Mechanisms of Plate Motion

Causes of Plate Motions -Scientists generally agree that convection currents in the mantle provide the basic driving force for plate motion. Convection Flow – the hot matter goes up as it is less dense and lighter in weight and the cold solid matter comes down as it is heavier and more dense. The slow movements of the plates and mantle are driven by the unequal distribution of Earth’s heat from the radioactive isotopes in the mantle such as uranium and thorium.

Convective Flow

Slab-Pull and Ridge-Push -Slab-Pull :The force of the gravity pulls old oceanic crust ( lithosphere) which is relatively cool and dense down into the deep mantle.

-Ridge-Push :causes the stiff oceanic lithosphere to slide down the sides of the oceanic ridge as a result of gravity. Ridge-push an slab-pull are acting together moving ocean lithosphere from mid-ocean ridges toward subduction zones and then down into the mantle. Downward flow of subducted ocean lithosphere must be equal to upward flow of rock back toward the surface.

Mantle Convection Mantle Plume is a raising column of hot solid mantle rock. Scientists have two opinions; Whole Mantle Convection When slabs of cold oceanic lithosphere descend into the lower mantle, at the same time, hot mantle plumes originating near the mantle-core boundary transfer heat toward the surface. 2. Deep-layer model; when a rock replaces sinking ocean lithosphere through a slow broad rise of rock throughout the mantle. The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion

Whole-Mantle Convection

Deep-Layer Model

Answer the Assessment questions on page 271 # 1-5.