Name _________________________________ Chapter 9 Study Guide
1. What is the difference between mechanical and chemical weathering?
Mechanical weathering of occurs when a rock is physically broken down
while chemical weathering occurs when a substance that makes up the rock is
chemically changed to a new substance.
2. List and describe the agents of mechanical weathering.
Abrasion – rock and sand particles in wind and water wear away other rock
particles.
Frost wedging – Splits rocks apart through the process of water freezing and
thawing.
Plant growth – Root growth in the cracks of rocks causes them to split and
some roots produce weak acids that chemically weather rocks.
Animal activity – Animals burrow in the ground and break rocks further down.
3. List and describe the agents of chemical weathering.
Carbon dioxide – Forms carbonic acid in rain which then chemically weathers
marble and limestone.
Oxidation – Oxygen reacts to cause rust on some rocks.
Acid rain – Forms from coal, oil and gas burning which then chemically
weathers rocks.
4. What two factors determine the rate of weathering?
The rate of weathering will be accelerated by a warmer and wetter climate and
also depends on the type of rock being weathered.
5. Why do permeable rocks weather easily? They contain connected air spaces, or
pores, which allow water to seep easily through them. These pores give increase opportunity for chemically weathering as well as mechanically weather
such as frost wedging to occur.
6. How is soil formed?
Exposed bedrock is broken down into various particle sizes (gravel, sand, silt
and clay) through the weathering process. The weathered sediment is transported to lower ground by erosion and deposited. Humus is introduced to the
soil through organic matter decaying.
7. What is the composition of soil? Gravel, sand, silt, clay and humus (decayed organic matter)
8. What soil particle is the smallest? Clay
9. What are soil horizons? A layer of soil that differs in color, texture, and composition from the layers above or below it.
10. What does the A horizon consist of? Topsoil - Dark brown soil that is a mixture of humus, clay, and other minerals.
11. What does the B horizon consist of? Subsoil – usually consists of clay and
other particles of rock, but little humus.
12. What does the C horizon consist of? Partly weathered rock.
13. Describe three ways soil can be damaged or lost.

Loss of moisture due to drought

Loss of topsoil due to erosion

Loss of nutrients due to continuous planting and harvesting of nutrient
stealing crops like cotton and corn.
14. Compare and contrast topsoil and subsoil.
Both topsoil and subsoil contain clay. The topsoil, however, contains an abundance of humus while the subsoil contains very little.
15. What is humus? Decayed organic matter.
16. What makes soil high in fertility? A nearly equal mixture of sand, silt and clay
(loam) as well as the presence of humus as moisture.
17. What is the type of soil called loam made of? A nearly equal mixture of sand,
silt and clay.
18. How do living organisms that live in soil help the soil? They decompose organic matter.
19. What can cause the loss of soil that is not protected by plant cover? Erosion.
20. What is the practice of plowing fields along the curves of a slope is called?
Contour plowing.
21. What is crop rotation and how does it conserve soil? Crop rotation is the
practice of planting nutrient stealing crops like cotton and corn, followed
by nutrient fixing crops like peanuts and beans. It conserves soil by restoring nutrients in the soil at varying seasons.
22. What are legumes? Small lumps on the roots of plants like beans that contain nitrogen-fixing bacteria and help restore soil fertility.
23. Which layer of soil in the soil development diagram is made up only of
partly weathered rock? What is this layer called? Layer 3 – C Horizon.
.
24. Which layer of soil in the soil development diagram contains clay and other
particles of rock, but little humus? What is this layer called? Layer 2 – B Horizon (subsoil)
25. Which layer of soil in the soil development diagram contains humus? What is
this layer called? Layer 1 – A Horizon (topsoil)