2. Sea Floor Spreading

3. So… now that we know that about 250 million years ago there was one big super continent called Pangaea. What caused it to move apart?

4.  Scientists in the 1950’s and 60’s discovered a large system of underwater mountains that have a deep crack (called a rift valley) running through the center. These underwater mountains are known as midocean ridges.

5.  Volcanic activity occurs at the midocean ridges. The longest chain of mountains in the world. Lava erupts from the rift valleys, hardens and forms new ocean floor. This process is called sea-floor spreading and it helps explain how continents drift.  http://www2.nature.nps.gov/geology/usgsnps/animate/A4 8.gif http://www2.nature.nps.gov/geology/usgsnps/animate/A4 8.gif http://www2.nature.nps.gov/geology/usgsnps/animate/A4 8.gif

6. Evidence to support this:  Rock samples from the ocean floor indicate that rocks next to a midocean ridge are younger than rocks farther away.  Magnetic stripes in the rocks on the ocean floor.  Molten material has erupted again and again along the central valley of the midocean ridge.

7.  Trenches are deep V-shaped valleys in the bottom of the ocean. Older ocean floor moves down along the trenches and plunges back into the Earth; this is called SUBDUCTION.

8. The end