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    Test Reading 1

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    Mahdi Khalili

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    Test Reading 1

    Uploaded by

    Mahdi Khalili
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    test reading 1

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    30 views2 pages

    Test Reading 1

    Uploaded by

    Mahdi Khalili

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    © All Rights Reserved
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    The Geologic History of the Mediterranean

    In 1970 geologists Kenneth J. Hsu and William B. F. Ryan were collecting


    research data while aboard the oceanographic research vessel Glomar
    Challenger. An objective of this particular cruise was to investigate the floor of
    the Mediterranean and to resolve questions about its geologic history. One
    question was related to evidence that the invertebrate fauna (animals without
    spines) of the Mediterranean had changed abruptly about 6 million years ago.
    Most of the older organisms were nearly wiped out, although a few hardy species
    survived. A few managed to migrate into the Atlantic. Somewhat later, the
    migrants returned, bringing new species with them. Why did the near extinction
    and migrations occur?

    Another task for the Glomar Challenger’s scientists was to try to determine
    the origin of the domelike masses buried deep beneath the Mediterranean
    seafloor. These structures had been detected years earlier by echo-sounding
    instruments, but they had never been penetrated in the course of drilling. Were
    they salt domes such as are common along the United States Gulf Coast, and if
    so, why should there have been so much solid crystalline salt beneath the floor of
    the Mediterranean?

    With questions such as these clearly before them, the scientists aboard the
    Glomar Challenger proceeded to the Mediterranean to search for the answers.
    On August 23, 1970, they recovered a sample. The sample consisted of pebbles
    of hardened sediment that had once been soft, deep-sea mud, as well as
    granules of gypsum [1] and fragments of volcanic rock. Not a single pebble was
    found that might have indicated that the pebbles came from the nearby continent.
    In the days following, samples of solid gypsum were repeatedly brought on deck
    as drilling operations penetrated the seafloor. Furthermore, the gypsum was
    found to possess peculiarities of composition and structure that suggested it had
    formed on desert flats. Sediment above and below the gypsum layer contained
    tiny marine fossils, indicating open-ocean conditions. As they drilled into the
    central and deepest part of the Mediterranean basin, the scientists took solid,
    shiny, crystalline salt from the core barrel. Interbedded with the salt were thin
    layers of what appeared to be windblown silt.

    The time had come to formulate a hypothesis. The investigators theorized


    that about 20 million years ago, the Mediterranean was a broad seaway linked to
    the Atlantic by two narrow straits. Crustal movements closed the straits, and the
    landlocked Mediterranean began to evaporate. Increasing salinity caused by the
    evaporation resulted in the extermination of scores of invertebrate species. Only
    a few organisms especially tolerant of very salty conditions remained. As
    evaporation continued, the remaining brine (salt water) became so dense that the
    calcium sulfate of the hard layer was precipitated. In the central deeper part of
    the basin, the last of the brine evaporated to precipitate more soluble sodium
    chloride (salt). Later, under the weight of overlying sediments, this salt flowed
    plastically upward to form salt domes. Before this happened, however, the
    Mediterranean was a vast desert 3,000 meters deep. Then, about 5.5 million
    years ago came the deluge. As a result of crustal adjustments and faulting, the
    Strait of Gibraltar, where the Mediterranean now connects to the Atlantic,
    opened, and water cascaded spectacularly back into the Mediterranean.
    Turbulent waters tore into the hardened salt flats, broke them up, and ground
    them into the pebbles observed in the first sample taken by the Challenger. As
    the basin was refilled, normal marine organisms returned. Soon layers of oceanic
    ooze began to accumulate above the old hard layer.

    The salt and gypsum, the faunal changes, and the unusual gravel provided
    abundant evidence that the Mediterranean was once a desert.

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