Today we know how fast plates move, which directions they move in, what happens to them at boundaries.  But, what makes them move?

Convection in the mantle
Convection occurs in circular units of movements called convection cells.  Heating of rock material causes it to move in convection cells.  Scientists agree that convection causes plate movement.  They do not agree on exactly how it occurs in the mantle.

Models for plate movement
    How convection occurs in the mantle:
            Convection occurs in the asthenosphere
            Convection occurs in the entire mantle

    How convection moves the plates:
            Plates are carried along by movement of convection
            (like a giant conveyor belt.)  If convection is pushing
            upward, then divergent boundaries form.  Where
            convection goes back down, convergent boundaries form.

            Each plate is part of a convection cell.  A cooler plate is
            denser and tends to sink.  Its sinking PULLS the rest of
            the plate down and helps create the circular movement of
            convection.

Convection in your house

• In the shower - warm shower, cold air from outside of the shower rushes in, hot air escapes from above the shower curtain, a convection cell has been created.
• Heating vents - warm air enters through vent, the warm air rises, colder air moves in to take its place, it is heated and then it rises, colder air moves in  …
• Cooking - heat from the burner transfers from the bottom of the pan, water at the bottom becomes hot, moves upward; colder (and denser) water moves downward; the convection cell spreads heat until all water is equally heated.


Convection cells heat matter in liquids and gases.