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?

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

   India moves at about 9 m a century. Using Card G, how does it rank against the San Andreas Fault at 5 cm a year, Jun asks?

   • India is faster: 9 m a century is 9 cm a year, about 1.8 times the San Andreas rate of 5 cm a year
   • India is faster by about 180 times, since 9 m is 900 cm and 900 divided by 5 is 180
   • India is slower: 9 m a century is under 1 cm a year, since 9 is smaller than 5 times 100
   • They are equal, since both figures are the speed at which a boundary slides and both are 'about' 5 to 9
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?

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

   • Rising at over 1 cm a year is over 10 km of uplift per million years, but erosion wears rock away
   • Yes: 1 cm a year is 10 km per million years, so the mountains gain that much height in a million years
   • No: erosion wears rock away faster than the mountains rise, so the Himalayas get lower each year
   • No: 1 cm a year is just 1 km per million years, so the claim is ten times too large
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 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
   • About 10 km, since 5 cm a year is 5 km per million years, and two million years doubles that to about 10
   • About 1,000 km, since 5 cm a year is 500 km per million years, and two million years doubles that
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?

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

   • About 2,500 km narrower if the rate had stayed the same; since rates differ over time, it is a rough estimate
   • 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
   • 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?

   • 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
   • 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
10. 9.

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

    • No: new crust forms at the same speed on each ridge, and spreading speed changes the width of the ridge itself, not the amount
    • 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
    • No: faster spreading means plates sink faster, so it destroys crust rather than making it
    • Yes, because the East Pacific Rise is a longer ridge than the Mid-Atlantic Ridge, so it makes more crust
11. 10.

    Card G says India's motion is slowing by about half over time. Which statement is supported, Ari asks?

    • India's motion has stopped, since a rate that has slowed by about half is close to zero now
    • Distances for India's past can be worked out exactly from today's speed, since slowing is a small effect on the scale of the motion
    • The Himalayas stopped rising when the motion slowed, since uplift follows the speed of India's motion
    • A distance worked out from today's speed over a long past would be off, since the speed has not stayed the same