1. Weathering and Erosion
Section 1: Weathering
Section 2: The Nature of Soil
Section 3: Erosion

2. Weathering and Its Effects
Together, surface processes that work to break down rock are called weathering.
Weathering breaks rock into smaller and smaller pieces, such as sand, silt, and clay. These smaller, loose pieces are called sediment.
The terms sand, silt, and clay are used to describe specific sizes of sediment.

3. Mechanical Weathering
Mechanical weathering occurs when rocks are broken apart by physical processes. This means that the overall chemical makeup of the rock stays the same.
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4. Mechanical Weathering
Growing plants, burrowing animals, and expanding ice are some of the things that can mechanically weather rock.
As rock is broken apart by mechanical weathering, the amount of rock surface exposed to air and water increases.

5. Plants and Animals Burrowing animals also cause mechanical weathering.
As these animals burrow, they loosen sediment and push it to the surface. Once the sediment is brought to the surface, other weathering processes act on it.

6. Ice Wedging
Ice wedging occurs in temperate and cold climates where water enters cracks in rocks and freezes.
When water enters cracks in rock and freezes, it expands, causing the cracks to enlarge and the rock to break apart.

7. Chemical Weathering
Chemical weathering occurs when chemical reactions dissolve the minerals in rocks or change them into different minerals.
This type of weathering changes the chemical composition of the rock, which can weaken the rock.

8. Natural Acids
Carbonic acid reacts with minerals such as calcite, which is the main mineral that makes up limestone.
Over many thousands of years, carbonic acid has weathered so much limestone that caves have formed.

9. Oxygen
Oxidation occurs when some materials are exposed to oxygen and water.
Oxidation of minerals gives some rock layers a red color.

10. Effects of Climate
Climate is the pattern of weather that occurs in a particular area over many years.
In cold climates, where freezing and thawing are frequent, mechanical weathering rapidly breaks down rock through the process of ice wedging.

11. Formation of Soil What is soil and where does it come from?
1. A layer of rock and mineral fragments produced by weathering covers the surface of Earth.
2. Weathering gradually breaks rocks into smaller and smaller fragments.
3. Plants and animals add organic matter, the remains of once-living organisms, to the rock fragments.
Soil is a mixture of weathered rock, decayed organic matters, mineral fragments, water, and air.

12. Composition of Soil Most organic matter in soil comes from plants.
Animals and microorganisms provide additional organic matter when they die.
The decayed organic matter turns into a dark-colored material called humus. Humus serves as a source of nutrients for plants.

13. Soil Profile
The top layer typically is darker than the soil layers below it. These different layers of soil are called horizons.
Most soils have three horizons—A, B, and C, with bedrock below, and humus or litter layer on top.
All the horizons of a particular soil form a soil profile.

14. A Horizon The A horizon is the top layer of soil.
In a forest, the A horizon might be covered with litter. Litter consists of leaves, twigs, and other organic material that eventually can be changed to humus by decomposing organisms.
The A horizon also is known as topsoil.

15. B Horizon
Leaching is the removal of minerals that have been dissolved in water. In soil, water seeps through the A horizon where it reacts with humus and carbon dioxide to form acid.
The acid dissolves some of the minerals in the A horizon and carries the material into the B horizon, or subsoil.

16. C Horizon
The C horizon consists of partially weathered rock and is the bottom horizon in a soil profile.
This horizon does not contain much organic matter and is not strongly affected by leaching.

17. Erosion and Deposition
Erosion is a process that wears away surface materials and moves them from one place to another.
An important erosional force is gravity. It’s power is seen in this landslide photo.
Other causes of erosion, also called agents of erosion, are water, wind, and glaciers.

18. