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Seafloor Spreading andMagnetic 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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TerrestrialGeologicEvidence

Only occur in rocks >145 Ma BP!

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Paleontological evidence•Nomechanismto transportacrossocean•Divergenceof speciesfollowingbreak-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, thetheory was not widely accepted

• Lack of testable hypothesis!!

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Development of Plate TectonicTheory

• Original evidence for continental drift wasfrom continental rocks

• Technological advances in the 1950’s and1960’s allowed investigation of the sea floor

• New data provided intriguing newinformation!

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Geology of theOcean Floor

• Doc Ewing doesNOT find granitewhen he dredges theocean 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 theentire globe

– Central rift valleywithin the ridge

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BathymetryRevealsOceanic

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 preservethis record of the magnetic field!

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Earth’sMagnetic Field Geodynamo System!

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02_10.jpgGeologic Magnetic Recorders

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Polarity Reversals

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02_23.jpgReversals & the Geologic Record

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GeomagneticTime Scale

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• Vine & Matthews (1963) tested Hess’shypothesis 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 TectonicTheory

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Oceanic Magnetometers

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SeafloorMagnetic

Anomalies

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Seafloor Ages