Seafloor Spreading and Magnetic Reversals Wegener (1915 ...w3. bhubeny/GLS100/c1. آ 

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    Seafloor Spreading and Magnetic Reversals

    Brad Hubeny

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    Continental Drift

    • Predecessor to modern plate tectonics • Shape and “fit” of the continents was

    the initial evidence – Snider-Pelligrini (1858) – Taylor (1908) – Wegener (1915)

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    Wegener (1915): Continental Drift • Proposed that all of the continents were once part of a

    large supercontinent - Pangaea • Based on:

    • Similarities in shorelines • Terrestrial Geologic Evidence • Distinctive fossil groups found in Africa & South

    America

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    Terrestrial Geologic Evidence

    Only occur in rocks > 145 Ma BP!

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    Paleontological evidence •No mechanism to transport across ocean •Divergence of species following break-up

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    Seafloor Spreading

    • Wegner proposed a mechanism for drift – Less dense silicic rocks plowed through more

    dense ocean floor – Earth’s rotation was driving force

    • Although supporting evidence existed, the theory was not widely accepted

    • Lack of testable hypothesis!!

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    Development of Plate Tectonic Theory

    • Original evidence for continental drift was from continental rocks

    • Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor

    • New data provided intriguing new information!

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    Geology of the Ocean Floor

    • Doc Ewing does NOT find granite when he dredges the ocean bottom!!

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    Geology of the Ocean Floor • Bathymetry of the

    ocean basins – Sonar revealed Ridge

    System in ocean basins – Ridge system is

    continuous around the entire globe

    – Central rift valley within the ridge

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    Bathymetry Reveals Oceanic

    Rift System

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    Earthquakes and Volcanoes

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    Harry Hess’ “Essay in Geopoetry”

    • Seafloor spreading proposed in 1960 paper • Considered new data on ocean floor • Proposed NEW more robust mechanisms • Great HYPOTHESIS, but needed to be

    tested

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    Enter Paleomagnetics!

    • Magnetic reversals – Earth’s magnetic field polarity has reversed

    through time • Normal polarity – Nmagnetic = Ngeographic • Reversed polarity - Nmagnetic = Sgeographic

    • Iron-rich basalts of ocean crust preserve this record of the magnetic field!

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    Earth’s Magnetic Field Geodynamo System!

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    02_10.jpg Geologic Magnetic Recorders

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    Polarity Reversals

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    02_23.jpg Reversals & the Geologic Record

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    Geomagnetic Time Scale

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    • Vine & Matthews (1963) tested Hess’s hypothesis using magnetism – Magnetic polarity reversals recorded in

    ocean floor basalt • Magma cools forming new crust • Polarity at time of cooling preserved • Old crust pushed aside

    • Became a Scientific Revolution!

    Development of Plate Tectonic Theory

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    Oceanic Magnetometers

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    Seafloor Magnetic

    Anomalies

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    Seafloor Ages