Transcript 8.3 Causes of Plate Movements

8.3 Causes of Plate
Movements
There is large body of
evidence that plate
tectonics move around.
WHY DOES THIS
HAPPEN????
Causes of Movement of Plates
Tectonics

1.
2.
3.
There are three main possible
causes responsible for the
movement of tectonic plates:
Convection currents in the upper
mantle
Ridge push
Slab Pull
1. Mantle Convection

The molten material under the crust is creates
circular currents, called convection cells.
Mantle Convection

Convection in a little more detail
Convection takes place primarily because buoyancy forces
are able to overcome viscous resistance. When a fluid in a
container is heated from a central source below, it expands
in the region of heating. In doing so it becomes less dense,
and hence wants to move upward toward the surface. The
surface above the heated region will also rise in response to
expansion of the heated fluid. This lighter fluid that has
risen to the surface will flow outward toward the edges of
the container where it will encounter the cold edge of the
container, cool down and, in doing so, become less dense
and sink toward the bottom of the container. The collective
effect is to set up a conveyer type motion with fluid rising in
the center above the heated region, moving outward at the
surface then down the sides. The overall effect is a
circulation of material in two relatively simple cells, as
illustrated in the cartoon below.
Convection made simple
2. Ridge Push (E)
2. Ridge Push
According to this hypothesis, the magma
that arrives at the surface of the crust
through the rift valley (on the bottom of
the ocean) cools down and solidifies; as it
becomes solid, it becomes denser and
heavier, and it slides away from the ridge
down the sloping astenosphere.
 The ridge push affects, therefore, the
divergent boundaries between tectonic
plates.

3. Slab Pull
3. Slab Pull
 At
convergent boundaries the denser
and heavier plate subducts beneath
the plate that is less dense.
 As this heavy plate sinks, it pulls the
rest of the plate along with it.
 The force that the sinking edge of
the plate exerts on the rest of the
plate is called
slab pull.