Resistance to Erosion
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he amount of erosion that occurs, whether on the banks of the
Mississippi River or on Green Hill in Boomtown, depends on
two major factors. The first factor is the strength of the force causing
the erosion. For example, a fast-flowing river has more force than a
trickle of water. Water flowing down a steep slope will erode the land
more quickly than the same amount of water flowing down a flatter
portion of the same hill, because the water travels faster on the steep
slope. The second factor that determines the effect of erosion is the
resistance of the material that is being eroded. For example, if granite
and sandstone are both undergoing the same erosional force, the
sandstone will erode faster because it is “softer,” or less resistant to
erosion.
In and around Boomtown, there are different kinds of soils and rocks. Green
Hill has loose and relatively soft soil. Seaside Cliff has more erosion-resistant, compact soil that is also sticky. In this activity you will investigate the
resistance of earth materials to erosion under similar forces.
CHALLENGE
Do all kinds of earth materials erode in the same way?
The rock layers shown above are made of limestone and shale. Notice how the two kinds of rock
erode differently.
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Resistance to Erosion • Activity 31
MATERIALS
For each group of four students
3
plastic basins
1
rainmaker
1
50-mL graduated cylinder
1
30-mL graduated cup
1
plastic spoon
supply of Earth Material A
supply of Earth Material B
supply of Earth Material C
supply of water
paper towels
masking tape
PROCEDURE
1. In your science notebook, make a table like the one below.
Observations of Erosion Resistance
Basin Observations
Erosion
Ranking
Water
Runoff
Ranking
A
B
C
2. Place the three plastic basins next to each other on the table. Use
the masking tape to label them A, B, and C.
3. Using the spoon, pack the 30-mL graduated cup with Earth
Material A.
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Activity 31 • Resistance to Erosion
4. Make a small “hill” by flipping the cup over in the center of Basin
A. Remove the 30-mL cup, but leave the material standing in the
basin as shown.
5. Repeat Steps 3 and 4 using Earth Materials B and C and their
labeled basins.
6. Place the empty rainmaker over the basin that contains Earth
Material A.
7. Add 50 mL of water to the rainmaker.
8. Observe any changes that take place as a result of the water on the
hill. Pay close attention to how much erosion occurs and how
much water runs off the hill. Record your observations in your
table.
9. Repeat Steps 3–8 for Earth Materials B and C.
10. Compare the three model hills. In your table, rank how much erosion occurred on each hill with 1 indicating the most erosion and
3 indicating the least erosion. Then rank how much water ran off
the hills with 1 indicating the most runoff and 3 indicating the
least runoff.
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Resistance to Erosion • Activity 31
ANALYSIS
1. How are the models you used:
a. like a real landform?
b. not like real landform?
2. In this investigation, what variables were:
a. kept the same?
b. tested?
c. not kept the same?
3. In your model, which earth material showed the least erosion?
Using evidence from this investigation, explain why this material
may have eroded less than the other two.
4. The hills made in this activity model the three kinds of earth materials listed below. Copy the list and then, next to each description,
write whether it describes Earth Material A, B, or C. For each one,
give the evidence from this activity that helped you make your
choice.
a. loose soil
b. sticky soil
c. loose soil with vegetation
5. Would you expect more erosion to be observed at Green Hill or
Seaside Cliff?
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