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Lecture #07- Mid-Ocean Ridges

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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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mid ocean ridges mor
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
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
slide5

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.
slide7

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.
slide8

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.
slide10

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.
rift zones
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)
slide12

The East-African Rift Zone:

A New Plate Boundary Being Developed Right Now!

Stage 1. Plume upwelling reaches surface

Stage 2. Continental breakup begins

slide17

The Age of the Sea Floor

Juan de Fuca Ridge

mors up close
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)
pillow basalts
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
mor summary
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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