EESA06H3 Chapter Notes - Chapter 2: Fracture Zone, Leading Edge, Geomagnetic Reversal

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12 Aug 2016
What is Sea-Floor Spreading?
Henry Hess (Geologist @ Princeton) proposed that the sea floor might be moving too
oContrasted with Wegneer’s idea who thought ocean floor remained stationary as continents
Hess’s 1962 proposal was named sea-floor spreading – it suggests the sea floor moves away from
the mid-oceanic ridge as a result of mantle convection
Initial concept of sea floor spreading
oSea floor is moving like a conveyor belt away from the crest of the deep ocean basin, finally
to disappear by plunging beneath a continent or island arc
Spreading Axis: ridge crust w/ sea floor moving away from it on either side
Subduction: sliding of sea floor beneath a continent or island arc
Hess’s original hypothesis was that sea floor spreading was driven by deep mantle convection
Convection is a circulation pattern driven by deep mantle rising of hot material and/ or sinking of
cold material
oConvection drives sea-floor spreading, hot mantle rock must be rising under mid-ocean
oMantle rock moves horizontally away from ridge crests on each side of the ridge; this
movement creates tension at the ridge crest, cracking open the oceanic crust to form rift
valleys and associated shallow-focus earthquakes
Downward plunge of cold rock accounts for the existence of the oceanic trenches as well as their
low flow values
oLarge negative gravity anomalies associated with trenches, for the sinking of the cold rock
provides a force that holds trenches out of isotactic equilibrium
Sea floor moves downward into the mantle along a Subduction zone; it interacts w/ the stationary
rock above it
oInteraction between moving sea floor rock and stationary rock can cause Benioff zones of
earthquakes associated with trenches
oCan also produce andesitic volcanism; forming volcanoes either on the edge of a continent
or along the side of an island arc
How old is the sea floor?
Marine geologists determine the age of the sea-floor rocks (isotopic dating) and sediments (by fossils)
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