1. 8.3 Causes of Plate Movements There is large body of evidence that plate tectonics move around. WHY DOES THIS HAPPEN????

2. Causes of Movement of Plates Tectonics  There are three main possible causes responsible for the movement of tectonic plates: 1. Convection currents in the upper mantle 2. Ridge push 3. Slab Pull

3. 1. Mantle Convection  The molten material under the crust is creates circular currents, called convection cells.

4. Mantle Convection Convection in a little more detail 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.

5. Convection made simple

6. 2. Ridge Push (E)

7. 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.

8. 3. Slab Pull

9.  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. slab pull.