How fast plates move

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Card (USGS)

   The Mid-Atlantic Ridge spreads about 2.5 cm a year, the San Andreas slips about 5, and the East Pacific Rise spreads over 15.

2. 1.

   A student tells Esme that every plate moves at one fixed speed. Which reply fits the card?

   • Not supported; rates range from about 2.5 to over 15 cm a year across boundaries
   • Supported; the plates share the Mid-Atlantic Ridge's rate of about 2.5 cm a year
   • Not supported; rates range from about 15 to 150 cm a year across boundaries
   • Supported; the San Andreas rate of 5 cm a year fits each plate in the world

3. Hawaiian chain (USGS ages; the bend's distance is example data)

   Place	Age	Distance from the Big Island
   Midway	28 million years	2,600 km
   The bend	43 million years	3,500 km
   Northernmost seamounts	80 million years	6,000 km

4. 2.

   A student tells Jun that the Hawaiian chain proves the Pacific Plate always moved 9.3 cm a year. Which reply fits the card?

   • Not supported; 9.3 averages one stretch, other stretches give about 6 to 7, and a fixed hotspot is an assumption
   • Not supported; hotspots are measured by GPS, which gives 6 cm a year today
   • Supported; 9.3 cm a year is the average rate along the whole of the chain
   • Supported; the bend shows the plate sped up to 9.3 cm a year for 80 million years
5. 3.

   The bend is 43 million years old and lies 3,500 km from the Big Island. 1 km = 100,000 cm. If the plate had kept moving 9.3 cm a year for 43 million years, how far would the bend lie, and how does that compare with 3,500 km, by Luz's math?

   • About 4,000 km, about 500 km beyond the measured 3,500 km
   • About 40,000 km, about 36,500 km beyond the measured 3,500 km
   • About 400 km, about 3,100 km short of the measured 3,500 km
   • About 2,600 km, about 900 km short of the measured 3,500 km

6. Card (USGS)

   The Himalayas rise more than 1 cm a year, which is 10 km per million years. Everest rises under 9 km above sea level.

7. 4.

   Rising 1 cm a year for 10 million years would be 100 km. Everest is under 9 km tall. What does that show, by Milo's reasoning?

   • The uplift rate is wrong by ten times, so the true rate must be 0.1 cm a year, not 1 cm
   • Plate motion pushes mountains down as quickly as it raises them, so there is no uplift to count
   • The Himalayas stopped rising after one million years, so the 1 cm a year rate no longer applies
   • Erosion and other processes remove much of the rise, so uplift rate alone does not give height

8. Pacific Plate rates by method (USGS)

   Method	Span	Rate
   GPS	today	6 cm a year
   Midway's age	28 million years	9.3 cm a year
   Whole chain	80 million years	7.5 cm a year

9. 5.

   A student says the rates show the plate has sped up from 6 to 9.3 cm a year. What does the table support, by Pia's reading?

   • Motion may have been faster in the past, but methods and spans differ, so a trend is not shown
   • The values show the plate is slowing at a steady rate each million years
   • The plate has sped up from 6 to 9.3 cm a year, as the table shows directly
   • The 6 cm a year value is wrong, since it differs from the other two values

10. Card (example data)

    Age 28 ± 1 million years; distance 2,600 ± 100 km. 1 cm a year is 10 km per million years.

11. 6.

    Midway's age is 28 ± 1 million years and its distance is 2,600 ± 100 km (example uncertainties). What range of rates results, by Ren's math?

    • About 8.6 to 10.0 cm a year
    • About 8.9 to 9.6 cm a year, from the distance uncertainty alone
    • About 7.5 to 11 cm a year
    • About 9.0 to 9.6 cm a year, from the age uncertainty alone

12. Card

    USGS's author's note says some newer studies question whether hotspots are fixed. The chain gives the plate's motion relative to the hotspot as 9.3 cm a year.

13. 7.

    Suppose the hotspot itself drifts 1 cm a year in the same direction the plate moves (example). What is the plate's motion relative to the deep mantle, by Tala's reasoning?

    • About 93 cm a year
    • About 8.3 cm a year
    • About 9.3 cm a year, unchanged by the hotspot's drift
    • About 10.3 cm a year

14. Pacific Plate rates by method (USGS)

    Method	Span	Rate
    GPS	today	6 cm a year
    Midway's age	28 million years	9.3 cm a year
    Whole chain	80 million years	7.5 cm a year

15. 8.

    Which summary of the Pacific Plate's speed fits the table, by Zee's reading?

    • About 6 to 9.3 cm a year, depending on method and span
    • Exactly 9.3 cm a year, whatever the method or span
    • Exactly 6 cm a year, whatever the method or span
    • About 60 to 93 cm a year, depending on method and span

16. Card A (USGS)

    Rates come from a distance and a time. Card B: dated stripes, dated volcanoes and offset rocks give long-term rates; GPS gives today's rate.

17. 9.

    Which summary of how to find a rate fits the cards, by Ari's reading?

    • Use a distance and the time it took, from two dated points, then match the units
    • Use a distance alone, since the time it took is already known from the map
    • Use GPS for any past span, since it measures any span of time that has gone by, even millions of years
    • Use a rock's age alone, since the age of a rock is already a rate of motion

18. Card (USGS)

    The Arctic Ridge spreads under 2.5 cm a year and the East Pacific Rise over 15. 1 cm a year is 10 km per million years.

19. 10.

    Which summary of plate speeds fits the cards, by Bo's reading?

    • A few meters a year, from under 2.5 to over 15, which is tens of km per million years
    • A few cm a year, from under 2.5 to over 15, which is tens of km per million years
    • A few cm a year, from under 2.5 to over 15, which is hundreds of km per million years
    • A few cm a year, from under 25 to over 150, which is tens of km per million years