Features of the ocean floor

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Card (example)

   Sea floor 500 km from a ridge crest is 20 million years old. Magma rises at the crest and hardens into new floor. Old floor sinks at trenches. Some continental rock is billions of years old.

2. 1.

   A student tells Esme the sea floor is as old as the continents. Which reply is best supported by the card?

   • Yes; both formed together when Earth cooled, and neither has changed since then
   • No; floor is made at ridges and recycled at trenches, so it is much younger
   • Yes; magma rising at ridges keeps adding new floor and no floor is lost anywhere
   • No; the sea floor is older, since it lies below the continents and formed first

3. Card

   Trenches are narrow and 8 to 10 km deep (USGS). They lie beside curved chains of volcanoes. Deep ocean currents move slowly (example).

4. 2.

   A student tells Jun that deep currents carved the trenches. Which reply is best supported by the card?

   • Yes; deep currents carve trenches the same way rivers cut canyons on land, but deeper
   • No; trenches form where plates part, and volcanoes then fill the trenches with rock
   • No; currents cannot cut a trench 8 to 10 km deep beside volcanoes; plate convergence fits
   • Yes; the volcanoes beside trenches are built from the rock that currents eroded

5. Card (NOAA)

   The mid-ocean ridge spans 40,389 miles, about 90% under the ocean. 1 mile is 1.609 km. Example, rounded: the Andes, the longest range on land, stretch about 7,000 km.

6. 3.

   A student tells Kira that ridges are small next to land mountain ranges. Which reply fits the card?

   • Yes; just the part above water counts as a mountain range, so the ridge is small
   • Yes; at about 6,500 km the ridge is shorter than the Andes, so it is small by comparison
   • No; the ridge is about 65,000 km long, though it lies entirely above the water line
   • No; at about 65,000 km the ridge is many times the Andes, but about 90% is underwater

7. Card (example data)

   a submersible descends at 0.5 m per second, explores for 1 hour, then rises at 1 m per second. The average ocean depth is 3,682 m (NOAA).

8. 4.

   Luz plans a dive to the average ocean depth. About how long is the whole trip?

   • About 5.1 hours
   • About 3.1 hours
   • About 4.1 hours
   • About 2.0 hours

9. Card (example, rounded)

   Water pressure rises 1 atmosphere (atm) for every 10 m of depth, on top of 1 atm at the surface. NOAA: average depth 3,682 m; deepest about 10,935 m.

10. 5.

    How much more pressure is there at the deepest point than at the average depth, by Milo's math?

    • About 1,094 atm more
    • About 72 atm more
    • About 7,253 atm more
    • About 725 atm more

11. 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.

12. 6.

    Floor spreads from a ridge at 4 cm a year. How far from the ridge is floor that is 5,300 m deep, by Noah's math? (1 km = 100,000 cm.)

    • About 25,600 km
    • About 256 km
    • About 2,560 km
    • About 320 km

13. Sea-floor sites (example data)

    Site	Age	Depth
    Site 1	25 million years	4,250 m
    Site 2	49 million years	4,950 m
    Site 3	81 million years	5,650 m
    Site 4	64 million years	4,600 m

14. 7.

    Pia tests the rule depth = 2,500 + 350 × √age (age in millions of years) on four sites. Which conclusion fits the table?

    • The rule fails at each of the four sites, so sea-floor depth does not depend on age
    • Site 4 is 700 m deeper than the rule predicts, so it is probably the oldest floor of the four
    • The four sites fit the rule within 100 m, so age explains each depth in the table
    • Sites 1 to 3 fit the rule; Site 4 is 700 m shallower than predicted, so something lifts it

15. Card (example data)

    a ridge flank deepens 1,400 m over 700 km. The unknown belt beside curved volcanic islands deepens from 4,000 m to 10,000 m over 60 km.

16. 8.

    Which feature is the belt, and why, by Ren's reading?

    • A trench: it drops about 5 times more steeply than a ridge flank
    • A trench: it drops about 50 times more steeply than a ridge flank
    • A continental slope, since it drops steeply beside the volcanic islands
    • A ridge flank, since depth changes with distance along both of them
17. 9.

    The average ocean depth is about 3,682 m. The deepest point is about 10,935 m. Which statement does the card support, by Ari's math?

    • The deepest point is about 30 times the average depth and about 7,250 m deeper
    • The deepest point is about 3 times the average depth and about 7,250 m deeper
    • The deepest point is about 3 times the average depth and about 14,600 m deeper
    • The deepest point is about one-third of the average depth and about 7,250 m deeper

18. Card (NOAA)

    Ridges form where plates push apart and magma rises; trenches form where plates converge and one moves under the other; seamounts are volcanic mountains. Sediment settles on abyssal plains.

19. 10.

    Which summary links each feature to how it forms, by Bo's reading?

    • Ridges: plates meet and one sinks. Trenches: plates part. Seamounts: volcanoes. Plains: settled sediment
    • Ridges: plates part. Trenches: plates meet, one sinks. Seamounts: settled sediment. Plains: volcanoes
    • Ridges: plates part. Trenches: wave erosion cuts them. Seamounts: volcanoes. Plains: settled sediment
    • Ridges: plates part. Trenches: plates meet, one sinks. Seamounts: volcanoes. Plains: settled sediment