1. Section 4.1 Notes: Mechanical & Chemical Forces Break Down Rocks
Big Idea: Natural forces break rocks apart and form soil, which supports life.

2. Weathering breaks rocks into smaller pieces
Weathering is the process by which natural forces break down rocks.
There are 2 types of weathering:
Mechanical weathering
Chemical weathering

3. Mechanical weathering produces physical changes in rocks
Mechanical weathering is the breaking up of rocks by physical forces.
There are 4 main types:
Ice wedging
Exfoliation
Plant root growth
Abrasion

4. Ice Wedging
When water freezes, it expands. When water freezes in the cracks and pores of rocks, the force of its expansion is strong enough to split the rocks apart.

5. Exfoliation
The process in which layers or sheets of rock gradually break off due to the expansion of the rock caused by a decrease in pressure.

6. Plant Root Growth
Trees, bushes, and other plants may take root in cracks of rocks. As the roots of these plants grow, they wedge open the cracks.

7. Abrasion
The process of wearing down by friction, the rubbing of one object or surface against another.

8. Chemical weathering changes the mineral composition of rocks
The breakdown of rocks by chemical reactions that change the rocks’ makeup, or composition.
There are 2 main types:
Dissolving
Rusting

9. Dissolving
Water is the main cause of chemical weathering.
Many more minerals dissolve in water that is slightly acidic – like lemonade.

10. Rusting
The oxygen in air is also involved in chemical weathering.
Many common minerals contain iron. When these minerals dissolve in water, oxygen in the air and the water combines to produce rust.

11. Weathering occurs at different rates
Most weathering occurs over long periods of time – hundreds, thousands, or even millions of years.
There are 3 Influential factors:
Surface area
Rock composition
Climate

12. Surface Area
The more of a rock’s surface that is exposed to air and water, the faster the rock will break down.

13. Rock Composition
Different kinds of rock break down at different rates.

14. Climate
Chemical weathering occurs faster in hot, wet regions than it does in cold, dry regions.
Mechanical weathering caused by freezing and thawing occurs more in cold regions than in hot regions.

15. Section 4.2 Notes: Weathering & Organic Processes Form Soil
Big Idea: Natural forces break rocks apart and form soil, which supports life.

16. What is soil?
Soil is a surface mixture of rocks, minerals, organic matter, water, and air.

17. Where do the ingredients come from?
The rocks and minerals come from weathering.
The organic matter is called humus, which comes from the decay of different life forms in or on the soil.

18. Where do the ingredients come from?
Water is from precipitation
Soil air is from the atmosphere.

19. What determines the soil in an area?
Type of rock
Climate and overall weather
Landforms – mountains & valleys
Plant cover
Organisms
Time

20. How are the soil parts arranged?
Rocks, minerals, and organic matter form soil particles (~ 50% of a soil by volume).
The spaces between soil particles are called soil pores
(~ 50% of a soil by volume).
Water and air fill the pores.

21. How does soil develop?
Over time, soils develop horizons, distinct layers of soil identified by color, texture, and particle structure.

22. How does soil develop?
O: Decaying organic matter on soil surface (fallen leafs).
A: Loose dark surface soil good for food production (topsoil).

23. How does soil develop?
B: Dense light-colored sub-surface soil not good for food production (contains rocks).
C: Mostly partially weathered rocks and minerals (bedrock).

24. Properties of soil can be observed & measured
Texture: determined by the size of the weathered rock particles.
Color: indicates the content of the soil and how well it drains.
Pore space: the spaces between soil particles.
Chemistry: determines how well nutrients dissolve in water.

25. Section 4.3 Notes: Human Activities Affect Soil
Big Idea: Natural forces break rocks apart and form soil, which supports life.

26. Soil is a necessary resource
Supports the growth of plants, which in turn supply us with many things, such as food, clothing, medicines, lumber, and even oxygen.
Purifies water as it drains through the ground and keeps water systems clean.

27. Soil is a necessary resource
Nutrient recycling by decomposers in soil that enable plants to grow.
Home to a variety of living things.

28. Land-use practices can harm soil
Farming: use of fertilizers & practices
farmers lose 5 metric tons of soil per 1 metric ton of grain
microorganisms have difficulty producing nutrients naturally.
pollutes water systems.
reduce plant cover and increases soil loss due to wind and water.
overgrazing by livestock.
Desertification!

29. Land-use practices can harm soil
Construction & Development: removal of vegetation to build
soil is blown or washed away.
makes water systems muddy and can harm the organisms living there.
causes flooding.

30. Land-use practices can harm soil
Mining: removal of soil
speeds up rates of chemical weathering by exposing rocks & minerals to rainwater and air.
contaminates soil (sulfuric acid).

31. Soil can be protected and conserved
Crop rotation: The practice of planting different crops on the same field in different years or growing seasons.
Conservation tillage: Reduces the number of times that fields are tilled in a year; therefore reducing the amount of erosion.

32. Soil can be protected and conserved
Terraces: Flat, step-like areas built on a hillside to hold rainwater and prevent it from running downhill.
Contour plowing: Plowing along the curves of a slope which helps channel rainwater resulting in less erosion.

33. Soil can be protected and conserved
Windbreaks: Rows of trees planted between fields to reduce the force of winds that can carry off soil.