Seafloor spreading and why ocean crust is young

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. 1.

   Conveyor from a ridge to a trench. A student says new crust at ridges makes Earth grow larger. Which reply fits USGS, by Jun's reading?

   • No; subduction recycles crust at about the same rate
   • Correct; Earth grows a little each year
   • Correct; that growth is why the ocean sediment is so thin
   • No; ridges do not make new crust
2. 2.

   Ridge cross-section with stripes. A student says the stripes formed because the field grew stronger and weaker. Which reply fits USGS, by Kira's reading?

   • No; Vine, Matthews and Morley tied them to field reversals
   • Correct; the stripes formed at random
   • Correct; changes in the field's strength made the ocean stripes
   • No; the stripes come from seawater
4. 3.

   A stripe boundary 75 km from a ridge marks a reversal 3 million years ago (model data). What is that side's spreading rate, by Luz's math?

   • About 25 cm a year
   • About 7.5 cm a year
   • About 225 cm a year
   • About 2.5 cm a year
5. 4.

   USGS: the Atlantic grew over the past 100 to 200 million years. At an example 2.5 cm a year, how wide would 150 million years of spreading make it, by Noah's math?

   Use the picture in question 3.

   • About 3,750 km
   • About 375 km
   • About 37,500 km
   • About 3.75 km
6. 5.

   Four evidence cards, labeled A to D. Which evidence most strongly supports seafloor spreading, by Pia's reading (USGS)?

   • Mirror-image stripes and rock ages that rise with distance from the ridge
   • Ocean floor as old as the continents
   • Stripes on one side of the ridge
   • Thick sediment piled at the ridge crest
7. 6.

   Model data: for the same span of reversals, stripes are 10 km wide at ridge P and 50 km wide at ridge Q. Which spreads faster, by Ren's reading?

   • Ridge P, since narrow stripes mean faster spreading
   • Neither; stripes say nothing about rate
   • They spread at the same rate
   • Ridge Q, since wider stripes mean faster spreading

8. Read this first

   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

9. 7.

   Why do continents keep rock as old as 4 billion years while the ocean floor does not, by Tala's reading (USGS)?

   • Ocean crust is lighter and drifts away
   • The continents formed later than the oceans
   • Light continental crust resists sinking; denser ocean crust subducts
   • Ocean crust grows older until it melts back at the ridges where it formed
10. 8.

    Conveyor from a ridge to a trench. Which summary of seafloor spreading fits USGS, by Zee's reading?

    • Crust forms at trenches and sinks at ridges
    • Crust forms at ridges, and stripes record field strength
    • New crust forms at ridges, ages outward, records reversals, and sinks at trenches
    • Crust forms at ridges, and Earth grows larger

11. 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

12. 9.

    Which summary of crust ages fits USGS, by Ari's reading?

    • Ocean floor up to about 180 million years; continental rock up to about 4 billion
    • Both up to about 4 billion years
    • Both up to about 180 million years
    • Ocean floor up to 4 billion; continents up to 180 million
13. 10.

    Which summary of how spreading rates are found fits USGS, by Bo's reading?

    • Measure the color of the sediment
    • Divide a stripe's distance by the dated age of its reversal
    • Multiply the stripe's distance from the ridge by its dated age
    • Count the stripes and call that cm a year