Seafloor spreading and why ocean crust is young

Grade 9 Science Worksheet

Circle the best answer.

2. 1.

   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?

   • About 400 years
   • About 40,000 years
   • About 4 million years
   • About 250,000 years
3. 2.

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

4. Read this first

   Use this

   Ridge cross-section with stripes.

5. 3.

   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
   • The stripes are oldest at the crest
   • Mirror-image stripes, with the youngest normal stripe at the crest
   • Spreading on one side of the ridge
6. 4.

   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 showed stripes come from changes in field strength
   • It put ages on the stripes, which gave spreading rates
   • It showed the stripes formed at random
7. 5.

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

   • The ocean floor is young, so little sediment has built up
   • Sediment is being made faster than before
   • Earth is expanding, which stretches the sediment layer thin
   • The ocean floor is billions of years old
8. 6.

   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?

   • Rock grows younger away from the ridge
   • The cores were as old as the continents
   • Rock grows older with distance from the ridge
   • The cores matched in age everywhere
9. 7.

   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
   • Inside the continents
   • Nowhere; Earth expands

10. 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

11. 8.

    About how many times older is the average continental crust than the average ocean crust, by Tala's math (USGS)?

    • About the same age
    • About 420 times
    • About 42 times
    • About 4 times
12. 9.

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

    • About 220 times
    • About 2 times
    • About the same age
    • About 22 times
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 dissolves in seawater
    • Earth had no oceans until about 150 million years ago
    • Old sea floor piles up at the ridges
    • Old sea floor subducts and is replaced by new crust