Earthguide Online Classroom

All-in-one map for plate tectonics

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

Key features to observe

  1. Recognizing plate margins (the edges of plates) because that's where key Earth processes and geologic hazards take place.
  2. Distinguishing between the margins of plates vs. margins of continents.
    Given that living things including humans tend to concentrate near continental margins, that the distance from plate margins imposes distinct conditions. Continental margins that happen to be near plate margins have certain geographic charactistics and are associated with higher-than-average risks from earthquakes and volcanism. Continental margins within the interior of plates have their own geographic tendencies and are associated with lower risks of earthquakes and volcanism.

    Plate margins are classified as - convergent, divergent or transform.
    Continental margins are classified as - active or passive.
  3. The Mid-Atlantic Ridge and East Pacific Rise are examples of midocean ridges.
  4. Midocean ridges reach a typical summit elevation of 2,700 meters below sealevel. They are the shallowest major features of the seafloor. Perhaps counter to expectation, the oceans are deeper closer to continents and farthest from midocean ridges.
  5. Seafloor spreading is one of the two major processes of plate tectonics, the other being subduction.
  6. Seafloor spreading is the continuous process of forming new igneous rock at midocean ridges by injection of magma that forms new seafloor. The process is continuous because forces cause opposite sides of the midocean ridge to constantly move apart, making new room for the process to repeat. This can result in an ever-widening seafloor at the expense of area lost elsewhere on the planet. Seafloor spreading ends when midocean ridges are subducted.
  7. Seafloor spreading produces major characteristics of the seafloor - 1) the age of the seafloor is progressively older away from midocean ridges, 2) the elevation of the seafloor is progressively lower away from midocean ridges, 3) the magnetic history of the seafloor bears the striped-pattern of the Earth's magnetic reversals, and 4) older parts of the seafloor have the greatest potential to have accumlated sediments over the longest time.
  8. Volcanoes and other features that grow or fall onto the seafloor move and sink with the seafloor over time.
  9. Since the seafloor on each side of a midocean ridge moves in opposite directions, each side belongs to a different lithospheric plate.
  10. Midocean ridges, transform faults and fracture zones are the key surface features produced by seafloor spreading.
  11. In detail, midocean ridges are made up of short offset segments.
  12. The curved linear features running perpendicular to midocean ridges are fracture zones and transform faults. Fracture zones and transform faults make up parts of each linear feature.
  13. The transform faults lie between two neighboring offset ridges. The motion of the plates on opposite sides of the transform fault are consistent with seafloor being produced at each ridge and moving away to each side.
  14. Seafloor spreading determines plate motion around a midocean ridge. Midocean ridges are always plate boundaries. Since many plates, such as the North American Plate, include continental portions attached to oceanic portions, seafloor spreading also the direction in which continents move. Continents don't drift independently. North America moves to the west with the western half of the Atlantic.

Questions for thought

  1. What are the shallowest major features of the Atlantic and Pacific Oceans?
  2. Why does the seafloor slope downward away from midocean ridges?
  3. How fast are plates moving apart at midocean ridges?
  4. What happens as the thickness of the lithosphere grows from zero at midocean ridges to 100 km away from midocean ridges?
© 2008 Earthguide at Scripps Institution of Oceanography.
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