|MadSci Network: Earth Sciences|
Paul Fresh water is lighter than salt water, and so fresh water floats on sea water. Gradually the two mix and most "pools" of fresh water on the sea do not persist for long. The most long-lived are probably the effluent from major rivers, where large amounts of fresh river water flow out onto the sea. Glacial ice is also fresh, and melting ice bergs produced short-lived freshened parts of the sea's surface. Blue holes, as I understand it, do not open on the sea floor, but open onto the land surface of Bahamian islands. The fresh water in the blue holes extends below sea level because the fresh ground water within an island forms a lens. The top of the fresh water lens is above sea level, and the weight of this "extra" water pushes the bottom of the lens below sea level. However, below the fresh ground water in an island there is salt water. Fresh water can be denser than sea water. The most common way this happens is to have a lot of sediment suspended in the fresh water, making it turbid. Turbid water can flow along the bottom of the sea, or even at some intermediate level within the sea, depending on the exact density relationships among the different water masses in the sea. Temperature also affects water density, but sea water is so much denser than clean fresh water that temperature by itself is not enough to make fresh water denser than sea water. You can find discussions of these questions in a sedimentology or oceanography textbook and many physical geology textbooks. This is one sedimentology textbook: Friedman, G. M., Sanders, J. E., and Kopaska-Merkel, D. C., 1992, Principles of Sedimentary Deposits, Macmillan Publ. Co., New York, 717 p. You can find many other books that will help you with this question at any university library and many public libraries. David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 www.gsa.state.al.us
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