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

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

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

4. Sonar line across a trench (example data)

   Distance along line	Depth
   0 km	5,000 m
   100 km	5,100 m
   160 km	9,600 m
   200 km	9,800 m
   260 km	2,000 m

5. 3.

   Kira compares the two sides of the trench. Which side is steeper, and by how much on average?

   • The 200 to 260 km side, at 7,800 m per km against 4,500 m per km
   • The 100 to 160 km side, at 75 m per km against 130 m per km
   • Neither; both sides span 60 km, so they are equally steep
   • The 200 to 260 km side, at 130 m per km against 75 m per km

6. Sonar line across a ridge (example data)

   Place	Depth
   West 300 km	3,900 m
   West 100 km	3,100 m
   Crest	2,600 m
   East 100 km	3,100 m
   East 300 km	3,900 m

7. 4.

   Luz reads the symmetrical profile. Which interpretation fits the table?

   • New floor forms at the crest and spreads both ways, deepening as it cools
   • The crest is the oldest floor, so the younger sides sank into deeper water
   • Old floor sinks at the crest, so both sides lie deeper than the crest
   • The sides are trenches, since depth increases away from the crest

8. Card

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

9. 5.

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

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

10. 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.

11. 6.

    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 140-million-year-old plate sinks, because it is colder and denser
    • Neither sinks; the trench is a crack that both plates fill
    • The 30-million-year-old plate sinks, because it is thinner and weaker

12. 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.

13. 7.

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

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

14. 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.

15. 8.

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

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

16. 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.

17. 9.

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

    • Everest, by about 15,232 feet, since it rises higher above sea level
    • Mauna Kea, by about 62,535 feet, adding Everest's height to its own
    • Mauna Kea, by about 4,470 feet
    • They are equal, since both stand on solid rock
18. 10.

    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 70%
    • About 14%
    • About 59%
    • About 41%