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 100 m
   • About 1,000 km
   • About 10 km
   • 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 250,000 years
   • About 4 million years
   • About 40,000 years
   • About 400 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 2.5 cm a year
   • About 25 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?

   • The stripes are oldest at the crest
   • Random stripes with no meaning
   • Mirror-image stripes, with the youngest normal stripe 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)?

   • Seawater resets them each year
   • They are the oldest rocks there
   • They cooled in today's magnetic field
   • Normal polarity means the rocks are still very cold
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 showed the ocean floor is 4 billion years old
   • 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
9. 7.

   In 1947, scientists found the Atlantic sediment layer much thinner than expected (USGS). What did that point to, by Noah's reading?

   • Sediment is being made faster than before
   • Earth is expanding, which stretches the sediment layer thin
   • The ocean floor is billions of years old
   • The ocean floor is young, so little sediment has built up
10. 8.

    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
    • Rock grows younger away from the ridge
    • Rock grows older with distance from the ridge
    • The cores were as old as the continents

11. Use this

    Conveyor from a ridge to a trench.

12. 9.

    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
    • Nowhere; Earth expands
    • Inside the continents
    • At the ridges themselves
13. 10.

    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 piles up at the ridges
    • Earth had no oceans until about 150 million years ago
    • Old sea floor dissolves in seawater