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, 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 250 km per million years, so the widening would be 2,500 km in 10 million
   • Yes, exactly 250 km, since USGS gives 2.5 cm a year as a fixed number that does not change
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 exactly six times as fast, since 15 divided by 2.5 is exactly 6
   • The East Pacific Rise spreads at least six times as fast, since over 15 divided by about 2.5 is more than 6
   • The Mid-Atlantic Ridge spreads faster, since the Atlantic Ocean is bigger than the Pacific's ridge
   • 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
4. 3.

   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?

   • They are equal, since both figures are the speed at which a boundary slides and both are 'about' 5 to 9
   • India is faster by about 180 times, since 9 m is 900 cm and 900 divided by 5 is 180
   • 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 slower: 9 m a century is under 1 cm a year, since 9 is smaller than 5 times 100
5. 4.

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

   • 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
   • Yes: 1 cm a year is 10 km per million years, so the mountains gain that much height in a million years
   • 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
   • Exactly 100 km, since the USGS rate for the fault is a constant that the card gives for any period
   • About 100 km, since 5 cm a year is 50 km per million years, if the rate held; rates differ over time
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
   • Change 2.5 to 15, since the East Pacific Rise's rate is the one that is fixed for ridges
   • 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
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?

   • 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
   • 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
9. 8.

   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 36 times, since 9 m is 900 cm and 900 divided by 25 gives 36, which fits the larger unit
   • About 6.5 times, since 9 plus 2.5 is 11.5 and the sum is the difference between their speeds
   • 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
   • 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
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?

    • 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: faster spreading means plates sink faster, so it destroys crust rather than making it
    • No: new crust forms at the same speed on each ridge, and spreading speed changes the width of the ridge itself, not the amount
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