Features of the ocean floor
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
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.
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
- 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
- No; floor is made at ridges and recycled at trenches, so it is much younger
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.
A student tells Kira that ridges are small next to land mountain ranges. Which reply fits the card?
- No; at about 65,000 km the ridge is many times the Andes, but about 90% is underwater
- Yes; just the part above water counts as a mountain range, so the ridge is small
- No; the ridge is about 65,000 km long, though it lies entirely above the water line
- Yes; at about 6,500 km the ridge is shorter than the Andes, so it is small by comparison
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).
Luz plans a dive to the average ocean depth. About how long is the whole trip?
- About 2.0 hours
- About 3.1 hours
- About 5.1 hours
- About 4.1 hours
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.
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 256 km
- About 320 km
- About 25,600 km
- About 2,560 km
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 Pia tests the rule depth = 2,500 + 350 × √age (age in millions of years) on four sites. Which conclusion fits the table?
- The four sites fit the rule within 100 m, so age explains each depth in the table
- Site 4 is 700 m deeper than the rule predicts, so it is probably the oldest floor of the four
- Sites 1 to 3 fit the rule; Site 4 is 700 m shallower than predicted, so something lifts it
- The rule fails at each of the four sites, so sea-floor depth does not depend on age
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.
Which feature is the belt, and why, by Ren's reading?
- A continental slope, since it drops steeply beside the volcanic islands
- A trench: it drops about 5 times more steeply than a ridge flank
- A ridge flank, since depth changes with distance along both of them
- A trench: it drops about 50 times more steeply than a ridge flank
Claim: new sea floor forms at a ridge crest and spreads away on both sides. Which evidence would most strengthen the claim, by Tala's reading?
- A photo of fresh black rock at the crest taken by a submersible, showing new lava
- Depth readings along one line that deepen away from the crest on both of its sides
- Dated rock samples showing that ages rise with distance from the crest on both sides
- The crest's height compared with the heights of other mountain ranges on land
Sonar line from shore (example data)
Distance from shore Depth 0 km 0 m 50 km 120 m 80 km 2,400 m 180 km 4,000 m 800 km 4,200 m Which summary fits the table, by Zee's math?
- A gentle shelf, a steep slope near 76 m per km, then a rise steeper than the slope
- A gentle shelf, a steep slope near 76 m per km, a rise near 16 m per km, then a nearly flat plain
- A gentle shelf, a steep slope near 760 m per km, a rise near 160 m per km, then a nearly flat plain
- A steep shelf, a gentle slope near 16 m per km, a rise near 76 m per km, then a nearly flat plain
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 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
- The deepest point is about 3 times the average depth and about 7,250 m deeper
- The deepest point is about 30 times the average depth and about 7,250 m deeper
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.
Which summary links each feature to how it forms, by Bo's reading?
- Ridges: plates part. Trenches: plates meet, one sinks. Seamounts: volcanoes. Plains: settled sediment
- Ridges: plates part. Trenches: wave erosion cuts them. Seamounts: volcanoes. Plains: settled sediment
- Ridges: plates part. Trenches: plates meet, one sinks. Seamounts: settled sediment. Plains: volcanoes
- Ridges: plates meet and one sinks. Trenches: plates part. Seamounts: volcanoes. Plains: settled sediment