Forces that build land up and wear it down

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Model card (example data)

   Crust 2.7 g/cm³ floats on mantle 3.3 g/cm³. As rock is removed, the crust floats up by 2.7/3.3 of the thickness removed.

2. 1.

   A student says removing 3,300 m of rock lowers a range's peaks by 3,300 m. Milo uses the model card. What is the net lowering?

   • About 600 m, since the crust rebounds about 2,700 m as the load is removed
   • About 2,700 m, since the rebound is the lowering
   • About 3,300 m, since peaks drop by the thickness eroded
   • The peaks rise 2,700 m, since erosion lifts the crust
3. 2.

   A student says a range rising 1 cm a year (USGS) will be 100 km taller in 10 million years. Luz checks the claim. What is its main flaw?

   • Uplift slows when erosion piles sediment on the peaks
   • Mountains cannot rise, since weathering wins over uplift in the long run
   • It ignores erosion and changing rates over millions of years
   • The rate is too small: 1 cm a year adds just 10 km in 10 million years, not 100 km

4. Model card (example data)

   Crust 2.7 g/cm³ floats on mantle 3.3 g/cm³. As rock is removed, the crust floats up by 2.7/3.3 of the thickness removed.

5. 3.

   Erosion removes 20 m of rock per million years from a range. Bo uses the model card. How long to lower its peaks by 600 m?

   • About 165 million years
   • About 1.65 million years
   • About 30 million years
   • About 37 million years
6. 4.

   Kira studies a basin of 2,000 km² that loses 1.6 million tonnes of rock a year (rock is 2.6 tonnes per m³; example data). What is the average lowering per year?

   • About 2.1 mm a year
   • About 0.3 mm a year
   • About 0.8 mm a year
   • About 0.3 m a year

7. River terraces above a river (example data)

   Terrace	Height above river (m)	Age (million years)
   Lower	40	0.8
   Upper	100	1.6

8. 5.

   Pia compares incision rates. How did the river's cutting rate change from 1.6 to 0.8 million years ago and since?

   • It slowed: 100 m per million years at first, then 40 m
   • It sped up: 50 m per million years at first, then 75 m
   • It stayed constant at 62.5 m per million years, the overall average
   • It slowed: 75 m per million years before 0.8 million years ago, then 50 m
9. 6.

   Sea-floor limestone now lies 2,700 m up. Noah weighs two explanations: the land rose, or the sea once stood that high. Which evidence would best decide?

   • The same marine layer lies at sea level on a coast 300 km away
   • More marine fossils found in the plateau rock, of many kinds and ages
   • Limestone layers that lie flat
   • A date showing the limestone formed on the sea floor
10. 7.

    A study measures 0.2 mm a year of erosion at one rock outcrop. A writer claims the whole range erodes at that rate. Tala grades the claim. What is the best verdict?

    • Strongly supported: a measured rate applies everywhere in a range
    • Not supported: erosion rates cannot be measured
    • Weakly supported: one outcrop cannot represent a varied range
    • Supported: a precise measurement at one place is precise everywhere

11. Three sites (example data)

    Site	Uplift (mm/yr)	Erosion (mm/yr)	Height trend
    P	5	5	steady
    Q	0	1	falling
    R	6	2	rising

12. 8.

    Zee reads the table. Which summary of what sets height fits all three sites?

    • Height follows erosion alone, so R at 2 mm a year is the lowest site
    • Height follows whichever rate is larger, so P is falling
    • Height follows uplift minus erosion: steady at P, falling at Q, rising at R
    • Height follows uplift alone, so P at 5 mm a year must be rising

13. Table, Three climates (example data), with columns Site, Mean temperature (°C), Rain (mm/yr), Weathering seen

    Alpine, -2, 900, frost-split blocks, little clay; Rainforest, 25, 3,000, deep clay soil, few fragments; Desert, 22, 100, thin rock crumbs, little clay.

14. 9.

    Esme reads the table. Which summary of weathering fits all three sites?

    • Wet sites break rock into blocks, while cold sites turn it to clay
    • The three sites show chemical weathering at the same rate
    • Cold sites favor physical breakup; warm, wet sites favor chemical change
    • Warmth alone drives chemical weathering, since the 22 °C desert made clay
15. 10.

    Ari runs uplift of 0.5 mm a year against erosion of 0.4 mm a year for 10 million years (example data). Which summary fits?

    • Net gain is 1,000 m, and a small rate error cannot change it much
    • Net gain is 1,000 m, the gap between 5,000 m and 4,000 m, so a 10% error in erosion shifts it by 400 m
    • Net gain is 5,000 m, the larger total, whatever the erosion rate
    • Net gain is 9,000 m, since both processes move rock