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Ch. 17 Sec. 2

Ch. 17 Sec. 2. Sea-Floor Spreading. Oceanic crust forms at ocean ridges and becomes part of the seafloor. Review Vocabulary. basalt: a dark-gray to black fine-grained igneous rock. I. Mapping the Ocean Floor.

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Ch. 17 Sec. 2

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  1. Ch. 17 Sec. 2 Sea-Floor Spreading

  2. Oceanic crust forms at ocean ridges and becomes part of the seafloor. Review Vocabulary basalt: a dark-gray to black fine-grained igneous rock

  3. I. Mapping the Ocean Floor Until the mid-1900’s, many scientists thought that the ocean floors were essentially flat and that oceanic crust was unchanging and was much older than continental crust. Advances in technology during the 1940’s and 1950’s showed that all of these widely accepted ideas were incorrect.

  4. A. Magnetometer - device that can detect small changes in magnetic fields

  5. B. Sonar 1. Bounces sound waves off underwater objects 2. Echoes determine distance to object 3. Maps ocean floor topography

  6. II. Ocean Floor TopographyA. Mid-Ocean Ridges

  7. B. Deep-sea trenches Pg. 474 Fig. 17.8

  8. The deepest trench, the Mariana Trench, is more than 11 km deep. Mount Everest, the world’s tallest mountain, stands at 9 km above sea level, and could fit inside the Mariana Trench with six Empire State buildings stacked on top.

  9. III. Ocean Rocks & Sediments A. Glomar Challenger (drilling ship) 1. Youngest rock sample near ridge 2. Oldest rocks near continents

  10. B. Evidence 1. Molten material a. Alvin (submarine) b. Rock samples

  11. C. Sediments 1. Thickness of ocean-floor sediments increases with distance from an ocean ridge

  12. Section 17.2 IV. Magnetism • Earth’s magnetic field 1. Flow of molten iron in the outer core 2. Magnetic reversal a. Flow in the outer core changes b. Earth’s magnetic field changes direction

  13. B. Magnetic polarity time scale 1. Paleomagnetism - the study of the history of Earth’s magnetic field 2. Lava solidifies a. iron-bearing minerals crystallize b. align with Earth’s magnetic field

  14. Pg. 476 Fig. 17.10 Periods of normal polarity alternate with periods of reversed polarity. Long-term changes in Earth’s magnetic field, called epochs, are named as shown here. Short-term changes are called events.

  15. Seafloor Spreading FIG 17. 12 Pg. 477 Regions of normal and reverse polarity form a series of stripes across the ocean floor parallel to the ocean ridges. The ages and widths of the stripes match from one side of the ridges to the other.

  16. 3. Isochron - imaginary line on a map a. points that have the same age

  17. V. Sea-floor spreading • Harry Hess studied mid-ocean ridges 1. Maybe Wegener was right

  18. B.New material added to ocean floor • Spreads apart at ridges • Moves ocean floor & continents

  19. C.Molten material rises • Pushes older rock outward • New material cools

  20. VI. Subduction at Trenches • Ocean floor sinks back into mantle 1. Gravity pulls old, dense rock down 2. Occurs at deep-ocean trenches

  21. B.Changes size & shape of oceans • Pacific Ocean is shrinking a. More subduction than new crust forming 2. Atlantic is expanding

  22. Oceanic crust forms at ocean ridges and becomes part of the seafloor. • Studies of the seafloor provided evidence that the ocean floor is not flat and unchanging. • Oceanic crust is geologically young.

  23. New oceanic crust forms as magma rises at ridges and solidifies. • As new oceanic crust forms, the older crust moves away from the ridges.

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