2.6

• Seafloor Spreading


  A geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other
• Mantle Convection


  The slow, churning motion of Earth's mantle
• Convection Currents


  Carry heat from the lower mantle and core to the lithosphere
• Seafloor Spreading


  1. Occurs at divergent plate boundaries

  2. As tectonic plates slowly move away from each other, heat from the mantle's convection currents makes the crust more plastic and less dense

  3. The less-dense material rises, often forming a mountain or elevated area of the seafloor

  4. The crust cracks

  5. Hot magma fuelled by mantle convection bubbles up to fill these fractures and spills onto the crust

  6. The bubbled-up magma is cooled by frigid seawater to form igneous rock, which becomes a new part of Earth's crust
• Seafloor Spreading


  • In 1950s and 1960s, marine geologists used echo sounding to map ocean ridges, or submarine mountain chains, in the North Atlantic Ocean and Pacific Ocean
  • They discovered that these were continuous on Earth's surface, resembling the stitches on a baseball
• Harry Hammond Hess


  • An American Biologist who conducted echo-sounding surveys on ocean floor and published his findings in his article, "The History of Ocean Basins"
  • He found out that magma oozes up from the Earth's interior along mid – oceanic ridges and this eventually solidifies and forms a new seafloor
• Subduction


  The seafloor on either side of the ridges spreads away form the crest of the ridge, until it sinks into the deep oceanic trenches
• Mid-Ocean Ridges


  • Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor
  • Mid-Atlantic Ridge separates the North American plate from the Eurasian plate, and the South American plate from the African plate
  • Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate
  • East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate
• Mid-Ocean Ridges


  1. At mid-oceanic ridge, magma rises to the surface and later forms a new crust

  2. The new oceanic crust pushes against the adjacent old oceanic crust

  3. The oceanic crust will be pushed away from the ridge

  4. As the process continues, seafloor spreads out thus moving the tectonic plates resulting in continental drift
• Ocean basin


  A depression of the earth's surface in which an ocean lies
• Ocean basins have developed as plate tectonics continued and date back to 2 billion years ago
• Evolution of the ocean basins through tectonic plate movements

  1. Break-up of the supercontinent Pangea

  2. Formation of the Atlantic Ocean about 200 million years ago

  3. Separation of North America from South America and Africa 180 million years ago

  4. Separation of South America from Africa 135 million years ago

  5. Shrinking of the Pacific Basin at the expense of the growth of the Atlantic and Arctic basins

  6. Opening of the Tethys Seaway and Southern Ocean
• Ocean basins


  • They are one of many vast undersea regions that together cover nearly three-quarters of the earth's surface
  • They contain the vast majority of all the water on the planet and have an average depth of nearly 4 km (about 2.5 miles)
  • They vary in size, shape, and characteristics due to the movement of the earth's crust (lithosphere)
• Continental shelf


  A shallow submerged margin of the continents that lies between the edge of the shoreline and the continental slope, composed of sediment or sedimentary rock
• Continental slope


  A steeply sloping portion of continental crust found between the continental shelf and continental rise
• Continental rise


  Thick layers of sediment found between the continental slope and the ocean floor
• Ocean floor


  A flat plain found at the bottom of the ocean, representing the surface of the oceanic crust, lying between the mid-oceanic ridges and the trenches, usually 5,000 to 7,000 meters below the ocean surface
• Mid-oceanic ridges


  A chain of submarine mountains where oceanic crust is created from rising magma plumes and volcanic activity, associated with plate divergence and a rift zone
• Stages of ocean basin formation (Wilson cycle)


  1. Crust thins due to extensional tectonics, an ocean basin forms and sediments accumulate

  2. Subduction is initiated at one of the margins of the ocean basin, closing the ocean basin

  3. Crust starts to thin again, a new cycle begins
• Seafloor Spreading


  A geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other
• Mantle Convection


  The slow, churning motion of Earth's mantle
• Convection Currents


  Carry heat from the lower mantle and core to the lithosphere
• Seafloor Spreading


  1. Occurs at divergent plate boundaries

  2. As tectonic plates slowly move away from each other, heat from the mantle's convection currents makes the crust more plastic and less dense

  3. The less-dense material rises, often forming a mountain or elevated area of the seafloor

  4. The crust cracks

  5. Hot magma fuelled by mantle convection bubbles up to fill these fractures and spills onto the crust

  6. The bubbled-up magma is cooled by frigid seawater to form igneous rock, which becomes a new part of Earth's crust
• Seafloor Spreading


  • In 1950s and 1960s, marine geologists used echo sounding to map ocean ridges, or submarine mountain chains, in the North Atlantic Ocean and Pacific Ocean
  • They discovered that these were continuous on Earth's surface, resembling the stitches on a baseball
• Harry Hammond Hess


  • An American Biologist who conducted echo-sounding surveys on ocean floor and published his findings in his article, "The History of Ocean Basins"
  • He found out that magma oozes up from the Earth's interior along mid – oceanic ridges and this eventually solidifies and forms a new seafloor
• Subduction


  The seafloor on either side of the ridges spreads away form the crest of the ridge, until it sinks into the deep oceanic trenches
• Mid-Ocean Ridges


  • Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor
  • Mid-Atlantic Ridge separates the North American plate from the Eurasian plate, and the South American plate from the African plate
  • Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate
  • East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate
• Mid-Ocean Ridges


  1. At mid-oceanic ridge, magma rises to the surface and later forms a new crust

  2. The new oceanic crust pushes against the adjacent old oceanic crust

  3. The oceanic crust will be pushed away from the ridge

  4. As the process continues, seafloor spreads out thus moving the tectonic plates resulting in continental drift