Multiple choice  

Long-Term Changes - Rising and Falling Sea Levels


    Changing Shorelines

    Note that about 20,000 years ago the eastern shoreline of the United States was quite displaced from its present positions. Sea level was considerably lower then than it is today. Note the position of the red line. During the last five million years ago sea level has been within a band nearly 300 miles wide. [from the U.S. Geological Survey]

    Animations Illustrating Changes in the Coastline

      Some of the animations that follow are quite large so be prepared for a wait if you are connected via a slow modem. The wait will be worth it.

      If you are using Netscape or Explorer you might want to open two windows -- one for this web page and one for the animations. Otherwise uses the back arrow key to switch between the two sites.

      Begin by looking at the Distribution of the Earth's Population. The small image should suffice. Answer the question about the population distribution raised by the authors.

      Review the influence of the North Atlantic Current.

      If you are using a modem to connect, then try this locally distributed short animation. The 'Super-Interglacial' Alternative allows you to increase sea level up to 200 meters. The population distribution is shown in red.

      Click on the click here for full size bar beneath the animation if you are using a high speed line.

      Clicking on the mouse button and moving the cursor vertically will cause sea level to rise. Moving the cursor horizontally will allow you to focus on some part of the surface of the Earth. Position the Earth so that North America is at the top of the animation screen.

      The questions that follow are designed to help you maximize the use of the animation and to focus your thinking about the consequences of a major sea level rise. Click on your selection for the multiple choice questions. For the fill in the blank questions, position your cursor in the answer "rectangle", type in your response, and then push the Return key.

      1. Let sea level rise to 50 meters. The large embayment that appears along the southern margin of North America is situated at the site of the present day .

      2. At about what sea level rise does Florida "disappear"?

        10 meters
        20 meters
        30 meters
        40 meters

      3. At about 80 meters the river basin in North and South America that shows the greatest change is that of the ______

        Mississippi
        Amazon
        Nile
        Colorado

      4. At about what sea level rise are North and South America separated?

        20 meters
        60 meters
        80 meters
        greater than 80 meters

      5. In terms of overall shape of the continents, which of the following appear to show the least change in shape if sea level were to rise to 80 meters?

        North America
        South America
        Africa
        Europe

      6. In which of the following areas would a rise in sea level of 80 meters have the greatest impact on the Earth's population?

        India and China
        South America
        Africa
        Europe

      7. Kirkby and his colleagues note that marine Cretaceous sedimentary deposits (about 100 million years old) cover large areas that are well over 2000 meters above sea level. If melting all the present glaciers would only raise sea level a maximum of 80 meters, how could sea level have been higher during the Cretaceous? Note also that there was no known widespread glaciation in the Cretaceous.

        an increase in the rate of sea floor spreading
        a decrease in the rate of sea floor spreading
        a tremendous increase in the amount of water stored in the ocean basins

      8. If the rate of sea floor spreading were to decrease, one would predict a lowering of sea level (True or False) .

      9. Thus, both melting of glacial ice and an increase in the rate of sea floor spreading can cause sea level changes. During the last one million years (approximately) there have been at least four major advances (and retreats) of continental ice sheets. Each retreat was accompanied by a (rise or fall) in sea level.

      10. The Cretaceous period lasted more than 75 million years. As noted by Kirkby and his colleagues, this was a time of an extensive sea level rise. Therefore, with respect to rates, an increase in the rate of sea floor spreading effects sea level for a much (longer or shorter) time than does the retreat of continental glaciers.

      11. In general, sea levels have been rising for nearly the last 20,000 years. On might expect the entire Texas Gulf Coast to be experiencing a transgression - a rise in sea level causing the beaches to migrate landward. However, suppose that there is a local area in which there is an abundance of sand; for example, near the mouth of a river. If the rate of supply of sand exceeds the rise in sea level, the beach might actually exhibit a migration away from the present coast line (True or False)

      As noted previously " we basically understand that it is the changing of sea level relative to the land and the increase and decrease in sand supply to the coast that cause the shoreline to retreat or advance over a period of about 50 years or more."

      The following figure (fromTexas Shoreline Change Project Gulf of Mexico Shoreline Change from the Brazos River to Pass Cavallo - Texas Shoreline Change Project) shows the Average Annual Rate of Shoreline Change (in meters per year).

      Note that near the Matagorda Ship Channel, Green's Bayou and the Colorado River, that the shoreline is advancing in spite of an overall rise in sea level. Near Sargent Beach, however, the shoreline is retreating at a rate of about 8 meters per year.

      In the next section we will explore variations in the supply of sand that may help explain what is happening along the Texas Gulf Coast.

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