1. The continents were once a single landmass surrounded by on large ocean.

2. Theory of Continental Drift Rocks have different density, so a less dense rock will ‘float’ on a more dense rock. The continental shelf (part of the lithosphere) is lighter than the asthenosphere.

3. Theory of Continental Drift This process is called buoyancy. This process is called buoyancy. The lighter and heavier materials will form a balance called an ‘isostatic equilibrium’. The lighter and heavier materials will form a balance called an ‘isostatic equilibrium’.

4. Theory of Continental Drift The continents ‘float’ on the asthenosphere like a boat floats on the ocean. The continents ‘float’ on the asthenosphere like a boat floats on the ocean. All they need is a ‘current’ to move them. All they need is a ‘current’ to move them.

5. Theory of Continental Drift Originally, there was one huge continent called Pangaea. Originally, there was one huge continent called Pangaea. It was surrounded by a single ocean called Panthalassa It was surrounded by a single ocean called Panthalassa

6. Theory of Continental Drift Alfred Wegener developed his theory in 1912 based on the fossil record. Alfred Wegener developed his theory in 1912 based on the fossil record. No one believed him. He was a meteorologist and couldn’t explain how it could happen No one believed him. He was a meteorologist and couldn’t explain how it could happen

7. Theory of Continental Drift In the 40’s, new technology revealed physical characteristics on the sea floor that showed how the continents might ‘drift.’

8. Theory of Continental Drift They revealed mid- ocean ridges, trenches and rift valleys.

9. How the Sea Floor Spreads

10. Harry Hess and Robert Dietz suggested an explanation of both how the sea floor could expand and ‘push’ the continents over the face of the Earth.

11. How the Sea Floor Spreads The denser (heavier) ocean crust slides under the less dense (lighter) continental crust forcing the continents further apart.

12. How the Sea Floor Spreads When the spreading seafloor reaches a continent, it is pushed below the continent in a subduction zone.

13. How the Sea Floor Spreads This brings the sea floor with it’s minerals and water below the continent.

14. How the Sea Floor Spreads Friction causes the sea floor to melt. The lighter magma flows upwards through cracks, forming volcanoes.

15. The Ring of Fire

16. This can be seen in the ‘ring’ of active volcanoes surrounding the Pacific Ocean.

17. The Ring of Fire These are the plates that form these subduction zones

18. The Ring of Fire