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 100 km
   • About 1,000 km
   • About 10 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 250,000 years
   • About 4 million 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?

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

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

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

   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 billions of years old
   • Sediment is being made faster than before
   • Earth is expanding, which stretches the sediment layer thin
   • The ocean floor is young, so little sediment has built up
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 the ridges themselves
   • At ocean trenches, where old crust sinks
   • 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 42 times
    • About 4 times
    • About 420 times
12. 9.

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

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