HOW DEEP IS THE OCEAN (AND WHERE)?
The mantle that is in contact with the crust cools and attaches to the bottom of the crust forming the lithosphere. As this cooling occurs the attached mantle is cooler and denser than the hot mantle from which it was formed. As the lithosphere continues to cool it gets thicker and heavier which causes the plate in this area to be denser and heavier than the supporting mantle (aesthenosphere). This thicker and heavier lithosphere presses down into the aesthenosphere, causing the ocean to be deeper in direct relation to the age of the floor underneath it. In other words, older ocean floor is found at the deeper parts of the ocean, and younger ocean floor is found at relatively shallower parts of the ocean. Deeper – subduction zones, shallow – mid ocean ridge spreading zones.)
When the older, and thus thicker and heavier, lithosphere reaches a subduction zone, it can and does sink into the aesthenosphere below due to the difference in density between the two layers. It is now thought that this sinking may be the driving force in plate tectonics. Once the edge of the plate at the subduction zone begins to sink, it pulls the rest of the plate along with it.
Research conducted at the University of Texas at Dallas by researchers and teachers aboard the RV Melville in March and April of 2001, collected data that supports the this current model.
Resources and Reference Materials
http://www.utdallas.edu/dept/SciMathEd/melville.htm (Subduction Factory Expedition from University of Texas at Dallas) http://www.amnh.org/home/index.html (American Museum of Natural History)
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