|MadSci Network: Earth Sciences|
I can think of two possible ways to answer your question and I don't know which kind of answer you are looking for, so I will give you both. First, the water in the ocean, and indeed all the water in the world, is involved in a continuous process called the water cycle. Most of the water that is involved in the water cycle is in the ocean (more than 97%). Water leaves the ocean chiefly by evaporation, and it returns to the ocean by precipitation, via rivers and streams, and via undersea springs. The total amount of water in the ocean doesn't change much. During the ice ages sea level fell by more than 100 meters (more than 300 feet), and if all of the existing glacial ice melted sea level would rise by an alarming but smaller amount. However, from year to year the ocean volume changes little. Second, one can ask the question: Where did all this water come from in the first place? As I understand it, there are two main sources: primary water, and water from comets. When the earth coalesced out of the primordial gas cloud that surrounded the young sun, hydrogen and oxygen were two of the chemical elements incorporated in the earth. Hydrogen is of course by far the most common element everywhere in the universe: you can't form a planet without hydrogen. Oxygen is less abundant in the universe, but it is a common element too, if you don't consider the "biggies", hydrogen and helium. Oxygen and hydrogen are chemically quite reactive, and so they both combined with various other elements to form chemical compounds. One of these compounds is water, and water quickly became an important part of the earth's surficial layer, along with many other familiar chemicals (silica - a major component of sand, calcium carbonate - what limestone is made of, etc.). If water existed on earth only as a solid, then there would be no oceans as we know them, but water can readily exist as a liquid under the pressure and temperature conditions that hold on many parts of the earth's surface. Liquids run downhill, and so liquid water accumulated in low places. There is a lot of water, so some low places contain huge amounts of it - we call these oceans. Over the past 4.5 billion years earth has been hit by many comets. We know that comets consist chiefly of chemical compounds that are either gases or liquids at earth-surface conditions: methane is one common component of comets and water is another. A significant amount of earth's water has probably been delivered by comets, especially in the first billion years or so after earth formed. During this time the solar system was filled with debris: small objects that coalesced from the primordial gas cloud. After a billion years or so, most of these objects had hit something, and airless bodies like our moon show many huge craters dating from this period. The Oort cloud of comets out past Pluto and the asteroid belt between Mars and Jupiter contain many of the small objects that have not yet slammed into planets or the sun. Hope this helps. Sincerely, David C. Kopaska-Merkel Head, Ground Water Section Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 web site www.gsa.state.al.us
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