Download Ch. 17 Sec. 2

Ch. 17 Sec. 2
Sea Floor Spreading
Technology
Until the mid 1900’s, scientists believe that the ocean
floor was flat
 Sonar helped change the ideas and map the ocean
floor
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Sound waves bouncing back from the floor
Magnetometer detects small changes in magnetic
fields
Ocean floor Topography
Maps made from sonar & Magnetometer really
stunned scientists
 Noticed many trenches and volcanoes
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Common along ocean ridges
Mariana Trench: 11 km deep
Ocean Rocks
Scientists collected samples of the deep-sea sediments
 1st: Ages of rocks vary from place to place
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Rocks near ocean ridges are younger that those taken by
trenches
2nd: Thickness of ocean-floor sediments is less than
predicted
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Continental Crust is extremely thick, 20 km or so
Thickness of the sediments increased further from ridges
Magnetism
Oceanic rock contains iron-bearing materials
 Paleomagnetism: study of magnetic record
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Basalt provides an accurate record of ancient magnetism
As Basalt cools, materials become parallel with the ridges
Magnetic Reversal: change in earth’s magnetic field
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As the sea floor spreads, so too the magnetic field is reversed
Long change in earth’s magnetic field is Epochs
Magnetic Symmetry
Noticed a series of + and – strips
 Help to create Isochron Maps
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Line on a map that connects points that have the same age
Sea-Floor spreading
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Harry Hess proposed the theory
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New ocean crust formed at the ocean ridges and destroyed
and deep trenches
Magma forced up, hotter & less dense than mantle material,
fills the gap created at the ocean ridges
Sea-floor spreading was the missing link for
Wegener’s Continental Drift theory
 Answers the how question, but not the why
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