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About 250 million years ago, Earth housed a supercontinent called Pangaea. The movement of this landmass is explained through sea-floor spreading, a process discovered in the 1950s and 60s. It occurs at mid-ocean ridges, where underwater volcanic activity creates new ocean floor as lava erupts and hardens. Evidence of this process includes rock samples revealing younger rocks near ridges and magnetic stripes in ocean floor rocks. Additionally, older ocean floor is recycled back into the Earth through subduction at oceanic trenches.
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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?
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.
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/A48.gif
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.
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.
