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 10 km
   • About 100 m
   • About 1,000 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 400 years
   • About 4 million years
   • About 40,000 years

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?

   • The stripes are oldest at the crest
   • Spreading on one side of the ridge
   • Mirror-image stripes, with the youngest normal stripe at the crest
   • Random stripes with no meaning
6. 4.

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

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

   • 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
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?

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

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

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

    • About 22 times
    • About 2 times
    • About the same age
    • About 220 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
    • 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