Seafloor spreading and why ocean crust is young

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. 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

2. 1.

   A classmate says the ocean floor must be as old as the continents. Which reply fits USGS, by Esme's reading?

   • Correct; the oceans formed first
   • Correct; both are about 4 billion years old
   • No; the ocean floor is older than the continents
   • No; ocean floor reaches about 180 million years, continental rock about 4 billion
3. 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; the stripes come from seawater
   • Correct; the stripes formed at random
   • Correct; changes in the field's strength made the ocean stripes
   • No; Vine, Matthews and Morley tied them to field reversals
5. 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 225 cm a year
   • About 7.5 cm a year
   • About 25 cm a year
   • About 2.5 cm a year
6. 4.

   USGS: the East Pacific Rise spreads more than 15 cm a year and the Arctic Ridge less than 2.5. How much crust does each add in 1 million years, by Milo's math?

   Use the picture in question 3.

   • More than 15 km versus less than 2.5 km
   • About the same width for both
   • More than 1,500 km versus less than 250 km
   • More than 150 km versus less than 25 km
7. 5.

   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 375 km
   • About 3.75 km
   • About 37,500 km
   • About 3,750 km
8. 6.

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

   • Thick sediment piled at the ridge crest
   • 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
9. 7.

   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?

   • Neither; stripes say nothing about rate
   • Ridge P, since narrow stripes mean faster spreading
   • They spread at the same rate
   • Ridge Q, since wider stripes mean faster spreading
10. 8.

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

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

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

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

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