Key Idea: When two plates are pulling apart, melted rock material rises up between the plates, creating new plate material.

Students are expected to know that:

  1. When two plates pull apart from each other, melted rock rises up between the plates.This rock solidifies as it cools, adding new oceanic plate material to the edges of both plates so that the plates are always in contact with each other and no space forms between them.
  2. The process of melted rock rising up between the plates can be gradual, with the melted rock material slowly welling up between the plates, and it can be sudden, with melted rock material suddenly projecting out from between the plates in volcanic eruptions.
  3. Melted rock can well up or erupt anywhere along the boundary where two plates are pulling apart.As the melted rock cools, it begins to form a row of mountains along the edges of both plates where they are pulling apart.
  4. When two plates move apart and split a continent in two, an ocean basin forms between them that widens over time.The ocean basin grows as new plate material is continuously added to the edge of the separating plates.
  5. At the same time that new plate material is being added as plates pull apart, other plate material is being folded upward or recycled into the interior of the earth so that the plate material that is lost in one place is balanced by the new plate material that is gained in another.


  1. Students are not expected to know why some oceanic plate material is denser than other oceanic plate material.
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New plate material forms where two plates pull apart.




The amount of new plate material that is added to a plate is balanced by the amount of plate material that is pushed into the earth's interior or is crumpled upward.




As two plates move apart new plate material forms between them.