Download c1. Seafloor Spreading - Salem State University

Continental Drift
Seafloor Spreading and
Magnetic Reversals
• Predecessor to modern plate tectonics
• Shape and “fit” of the continents was
the initial evidence
– Snider-Pelligrini (1858)
– Taylor (1908)
– Wegener (1915)
Brad Hubeny
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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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Geology of the
Ocean Floor
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!
• Doc Ewing does
NOT find granite
when he dredges the
ocean bottom!!
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Geology of the Ocean Floor
Bathymetry
Reveals
Oceanic
Rift System
• 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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Harry Hess’ “Essay in Geopoetry”
Earthquakes and Volcanoes
•
•
•
•
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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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Earth’s
Magnetic Field
Enter Paleomagnetics!
• Magnetic reversals
Geodynamo System!
– Earth’s magnetic field polarity has reversed
through time
• Normal polarity – N magnetic = N geographic
• Reversed polarity - Nmagnetic = S geographic
• Iron-rich basalts of ocean crust preserve
this record of the magnetic field!
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Geologic Magnetic Recorders
02_10.jpg
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Polarity Reversals
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Reversals & the Geologic Record
02_23.jpg
Geomagnetic
Time Scale
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Oceanic Magnetometers
Development of Plate Tectonic
Theory
• 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!
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Seafloor Ages
Seafloor
Magnetic
Anomalies
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