1. 9.2: Sea-Floor Spreading
in the early 1900s, scientists using sonar (SOund Navigation And Ranging) discovered deep-ocean trenches
deep-ocean trenches: long, curved valleys along the edges of some ocean basins

2. trenches form the deepest parts of the ocean

3. deepest is the Mariana Trench

4. ridges form as magma rises to create new ocean floor made of basalt
in the late 1950s, scientists discovered mid-ocean ridges
mid-ocean ridges: long, undersea mountain chains that have a steep, narrow valley at its center
ridges form as magma rises to create new ocean floor made of basalt

5. at the center of some mid-ocean ridges are a crack in Earth’s crust where magma can rise
this crack is a rift valley

7. Harry Hess (1963) proposed the idea of sea-floor spreading where
a) magma rises at mid-ocean ridges to create new ocean floor
b) ocean floor slowly moves outward away from the rift
c) ocean floor sinks back into the mantle at deep-ocean trenches

8. when ocean plates sink back into the mantle, it is called subduction
rift and then cools to form new rock.

9. trenches where the ocean plates sink back into the mantle are called subduction zones
rift and then cools to form new rock.

10. 1960s brought more data to support the idea of sea-floor spreading
scientists learned that Earth’s magnetic field occasionally reverses polarity
north magnetic pole becomes the south magnetic pole – called reversed polarity

11. paleomagnetism: study of the alignment of magnetic minerals that rock gains during formation
scientists discovered a striped magnetic pattern on the ocean floor on each side of a mid-ocean ridge

12. pattern on one side of the ridge is a mirror image of the pattern on the other side

13. mid-ocean ridges contain young rocks – none older than 180 million years
rocks get older towards the trenches
thin rocks – covered with less sediment – at top of ridge, getting thicker with more sediment as move towards the trenches

14. sea-floor spreading was the mechanism to explain continental drift!
rift and then cools to form new rock.