Earthguide Online Classroom

Understanding consequences of plate motion

1. How plates change size and cause oceans to open and close
2. How the San Andreas Transform Fault replaced a subduction zone

* This diagram may be better viewed in full-screen mode.

Questions for thought and understanding

  1. Which oceans are currently producing new seafloor?
  2. Which oceans are currently growing in area?
  3. Can an ocean grow new seafloor and close at the same time?

Notes: See how oceans open and close as a result of plates changing in size

  1. Look at a plate such as North America. Note that it is made up of areas of continental AND oceanic lithosphere.
    Because a plate consists of the lithosphere (which includes both crust and part of the underlying mantle),
    we use the term "oceanic lithosphere" to refer to lithosphere topped by oceanic crust,
    and "continental lithosphere" to refer to lithosphere topped by continental crust.
  2. Useful generalization - when plates move, areas of continental crust do not change area as much; however, areas of oceanic crust can change area signficantly.
  3. Look at the North American plate as the Atlantic opens, grows wider and gains area. The area of gain is in the area of oceanic crust, not the continent.
  4. Seafloor spreading at midocean ridges produces and adds new area to the oceanic lithosphere.
  5. Thus, plate change size over time. Some grow, some shrink and even disappear if mostly oceanic.
  6. Since areas of continental do not shrink and some oceans open, some area must be lost because the Earth isn't changing size overall. Again, the accommodation is in the ocean - as much as one ocean grows in area, another must be losing area.
  7. Oceanic lithosphere is lost by subduction at some plate margins.
  8. Today, the Atlantic is growing and the Pacific is losing area to subduction around its perimeter.

Notes: See how the San Andreas Transform has replaced what was previously a subduction setting

  1. Pending modification to our diagram to show the situation before the start of the San Andreas Fault.

Questions that remain

  1. While it might seem as simple as a denser plate sinking when pushed against one that is less dense, what actually goes on must be more complex.

    Around the perimeter of the Atlantic, oceanic crust does not subduct against continents. Why?

    Even relatively young oceanic crust subducts against the west coast of South America.
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