1. Table of Contents Rocks and Weathering How Soil Forms
Soil Conservation

2. Weathering and Erosion
- Rocks and Weathering
Weathering and Erosion
Weathering – the chemical and physical processes that break down rock at Earth’s surface.
Agents of weathering include: Heat, cold, water, ice, and gases in the atmosphere
Erosion – process by which water, ice, wind, or gravity moves weathered rock and soil.
Weathering and erosion work together continuously to wear down and carry away the rocks at Earth’s surface.

3. Mechanical Weathering
- Rocks and Weathering
Mechanical Weathering
There are two kinds of weathering: mechanical weathering and chemical weathering.
Mechanical weathering – type of weathering in which rock is physically broken into smaller pieces.
The smaller pieces of rock have the same composition as the rock they came from.
A very slow process, but eventually can wear even the tallest mountains down

4. Mechanical Weathering
- Rocks and Weathering
Mechanical Weathering
Causes of mechanical weathering include:
freezing, thawing, release of pressure, plant growth, actions of animals, and abrasion.
Abrasion - the grinding away of rock by rock particles carried by water, ice, wind, or gravity.
Ice wedging – process that splits rock when water seeps into cracks, then freezes and expands.

5. - Rocks and Weathering
Chemical weathering
Chemical weathering – process that breaks down rock through chemical changes.
Causes of chemical weathering include:
action of water, oxygen, carbon dioxide, living organisms, and acid rain.
Creates holes or soft spots in rock, causing the rock to break apart more easily.

6. - Rocks and Weathering
As weathering breaks apart rock, the surface area exposed to weathering increases.

7. Chemical weathering Water - weathers rock by dissolving it.
- Rocks and Weathering
Chemical weathering
Water - weathers rock by dissolving it.
Oxidation – when a substance combines with oxygen a chemical reaction occurs.
Ex: Iron + Oxygen = Rust
Carbon Dioxide - dissolves in rainwater and creates a weak acid called carbonic acid that breaks down rocks.

8. - Rocks and Weathering
Chemical weathering
Living Organisms – roots from plants push into cracks of rocks and produce a weak acid.
Acid Rain – the result of gases from burning fossil fuels combining with water vapor (clouds) creating weak acid droplets.

9. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
The graph shows the rate of weathering for two identical pieces of limestone that weathered in different locations.

10. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
Reading Graphs:
What does the x-axis of the graph represent?
Time in years

11. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
Reading Graphs:
What does the y-axis of the graph represent?
The thickness of stone lost to weathering

12. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
Reading Graphs:
How much thickness did Stone A lose in 1,000 years? Stone B?
Stone A lost about 8.5 millimeters; Stone B lost slightly more than 4 millimeters.

13. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
Drawing Conclusions:
Which stone weathered at a faster rate?
Stone A weathered at a faster rate.

14. Which Weathered Faster?
- Rocks and Weathering
Which Weathered Faster?
Inferring:
Since the two identical pieces of limestone weathered at different rates, what can you infer caused the difference in their rates of weathering?
They were exposed to different climate conditions.

15. - Rocks and Weathering
Rate of Weathering
The most important factors that determine the rate at which weathering occurs are the type of rock and the climate.
Type of rock – the mineral composition determines how fast a rock weathers.
Minerals that do not dissolve easily in water weather slowly.
Permeable rocks – contain tiny, connected air spaces that water can seep through.
Causes faster weathering
More surface area

16. - Rocks and Weathering
Rate of Weathering
Climate – both chemical and mechanical weathering occur faster in wet climates.
Chemical reactions also occur faster at higher temperatures.
Tropical rainforests have the highest rate of weathering.

17. Click the PHSchool.com button for an activity about weathering.
- Rocks and Weathering
More on Weathering
Click the PHSchool.com button for an activity about weathering.

18. - How Soil Forms
What Is Soil?
Soil – loose, weathered material on Earth’s surface in which plants can grow.
Created from bedrock below
Bedrock – the solid layer of rock beneath the soil.

