Download Mechanisms of Plate Motion

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;
1. 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.