Features of the ocean floor

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Card (approximate model, example)

   Sea-floor depth in meters is about 2,500 + 350 × √(age in millions of years). Young floor near a ridge is hot and shallow; it deepens as it cools.

2. 1.

   Bo uses the card for sea floor 36 million years old. About how deep is it?

   • About 15,100 m
   • About 2,850 m
   • About 2,506 m
   • About 4,600 m
3. 2.

   Using the same card, how old is sea floor that is 6,000 m deep, by Esme's math?

   • About 10 million years
   • About 17 million years
   • About 294 million years
   • About 100 million years
4. 3.

   Jun compares Site A, 16 million years old, with Site B, 64 million years old. How much deeper is Site B?

   • About 16,800 m deeper
   • About 1,400 m deeper
   • About 5,300 m deeper
   • About 700 m deeper

5. Card

   NOAA says abyssal plains cover about 70% of the ocean floor. Example, rounded: ocean floor 361 million km²; all land 149 million km².

6. 4.

   Milo compares plains with land. About how many times larger than all the land are the abyssal plains?

   • About 2.4 times
   • About 70 times
   • About 0.6 times
   • About 1.7 times

7. Card (example data)

   On one side of a trench the sea floor is 140 million years old. On the other side, beneath an island arc, it is 30 million years old. Older sea floor is colder and denser.

8. 5.

   Two ocean plates meet at a trench. Which one sinks, by Noah's reasoning?

   • Both sink at the same rate, because ocean plates have equal density
   • The 30-million-year-old plate sinks, because it is thinner and weaker
   • Neither sinks; the trench is a crack that both plates fill
   • The 140-million-year-old plate sinks, because it is colder and denser

9. Card (example)

   Fine sediment settles from the water onto the sea floor, filling low spots first and burying small hills. Young floor near a ridge has had little time to collect sediment.

10. 6.

    Why is an abyssal plain flatter than a mid-ocean ridge, by Pia's reasoning?

    • Thick sediment buries the bumpy rock on the plain, while the young ridge is bare
    • Waves from the surface wear the plain flat over millions of years of motion
    • Plates move faster under a plain than under a ridge, which smooths the rock flat
    • A plain forms where plates collide and press the floor flat under their weight

11. Card (example data)

    Young, hot rock near a ridge has a density of 3.28 g/cm³. Cool, old rock far from the ridge has a density of 3.34 g/cm³. Both float on the same mantle.

12. 7.

    Why does young floor stand higher than old floor, by Ren's reasoning?

    • Young floor is pushed up by plates colliding at the ridge crest and piling up
    • Young rock is hotter and less dense, so it floats higher on the mantle
    • Old floor is thinner than young floor, so it rides lower on the mantle below it
    • Young floor is heavier than old floor, so it presses the mantle upward and rises

13. Card (NOAA)

    Mauna Kea rises 13,803 feet above sea level and extends 19,700 feet below it. Mount Everest rises 29,035 feet above sea level.

14. 8.

    Which mountain is taller from base to summit, by Tala's reading, and by how much?

    • They are equal, since both stand on solid rock
    • Mauna Kea, by about 62,535 feet, adding Everest's height to its own
    • Mauna Kea, by about 4,470 feet
    • Everest, by about 15,232 feet, since it rises higher above sea level
15. 9.

    Mauna Kea rises 13,803 feet above sea level and extends 19,700 feet below it. What share of Mauna Kea's height from base to summit is above sea level, by Zee's math?

    • About 59%
    • About 14%
    • About 70%
    • About 41%
16. 10.

    The Challenger Deep is 35,876 feet deep. Mount Everest is 29,035 feet tall. By what percent is the Challenger Deep's depth greater than Everest's height, by Ari's math?

    • About 24%
    • About 19%
    • About 124%
    • About 6,841%