1. Chapter 7: Weathering & Soil

2. Aim: How does weathering change Earth’s surface?

3. Weathering
Is surface processes that work to breakdown rocks into smaller pieces
Smaller loose pieces are sediments
Gravel, Sand, silt and clay describe sizes of sediments
Size order: gravel, sand, silt, clay (largest to smallest)
Wears mountains down to hills
Produces strange rock formations
2 Types of weathering:
Mechanical Weathering
Chemical Weathering

4. Aim: How do mechanical weathering and chemical weathering differ?

5. Mechanical Weathering
Occurs when rocks are broken apart by physical processes
Chemical makeup of the rock stays the same
2 ways this can happen:
Plants and Animals
Water and nutrients that collect in the cracks or rocks enable plants to grow
As the roots grow they enlarge the cracks
Ex: tripped on crack in the sidewalk near a tree

6. Burrowing animals loosen sediments and push them to the surface
Sediments reach the surface and other weathering processes occur
Ice Wedging
Occurs in temperate and cold climates
Water enters cracks in rocks and freezes
Pressure builds up in the cracks causing them to expand and break apart the rock
The ice melts and this happens all over again
The process of freezing and thawing occurs over and over, especially in mountains

7. Ice wedging wears mountain peaks
It can also break up roads and highways
Mechanical weathering reduces rocks to smaller pieces giving the rock more surface area
As the surface area increases more rock is exposed to water and oxygen which aids in chemical weathering

8. Chemical Weathering
Chemical reactions dissolve the minerals in rocks or change them to different minerals
Changes the chemical composition of the rock
Naturally occurring acids, such as carbonic acid, react with calcite in limestone
The acid weathers away the limestone to form caves
Kaolinite clay is created when acids react with feldspar in granite
Clay is an end product in weathering

9. Plant acids can dissolve minerals in rocks and breaks the rocks into smaller pieces
This weathering also enables nutrients to be available for plants
Oxygen also causes weathering
Oxidation occurs when some materials are exposed to oxygen and water
Ex: rust

10. Effects of Climate on Weathering
Chemical weathering is more rapid in warm, moist climates
Mechanical weathering is important in dry climates and in cold climates

11. Aim: How does soil form?

12. Formation of Soil
Is a mixture of weathered rock, decayed organic matter, mineral fragments, water and air
Can take thousands of years to form
Climate, slope, types of rock, types of vegetation and length of time of weathering affect rock formation
There are different kinds of soil all over the world based on these factors
4 steps

13. Weathering of rocks
Natural acids in rainwater weather the surface of exposed bedrock
Water can freeze in cracks to break them apart
Plant Growth
Plants begin growing in cracks of rocks
As they grow they continue the process of breaking down rocks
Then a thin layer of soil begins to form

14. Living Organsims
Insects & worms live underground with the plant roots
Their wastes and dead material add organic matter to the soil
Organic Matter
as organic matter increases, the bedrock continues to breakdown
This causes the soil layer to thicken
Rich topsoil supports trees and plants with large root systems

15. Aim: What are the characteristics of soil?

16. Composition of Soil Most organic matter in soil comes from plants
Animals and microorganisms remains also add organic matter to the soil
Decayed organic matter over time turns into a dark-colored material called humus
Humus is a source of nutrients for plants
Animals burrowing in the ground mix the humus with fragments of rocks
Good quality soil have equal amounts of humus and weathered rock material

17. Layers of Soil Layers of soil are called horizons
All soil horizons form a soil profile
Most soils have 3 horizons
A Horizon
top layer-AKA topsoil
In a forest it is covered with litter
Litter helps prevent erosion and evaporation of water from the soil
Topsoil has more humus and fewer rock and mineral fragments than the other horizons

18. Is dark and fertile
B Horizon
is below the A horizon
Lighter in color (less organic matter)
Less fertile
Leaching moves minerals that have been dissolved in water from A horizon down into the B horizon

19. C Horizon
Bottom layer
Contains partially weathered rock
Thickest horizon
Not affected by leaching and doesn’t have much organic material
Coarser sediments
Most like the parent material, which is rock

20. Aim: What determines the different types of soil?

21. Types of Soil Many different types of soil exist Climate affects soil
Desert soil has a small amount of organic matter and is thin
Prairies have thick, dark A horizon soil due to the organic matter from the large amount of grasses present
Forest soil has a thinner A horizon than prairies because trees and leaves don’t supply as much organic matter as grasses do

22. The type of rocks soil comes from affects the type of soil
Sandstone turns into a sandy soil
Clay soil comes from basalt
Type of vegetation that grows in an affects soil formation
Slope of the land affects soil development
The soil on the slope is poorly developed
Soil in a valley is rich in organic matter

23. Aim: How has human activity affected Earth’s soil?

24. Soil Erosion
Soil erodes when it is moved from the place where it formed
Water and wind move sediments and cause erosion
More severe on steep slopes
Humans cause erosion to form faster by removing vegetation (ground cover) which helps increase soil erosion
Serious problem for agriculture
Soil loses nutrients, which plants need to grow

25. When soil erodes farmers compensate for the nutrient lose by buying fertilizer
Forest harvesting also causes soil erosion
Removing forests creates severe problems for the environment, especially rain forests
Forests are cleared for lumber, farming and grazing
Overgrazing also increases soil erosion
Sheep and cattle are grazed on land until no ground cover remains
Without protection from plants that soil is carried away by wind and water

26. Too much soil erosion, sediments can damage the environment
This can cause streams to be covered, eggs from organisms die, or reduce downstream water quality

27. Aim: How can humans prevent soil erosion?

28. Preventing Soil Erosion
Plant shelter belts of trees-this blocks the strength of the wind
No-till farming-farmers do not plow the fields
This practice provides cover for the soil all year round and reduces water runoff and soil erosion
Contour farming used on slopes-this slows the flow of water down the slope and reduces soil erosion
Terracing-leveled areas are built on slopes (looks like steps)

29. This practice reduces runoff by creating flat areas and shorter sections of slope