2. Plate Tectonics Tectonic plates move or float on top of the upper mantle. However they do not float freely. The plates are forced in specific directions by the flow of magma beneath.

3. Plate Tectonics – cont’d Plates move with the flow of magma. Plates move with the flow of magma. The magma closer to the core heats and then rises towards the surface as its density decreases. The magma closer to the core heats and then rises towards the surface as its density decreases. Once the rising magma reaches the lithosphere it moves in opposite directions. Once the rising magma reaches the lithosphere it moves in opposite directions. The magma forms convectional currents. The magma forms convectional currents.

4. Plate Tectonics – Convection Currents

6. Divergent Boundaries Tensional Forces occur where two tectonic plates are pulled apart. The tension is created as the plates move away from each other. Ridge Zones sometimes occur where two plates move apart. The magma rises between the plates and forms a ridge. Again caused by convectional currents in the magma

7. Tensional Forces – Ridge Zones This diagram above shows “Sea Floor Spreading”

8. Where’s the TENSION?

9. Convergent Boundaries Compressional Forces occur where two tectonic plates come together. They compress against each other. Compressional Forces occur where two tectonic plates come together. They compress against each other. Subduction Zones sometimes occur where compressional forces result from two plates colliding and one plate slips under the other. Subduction Zones sometimes occur where compressional forces result from two plates colliding and one plate slips under the other. Again caused by convectional currents in the magma Again caused by convectional currents in the magma

10. Compressional Forces – Subduction

11. Where’s the Subduction / Compression?

12. More about Plate Tectonics our text p. 10 - 12 watch it at: http://www.pbs.org/wgbh/aso/tryit/t ectonics/intro.html http://www.pbs.org/wgbh/aso/tryit/t ectonics/intro.html