19. - How Soil Forms
What Is Soil?
Soil is a mixture of rock particles, minerals, decayed organic material, water, and air.
sand, silt, and clay make up the portion of soil that comes from weathered rock.
Humus – a dark-colored substance that forms as plant and animal remains decay.
Provides nutrients for plants to grow
Fertility of soil – a measure of how well the soil supports plant growth
Soil rich in humus has a higher fertility

20. Soil Texture Soil texture depends on the size of the soil particles.
- How Soil Forms
Soil Texture
Soil texture depends on the size of the soil particles.
Gravel – largest > 2mm
Sand – between 1/16 – 2 mm
Silt – between 1/256 – 1/16 mm
Clay – smallest < 1/256 mm

21. - How Soil Forms
What Is Soil?
Loam – type of soil, is made up of air, water, and organic matter as well as materials from weathered rock.
Best for growing most types of plants.

22. The Process of Soil Formation
- How Soil Forms
The Process of Soil Formation
Soil forms when rock is broken down by weathering and mixes with other materials on the surface.
Soil is constantly being formed wherever bedrock is exposed.

23. The Process of Soil Formation
Soil horizon – layer of soil that differs in color and texture from the layers above or below it.
Topsoil – mixture of humus, clay, and other minerals that forms the crumbly, topmost layer of soil. (Horizon A)
Subsoil – layer of soil beneath the topsoil that contains mostly clay and other minerals. (Horizon B)
Horizon C – contains only partly weathered rock

24. - How Soil Forms
Soil Layers Activity
Click the Active Art button to open a browser window and access Active Art about soil layers.

25. - How Soil Forms
Soil Types
Scientists classify the different types of soil into major groups based on climate, plants, and soil composition.

26. Living Organisms in Soil
- How Soil Forms
Living Organisms in Soil
Some soil organisms make humus, the material that makes soil fertile. Other soil organisms mix the soil and make spaces in it for air and water.

27. Living Organisms in Soil
Most humus is formed from plant litter.
Litter – loose layer of dead plant leaves and stems on the surface of the soil.
Humus forms from decomposition
Decomposers – organisms in the soil that breaks down the remains of dead organisms and digests them.
The remaining organic material is put back into the soil.

28. Living Organisms in Soil
Earthworms are responsible for mixing up the soil and making it rich for plants.
Many burrowing mammals such as mice, moles, prairie dogs, and gophers also break up hard, compacted soil and mix humus through it.

29. Click the Video button to watch a movie about soil.
- How Soil Forms
Soil
Click the Video button to watch a movie about soil.

30. - Soil Conservation
Value of Soil
Natural resource – anything in the environment that humans use.
Soil is one of Earth’s most valuable natural resources because everything that lives on land, including humans, depends directly or indirectly on soil.

31. Soil Damage and Loss Wind erosion is one cause of soil loss.
- Soil Conservation
Soil Damage and Loss
Wind erosion is one cause of soil loss.
Ex: Wind erosion created the Dust Bowl on the Great Plains.
Farmers removed grasses containing sod causing soil to dry and blow away
Sod - thick mass of tough roots at the surface of the soil that keeps soil in place

32. Soil Damage and Loss Read pages 56-58 on “The Dust Bowl”
- Soil Conservation
Soil Damage and Loss
Read pages on “The Dust Bowl”
While reading think about these questions:
Could this problem happen again?
How would you be affected?

33. - Soil Conservation
Soil Conservation
Soil conservation – the management of soil to prevent its destruction.
These methods include: contour plowing, conservation plowing, and crop rotation.
Contour plowing – plowing fields along the curves of a slope to prevent soil loss.

34. Soil Conservation
Conservation plowing – dead stalks from the previous year’s crop are left in the ground to hold the soil in place.
Crop rotation – planting of different crops in a field each year to maintain the soil’s fertility.

35. Links on Soil Conservation
Click the SciLinks button for links on soil conservation.

36. Click the Video button to watch a movie about soil conservation.

37. Bedrock begins to weather
Soil Horizons
Bedrock begins to weather
The C horizon forms as bedrock breaks apart.
The A horizon develops from the C horizon where plants grow in it.
The B horizon develops as clay and minerals wash down from the A horizon.