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?

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

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?

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

   GPS gives today's rates, and GPS agrees with long-term rates. Different spans and methods can give different values. A student tells Kira that GPS must be wrong, since its rate differs from the long-term rate. Which reply fits the card?

   • Supported; just dated rocks give true rates, so GPS must be the wrong method
   • Not supported; GPS measures the age of stripes and so matches the stripes
   • Supported; GPS and long-term rates must differ whenever the two are compared
   • Not supported; USGS says GPS agrees with long-term rates, so differences come from span and method
6. 4.

   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 2,600 km, about 900 km short of the measured 3,500 km
   • 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
7. 5.

   The stripe on each side lies 100 km from the ridge crest and is 4 million years old. Stripes 100 km on each side of a ridge are 4 million years old (example). At what rate do the two plates separate, by Noah's math? (1 km = 100,000 cm.)

   • 2.5 cm a year, the rate of one side
   • 0.5 cm a year
   • 5 cm a year
   • 10 cm a year

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. 6.

   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

    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.

11. 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 10.3 cm a year
    • About 9.3 cm a year, unchanged by the hotspot's drift
    • About 93 cm a year
    • About 8.3 cm a year

12. 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

13. 8.

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

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

14. 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.

15. 9.

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

    • Use a distance alone, since the time it took is already known from the map
    • Use a distance and the time it took, from two dated points, then match the units
    • 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

16. 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.

17. 10.

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

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