Seafloor Spreading

• SEAFLOOR SPREADING THEORY proposed by American geophysicist Harry Hess (1960)
• SEAFLOOR SPREADING THEORY states that the intense heat produced by radioactive substances in the mantle, searches for a route out and induces convection currents to form
• How does seafloor spreading occur?

  • volcanic eruptions at mid-ocean ridges cause the oceanic crust to crack

  • new lava fills these cracks, pushing the crust apart on either side.

  • at subduction zones, oceanic crust sinks beneath continents or other oceanic plates and returns to the mantle
• As molten material erupts from the mantle, spreads out, and pushes older materials to the sides of the rift, a new ocean floor forms along fractures in the ocean crust. The additional ocean floor is being added as a result of sea-floor spreading.
• EVIDENCE FOR SEAFLOOR SPREADING

  • Evidence from Molten Material
  • Evidence from Magnetic Strip
  • Evidence from Drilling Sample
• Mapping the Mid-Ocean Ridge • longest chain of mountains in the world
• 1950 - mapped using SONAR
• Found out that the ocean is not flat and that there are huge mountain ranges called ridges
• EVIDENCE FROM MOLTEN MATERIAL

  • molten material burst out of cracks along the mid-ocean ridge and quickly cooled, creating rock pillows
  • rock pillows show that molten materials have erupted again and again from cracks along the mid-ocean ridge and cooled quickly
  • Scientist aboard a submersible called Alvin
• EVIDENCE FROM MAGNETIC STRIP

  • ocean floor lie in a pattern of magnetized stripes which hold a record of the reversals in magnetic field
  • it shows a symmetrical pattern of alternating magnetic polarities on either side of a mid-ocean ridge.
• MAGNETIC REVERSAL

  • change in the Earth’s magnetic field in which the North magnetic pole becomes the south magnetic pole and vice versa
  • has occured on average every 500,000 years over the past 65 million years
• Normal Polarity - magnetic orientation is the same as that of Earth’s current field
• Reversed Polarity- magnetic orientation in rocks that are opposite to the current orientation of Earth’s magnetic field
• sonar • uses sound waves to measure distance
• Magnetic polarity is normal at the ridge crest but reversed in symmetrical patterns away from the ridge center. This normal and reversed pattern continues across the seafloor
• Seafloor Age • scientists came up with a time scale for the magnetic reversals by combining magnetic polarity data from rocks on land and on the seafloor with radiometric age dating and fossil ages
• First 4 Magnetic Periods

  1. Brunhes normal
  2. Matuyama reverse
  3. Gauss normal
  4. Gilbert reverse
• 1. Brunhes normal - present to 730,000 years ago.
• 1. Matuyama reverse - 730,000 years ago to 2.48 million years ago.
• 1. Gauss normal - 2.48 to 3.4 million years ago.
• 1. Gilbert reverse – 3.4 to 5.3 million years ago
• EVIDENCE FROM DRILLING SAMPLES

  • show evidence of sea floor spreading through the age of the oceanic crust, retrieved sediment fossils, and recorded rock magnetism
• EVIDENCE FROM DRILLING SAMPLES

  • In the year 1968, a drilling ship called Glomar Challenger was used to gather samples of rocks from the ocean floor.
  • Scientist discovered that the further the rocks were from the mid ocean ridges, they older they were.
  • The youngest rocks were always at the center of the ridge.
• Subduction • process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle
• Deep Ocean Trench

  • occurs at subduction zones
  • deep, underwater canyons form when oceanic crust bends downwards
• SEAFLOOR SPREADING EFFECTS

  • Seafloor spreading impacts the carbon cycle and sea level.
  • The ridge expands as the seafloor spreads more quickly.
  • Hot, young lithosphere forms and moves away from the ridge before cooling and shrinking., This process causes sea level to rise.
  • Faster spreading leads to increased volcanic activity.
  • Volcanic activity releases greenhouse gases, affecting the carbon cycle.
• SIGNIFICANCE OF SEAFLOOR SPREADING THEORY

  1. Foundation of Plate Tectonics
  2. Understanding Oceanic Crust Formation
  3. Explanation of Magnetic Stripping
  4. Insight into Geological Activity
  5. Support for Continental Drift
  6. Impact on Geologic Mappin
  7. Advancement in Geophysical Techniques
• LIMITATIONS OF SEAFLOOR SPREADING THEORY

  1. Incomplete Mantle Dynamics
  2. Variable Spreading Rates
  3. Complex Plate Interaction
  4. Subduction Processes
  5. Measurement Challenge
  6. Evolving Theory