Plate tectonics and how landforms are made

Grade 9 Social Studies Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Card G, plate speeds (USGS)

   Mid-Atlantic Ridge, plates pulling apart at about 2.5 cm a year; East Pacific Rise, over 15 cm a year; San Andreas Fault, plates sliding at about 5 cm a year; India moving into Asia at about 9 m a century, slowing by about half over time; the Himalayas rising more than 1 cm a year. Card H, notes: 1 cm a year is 10 km per million years. Rates differ by place and over time. Rock on rising mountains is worn away by erosion. Most of India's motion is taken up by squeezing and thickening of the crust, and only part of it shows as height. At divergent boundaries, new crust forms in the gap as the plates move apart.

2. 1.

   Bo says the Mid-Atlantic Ridge's 2.5 cm a year means the Atlantic could widen about 250 km in 10 million years. Is that supported by Cards G and H?

   • Yes, exactly 250 km, since USGS gives 2.5 cm a year as a fixed number that does not change
   • Yes, as an estimate: 2.5 cm a year is 25 km per million years, so about 250 km in 10 million
   • No: 2.5 cm a year is 2.5 km per million years, so the widening would be just 25 km in 10 million
   • No: 2.5 cm a year is 250 km per million years, so the widening would be 2,500 km in 10 million
3. 2.

   The East Pacific Rise spreads at over 15 cm a year and the Mid-Atlantic Ridge at about 2.5. Which comparison is supported, Esme asks?

   • The East Pacific Rise spreads about twice as fast, since 15 is a little more than 2.5 plus 2.5, so the ratio is about 2 and the gap is small
   • The East Pacific Rise spreads exactly six times as fast, since 15 divided by 2.5 is exactly 6
   • The Mid-Atlantic Ridge spreads faster, since the Atlantic Ocean is bigger than the Pacific's ridge
   • The East Pacific Rise spreads at least six times as fast, since over 15 divided by about 2.5 is more than 6
4. 3.

   Kira's note: 'The Himalayas rise 9 cm a year because India moves 9 cm a year.' Which fix fits Cards G and H?

   • Keep it, since the Himalayas are the result of India's motion and the two rates must therefore be equal in each year
   • Change 'rise' to 'sink', since squeezing and thickening of crust makes the mountains lower with time
   • India moves about 9 cm a year, but the mountains rise more than 1 cm a year, since most motion is squeezing
   • Change 9 cm to 1 cm for India, since the mountains rise as fast as the plate moves toward Asia
5. 4.

   Luz says the Himalayas gain 10 km of height every million years. What do Cards G and H support?

   • Yes: 1 cm a year is 10 km per million years, so the mountains gain that much height in a million years
   • No: 1 cm a year is just 1 km per million years, so the claim is ten times too large
   • No: erosion wears rock away faster than the mountains rise, so the Himalayas get lower each year
   • Rising at over 1 cm a year is over 10 km of uplift per million years, but erosion wears rock away
6. 5.

   At about 5 cm a year, how far could two points across the San Andreas Fault slide apart in 2 million years, using Cards G and H, Milo asks?

   • About 1,000 km, since 5 cm a year is 500 km per million years, and two million years doubles that
   • About 10 km, since 5 cm a year is 5 km per million years, and two million years doubles that to about 10
   • About 100 km, since 5 cm a year is 50 km per million years, if the rate held; rates differ over time
   • Exactly 100 km, since the USGS rate for the fault is a constant that the card gives for any period
7. 6.

   Noah's note: 'Plate speeds are fixed numbers, so the Atlantic always widened at 2.5 cm a year.' Which fix fits Card H?

   • Delete the note, since a rate that changes over time gives no information about the past or the future of the plates
   • Change 2.5 to 15, since the East Pacific Rise's rate is the one that is fixed for ridges
   • Plate speeds are estimates that differ by place and over time, so 2.5 cm a year is about the rate today
   • Keep it, since a rate given by USGS is the measured speed and measured speeds do not change over time
8. 7.

   Pia's note: 'At 2.5 cm a year, the Atlantic was exactly 2,500 km narrower 100 million years ago.' Which fix fits Cards G and H?

   • Keep it, since 2.5 cm a year over 100 million years makes exactly 2,500 km and the USGS rate is exact, not an estimate
   • Change 2,500 to 250, since 2.5 cm a year is 2.5 km per million years and 100 million years gives 250
   • About 2,500 km narrower if the rate had stayed the same; since rates differ over time, it is a rough estimate
   • Change it to say no estimate can be made, since the rate may have been different at some time
9. 8.

   Ren says divergent boundaries are slower than transform ones, since 2.5 cm a year is less than 5. What do the cards support?

   • Wrong: transform boundaries are the slowest, since sliding plates rub against each other and slow down
   • Wrong: plate speeds are the same, since each of the rates is described as 'about' a small number
   • Right: divergent boundaries are slower, since the Mid-Atlantic Ridge's 2.5 is below the San Andreas's 5, which settles it
   • Divergent rates vary too: the East Pacific Rise spreads at over 15 cm a year, faster than the San Andreas
10. 9.

    India closes on Asia at about 9 cm a year. The Atlantic widens at about 2.5 cm a year. About how many times faster is India's motion, Tala asks?

    • About 6.5 times, since 9 plus 2.5 is 11.5 and the sum is the difference between their speeds
    • About 36 times, since 9 m is 900 cm and 900 divided by 25 gives 36, which fits the larger unit
    • About 3.6 times as fast, since 9 divided by 2.5 is about 3.6 once 9 m a century is read as 9 cm a year
    • About 0.36 times, since India's 9 m a century is slower than a ridge that moves at 2.5 cm a year, by a large margin
11. 10.

    A student says faster spreading means more new crust forms each year along a ridge. What do Cards G and H support, Zee asks?

    • New crust fills the gap as plates move apart, so a ridge at over 15 cm a year makes more per year than one at 2.5
    • Yes, because the East Pacific Rise is a longer ridge than the Mid-Atlantic Ridge, so it makes more crust
    • No: new crust forms at the same speed on each ridge, and spreading speed changes the width of the ridge itself, not the amount
    • No: faster spreading means plates sink faster, so it destroys crust rather than making it