• H. Harry Hess put forward the hypothesis of seafloor spreading in 1960.
• Hess mapped the ocean floor using sonar and discovered the mid-Atlantic ridge (mid-ocean ridge).
• The temperature near the mid-Atlantic ridge was found to be warmer than the surface away from it.
• Hess believed that the high temperature was due to magma leaking out from the ridge.
• This hypothesis supports the Continental Drift Theory of Alfred Wegener in 1912 on the shift position of the earth’s surface.

Convection Current Theory

• The Convection Current Theory is essential to the Seafloor Spreading Theory.
• Arthur Holmes discussed the possibility of convection currents in the mantle during the 1930s.
• Convection currents in the mantle are generated by thermal differences caused by radioactive elements.
• According to this theory, the intense heat generated by radioactive substances in the mantle (100-2900 km below the earth’s surface) seeks a path to escape and gives rise to the formation of convection currents in the mantle.
• Wherever the rising limbs of these currents meet, oceanic ridges are formed on the seafloor due to the divergence of the lithospheric plates (tectonic plates).
• Wherever the falling limbs of these currents meet, trenches are formed due to the convergence of the lithospheric plates (tectonic plates).
• The movement of the magma in the mantle causes the movement of the lithospheric plates.
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Paleomagnetism

• Paleomagnetism is the study of the earth’s magnetic field record in rocks, sediment, or archaeological materials.
• Magnetic fields recorded in rocks help detect the polarity of the Earth’s magnetic field and magnetic field reversals.
• Rocks formed from underwater volcanic activity, such as basalt, contain magnetic minerals that align in the direction of the magnetic field as the rock solidifies.
• Paleomagnetic studies of rocks have shown that the orientation of the earth’s magnetic field has frequently alternated over geologic time (geomagnetic reversal).
• Paleomagnetism played a critical role in developing the theories of Sea Floor Spreading and Plate Tectonics from the continental drift hypothesis.
• Paleomagnetic rocks on either side of the mid-ocean ridges provide the most important evidence of Sea Floor Spreading.
• Magnetic field records can provide information on the past location of tectonic plates.
• Oceanic ridges are boundaries where tectonic plates are diverging, and the rising magma assumes the polarity of the Earth’s geomagnetic field before it solidifies on the oceanic crust.
• As the conventional currents pull the oceanic plates apart, the solidified band of rock moves away from the vent (or ridge), and a new band of rock takes its place a few million years later when the magnetic field was reversed. This results in alternating magnetic striping on the seafloor.

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Sea Floor Spreading Theory

• Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
• Harry Hess proposed the idea that the seafloor moves and carries the continents with it.
• Convection currents are generated in the mantle due to intense heat from radioactive substances.
• Oceanic ridges are formed wherever rising limbs of these currents meet and trenches are formed wherever the failing limbs meet.
• New material is added to the ocean floor while pushing older rocks away from the ridge.
• New ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle and spreads out, pushing older rocks to the sides of the crack.
• The process of sea-floor spreading continually adds new ocean floor.
• Mid-Ocean Ridge is the longest chain of mountains in the world and these are divergent plate boundaries.
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Evidence of Sea Floor Spreading

• Evidence from Molten material:
  • Rocks shaped like pillows(rock pillows) show that molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly.
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• Evidence from Magnetic Strips:
  • Rocks that make up the ocean floor lies in a pattern of magnetizing stripes that hold a record of the reversals in Earth’s magnetic field.
• Evidence from Drilling Samples:
  • Core samples from the ocean floor show that older rocks are found farther from the ridge; the youngest rocks are in the center of the ridge.
• Subduction:
  • Process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle; allows part of the ocean floor to sink back into the mantle.
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• Deep-Ocean Trench:
  • This occurs at subduction zones. Deep underwater canyons form where oceanic crust bends downward.

Distribution of Earthquakes and Volcanos along the Mid-Oceanic Ridges

• The normal temperature gradient on seafloor is 9.4° C/300 m, but near the ridges it is higher, indicating upwelling of magmatic material from mantle.
• Dots in central parts of oceans are almost parallel to coastlines, indicating widening of seafloor over time.
• Earthquake foci in mid-oceanic ridge areas are shallow, whereas along Alpine-Himalayan belt and Pacific rim are deep-seated.
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Conclusion

• Sea-floor spreading solves the problem of younger age crust at mid-oceanic ridges and older rocks being found farther away.
• It explains why sediments at central parts of oceanic ridges are thin.
• It also supports Alfred Wegener’s theory of continental drift and contributes to the development of plate tectonics theory