earth science 9.4 mechanisms of plate motion mechanisms of plate motion

Earth Science 9.4 Mechanisms of Plate Motion

Mechanisms of Plate Motion


Convection Currents

When you heat your home you notice warm air rises and cool air sinks. This is an example of convection currents.


Convection Currents

A convection current is a continuous flow that occurs in a fluid or gas because of differences in density. Warm material is less dense, so it rises. Cooler material is more dense, so it sinks.


Convection Currents

Convection currents in the mantle of the Earth provide the basic driving forces for plate motion.

The hot but solid rock of the mantle behaves in a plastic way over long periods of geologic time. It can flow very slowly circulating within the Earth.


Convection Currents

The main heat source for mantle convection is the energy released by radioactive isotopes in the mantle, such as uranium, thorium, and potassium.

Another source is heat from the Earth’s core.

Since most of the heat comes from the mantle, a bowl of soup in a microwave oven is a better analogy for this process than a pot on a stove.


Convection Currents

But how does mantle convection produce plate motions?

The plates are simply the top part of the mantle convection currents.

The weakness of the asthenosphere allows the stiff lithosphere to slide across it.


Convection Currents

At the “top” of these convection currents, ocean plates cool and become denser than the mantle rock beneath them.

As a result, an ocean plate will begin to subduct beneath another plate.


Convection Currents

The greater density of the cold ocean plate causes it to sink all the way down to the base of the mantle.

Rock from the lower mantle rises into the upper mantle and reaches the surface at mid-ocean ridges, where new ocean floor is formed.


Convection Currents

This cyclic flow of mantle rock, which may take a half-billion years, is called whole mantle convection.


Convection Currents

The sinking of cold ocean lithosphere directly drives the motions of mantle convection through slab-pull and ridge-push.

In slab-pull, the force of gravity pulls old ocean lithosphere , which is relatively cold and dense, down into the deep mantle.


Convection Currents

In ridge-push, the newly formed lithosphere slides down the sides of the mid-ocean mountains after it emerges from the Earth as undersea lava.

This downward slide is the result of gravity acting on the cool and dense oceanic lithosphere ( the newly created ocean floor).


Convection Currents

Acting together, slab-pull and ridge-push move ocean lithosphere from mid-ocean ridges toward subduction zones and than down into the mantle.


Convection Currents

Because Earth is not growing or shrinking in size, the downward flow of subducted ocean lithosphere must equal the upward flow of rock back up toward the surface.

Scientists are presently debating how this happens.


Convection Currents

Some scientists think that most upwelling of mantle rocks occurs in the form of hot-spot mantle plumes.

A mantle plume is a rising column of hot solid mantle rock.


Convection Currents

Other scientists think that large mantle plumes DO NOT EXIST. They think that rock replaces sinking ocean lithosphere through a slow, broad rise of rock throughout the mantle .

Most scientists think that both process are involved.

Computer Lab Assignment: Use the internet to research and find the answer to

the following questions: DO NOT COPY CUT OR PASTE Write one paragraph (five full sentences) one each question.

How are plate motions connected with motions in the other parts of the Earth’s mantle?

How are the forces of slab-pull and ridge-push related to plate motions?