(PT) Seafloor Spreading Lesson

Seafloor Spreading

The features of the seafloor and the patterns of magnetic polarity symmetrically about the mid-ocean ridges were the pieces that Hess needed. He resurrected Wegener's continental drift hypothesis and also the mantle convection idea of Holmes.

Hess wrote that hot magma rose up into the rift valley at the mid-ocean ridges. The lava oozed up and forced the existing seafloor away from the rift in opposite directions. Since magnetite crystals point in the direction of the magnetic north pole as the lava cools, the different stripes of magnetic polarity revealed the different ages of the seafloor. The seafloor at the ridge is from the Brunhes normal; beyond that is basalt from the Matuyama reverse; and beyond that from the Gauss normal. Hess called this idea seafloor spreading.

Since new oceanic crust is created at the mid-ocean ridges, either Earth is getting bigger (which it is not) or oceanic crust must be destroyed somewhere. Since the oldest oceanic crust was found at the edges of the trenches, Hess hypothesized that the seafloor subducts into Earth's interior at the trenches to be recycled in the mantle.

• As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis.
• If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle.
• The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.

Hess could also use seafloor spreading to explain the flat topped guyots. He suggested that they were once active volcanoes that were exposed to erosion above sea level. As the seafloor they sat on moved away from the ridge, the crust on which they sat become less buoyant and the guyots moved deeper beneath sea level.

Watch the following video to learn more about seafloor spreading.

 

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