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Lecture #07- Mid-Ocean Ridges. Mid-Ocean Ridges (MOR). Mid-Ocean ridges are divergent plate boundaries; these are regions where plates move away from one another As you might imagine, they occur only between oceans and oceans (continents are not involved). Mid-Ocean Ridges.

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Lecture 07 mid ocean ridges l.jpg

Lecture #07- Mid-Ocean Ridges


Mid ocean ridges mor l.jpg
Mid-Ocean Ridges (MOR)

  • Mid-Ocean ridges are divergent plate boundaries; these are regions where plates move away from one another

  • As you might imagine, they occur only between oceans and oceans (continents are not involved)


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Mid-Ocean Ridges

  • MORs are the “birthplace” of oceanic lithosphere

  • The newly formed oceanic crust and lithosphere are pushed sideways to make room for hotter upwelling material

  • This sideways motion continues as the plates age and become cooler and heavier



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Rocks & Magnetism

  • When rocks form in the presence of a magnetic field (like that on Earth), they are influenced by the field.

  • In an igneous rock, which forms from cooling lava or magma, the iron rich atoms are aligned with Earth’s field, and then frozen into the rock.


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Magnetic Time Scales

  • By studying many rocks of different ages, they were able to map out the “polarity” of the geomagnetic field back into Earth’s history.


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Irregular Patterns

  • Note that the pattern is irregular - that makes it like a fingerprint. We can use the pattern to estimate the age of rocks that show similar magnetic signatures.



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Sea Floor Spreading

  • One way to explain these observations was to have a system operating that created new ocean floor at the mid-ocean ridges.

  • As the material cooled, it inherited the polarity of Earth’smagnetic field, & then the sea floor spread apart like a conveyor belt.


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Rift Zones

  • The continental analog to a MOR is called a rift zone

    • A major rift zone exists in east Africa; some have already named the new plate the “Somalian Plate”

  • At a rift zone a continent is essentially being broken apart

    • If the rifting is successful a spreading ridge (MOR) will develop and ocean basins will form

    • If the rifting does not succeed the continent will remain whole and no new plate boundary will be formed (failed rift)


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The East-African Rift Zone:

A New Plate Boundary Being Developed Right Now!

Stage 1. Plume upwelling reaches surface

Stage 2. Continental breakup begins


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MORs – Like Seams on a Baseball

East Africa

Rift Zone





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The Age of the Sea Floor

Juan de Fuca Ridge



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MORs Up Close

  • When we look at mid-ocean ridges in detail we find some interesting things:

    • Hydrothermal systems

    • Pillow basalts

    • Plumes of chemical precipitation (black smokers)



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Pillow Basalts

  • When magma directly encounters water it is supercooled into pillow basalt

  • Basalt refers to the chemical composition of the rocks, and pillow refers to their smooth, rounded shapes







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MOR Summary

  • Mid-ocean ridges are divergent plate boundaries where new oceanic lithosphere is being created

  • Rifting refers to the breaking up of a continent; rifts may or may not turn into spreading ridges


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