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Lecture #07- Mid-Ocean Ridges

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

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  1. Lecture #07- Mid-Ocean Ridges

  2. 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)

  3. 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

  4. Mid-Ocean Ridge (MOR)

  5. 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.

  6. 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.

  7. 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.

  8. Magnetic Stripes

  9. 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.

  10. 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)

  11. The East-African Rift Zone: A New Plate Boundary Being Developed Right Now! Stage 1. Plume upwelling reaches surface Stage 2. Continental breakup begins

  12. MORs – Like Seams on a Baseball East Africa Rift Zone

  13. The Age of the Sea Floor Iceland

  14. Iceland - A “Dry” Ridge

  15. Icelandic Fissure Eruptions

  16. The Age of the Sea Floor Juan de Fuca Ridge

  17. Closer to Home – The Juan de Fuca Ridge

  18. 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)

  19. Hydrothermal System

  20. 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

  21. Direct Water-Magma Interaction (Hawaii)

  22. Pillow Basalts

  23. More Pillow Basalts …

  24. Deep Sea Vents (Smokers)

  25. Deep Sea Vents (Smokers)

  26. 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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