1. Lecture-7 1 Lecture #07- Mid-Ocean Ridges

2. Lecture-7 2 Mid-Ocean Ridges (MOR) F Mid-Ocean ridges are divergent plate boundaries; these are regions where plates move away from one another F As you might imagine, they occur only between oceans and oceans (continents are not involved)

3. Lecture-7 3 Mid-Ocean Ridges F MORs are the “birthplace” of oceanic lithosphere F The newly formed oceanic crust and lithosphere are pushed sideways to make room for hotter upwelling material F This sideways motion continues as the plates age and become cooler and heavier

4. Lecture-7 4 Mid-Ocean Ridge (MOR)

5. Lecture-7 5 Rocks & Magnetism F When rocks form in the presence of a magnetic field (like that on Earth), they are influenced by the field. F 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. Lecture-7 6

7. Lecture-7 7 Magnetic Time Scales F By studying many rocks of different ages, they were able to map out the “polarity” of the geomagnetic field back into Earth’s history.

8. Lecture-7 8 Irregular Patterns F 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.

9. Lecture-7 9 Magnetic Stripes

10. Lecture-7 10 Sea Floor Spreading F One way to explain these observations was to have a system operating that created new ocean floor at the mid-ocean ridges. F As the material cooled, it inherited the polarity of Earth’s magnetic field, & then the sea floor spread apart like a conveyor belt.

11. Lecture-7 11 Rift Zones F 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” F 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)

12. Lecture-7 12 The East-African Rift Zone: A New Plate Boundary Being Developed Right Now! Stage 1. Plume upwelling reaches surface Stage 2. Continental breakup begins

13. Lecture-7 13 MORs – Like Seams on a Baseball East Africa Rift Zone

14. Lecture-7 14 The Age of the Sea Floor Iceland

15. Lecture-7 15 Iceland - A “Dry” Ridge

16. Lecture-7 16 Icelandic Fissure Eruptions

17. Lecture-7 17 The Age of the Sea Floor Juan de Fuca Ridge

18. Lecture-7 18 Closer to Home – The Juan de Fuca Ridge

19. Lecture-7 19 MORs Up Close F When we look at mid-ocean ridges in detail we find some interesting things: FHydrothermal systems FPillow basalts FPlumes of chemical precipitation (black smokers) F

20. Lecture-7 20 Hydrothermal System

21. Lecture-7 21 Pillow Basalts F When magma directly encounters water it is supercooled into pillow basalt F Basalt refers to the chemical composition of the rocks, and pillow refers to their smooth, rounded shapes

22. Lecture-7 22 Direct Water-Magma Interaction (Hawaii)

23. Lecture-7 23 Pillow Basalts

24. Lecture-7 24 More Pillow Basalts …

25. Lecture-7 25 Deep Sea Vents (Smokers)

26. Lecture-7 26 Deep Sea Vents (Smokers)

27. Lecture-7 27 MOR Summary F Mid-ocean ridges are divergent plate boundaries where new oceanic lithosphere is being created F Rifting refers to the breaking up of a continent; rifts may or may not turn into spreading ridges