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Sea Level DOES Change Saturday, April 19, 2008

    The more we study it, the more the Earth gets, "curioser and curioser", (to quote Poo and Piglet).  Take for example, sea level change.  Mystery #3 asks us why shark's teeth are found in a muddy rock layer exposed in a cliff way above a modern beach.  We learn that water deep enough for sharks to swim in once covered the land where today the shark's teeth layer is found.   So how can this happen?

    Well, if the amount of water in the ocean were to increase  while the ocean basin stayed the same size - then, like water in a pot, the sea level would rise.  And as we are learning, melting glaciers can cause the amount of water in our ocean basins to increase.

    But there's more.  Let's say there are huge pressures slowly  pushing the land up.  We know this happens because satellites swinging around the Earth measure land elevation very, very accurately and we have learned that the land surface moves vertically up and down!

    OK, so we've got two reasons for sea level to change - can anyone think of others?   How about changing the volume of the ocean basins?  Imagine, not a pot, but water ponded in the middle of a large waterproof sheet.  Now, let's push the sheet up from underneath.  The water level rises because we have made the "ocean basin" smaller!  Yep.  Many scientists think that's what happened about 100 million years ago when much of Europe was under water some 250 to 300 meters (about 750 to 900 feet) deep; and, what is now the United States was two large islands with a shallow sea between them.

      This was kind of an unusual time in  the history of the Earth.  The climate was warm and the sea was teeming with little plants called  coccolithophorids.  Shells from these critters accumulated on the ocean bottom and have made thick layers of white chalk including the White Cliffs of Dover in southern England and a well-known Texas rock called the Austin Chalk.  There's more about what happened 100 million years ago in Mystery #161.

    So now we know there are several ways that sea level can change.  Is your head buzzing and your imagination stretched thin?  Well, check out Mystery #3, and welcome to geology - where there are always scientists to suggest why the Earth works the way it does.

posted by Ruth Deike at 10:27 am

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