Seafloor spreading and why ocean crust is young

Grade 9 Science Worksheet

Circle the best answer.

2. 1.

   USGS: the Mid-Atlantic Ridge spreads about 2.5 cm a year, or 25 km in a million years. How much new crust forms in 4 million years, by Bo's math?

   • About 1,000 km
   • About 10 km
   • About 100 m
   • About 100 km
3. 2.

   At the Mid-Atlantic Ridge's 2.5 cm a year (USGS), how many years does it take to add 1 km of new crust, by Esme's math?

   Use the picture in question 1.

   • About 40,000 years
   • About 400 years
   • About 4 million years
   • About 250,000 years
4. 3.

   Model data: a stripe edge sits 50 km from a ridge, in rock 2 million years old. What is the spreading rate on that side, by Jun's math?

   Use the picture in question 1.

   • About 100 km a year
   • About 0.25 cm a year
   • About 25 cm a year
   • About 2.5 cm a year

5. Read this first

   Use this

   Ridge cross-section with stripes.

6. 4.

   A ship crossing a ridge records stripes: normal, reversed, normal at the crest, reversed, normal (model data). What does this show, by Kira's reading?

   • Random stripes with no meaning
   • Mirror-image stripes, with the youngest normal stripe at the crest
   • The stripes are oldest at the crest
   • Spreading on one side of the ridge
7. 5.

   Why do the youngest rocks at a ridge crest have normal polarity, by Luz's reading (USGS)?

   • Normal polarity means the rocks are still very cold
   • Seawater resets them each year
   • They cooled in today's magnetic field
   • They are the oldest rocks there
8. 6.

   Cox, Doell and Dalrymple dated the magnetic reversals of the past 4 million years by potassium-argon (USGS). Why did that matter for the stripes, by Milo's reading?

   • It put ages on the stripes, which gave spreading rates
   • It showed stripes come from changes in field strength
   • It showed the stripes formed at random
   • It showed the ocean floor is 4 billion years old
9. 7.

   Ridge cross-section with core sites. In 1968 the Glomar Challenger drilled cores across the Mid-Atlantic Ridge (USGS). What did the core ages show, by Pia's reading?

   • The cores matched in age everywhere
   • The cores were as old as the continents
   • Rock grows younger away from the ridge
   • Rock grows older with distance from the ridge
10. 8.

    Conveyor from a ridge to a trench. Hess reasoned that if crust grows at ridges, it must shrink somewhere else (USGS). Where, by Ren's reading?

    • At ocean trenches, where old crust sinks
    • At the ridges themselves
    • Nowhere; Earth expands
    • Inside the continents

11. Crust ages (USGS)

    Crust	Age
    average ocean crust	about 55 million years
    average continental crust	about 2.3 billion years
    oldest ocean floor	about 180 million years
    oldest continental rock	about 4.03 billion years

12. 9.

    About how many times older is the oldest continental rock than the oldest ocean floor, by Zee's math (USGS)?

    • About 2 times
    • About 220 times
    • About 22 times
    • About the same age
13. 10.

    Conveyor from a ridge to a trench. NOAA says very little sea floor is older than 150 million years. Why, by Ari's reading?

    • Old sea floor subducts and is replaced by new crust
    • Old sea floor dissolves in seawater
    • Old sea floor piles up at the ridges
    • Earth had no oceans until about 150 million years ago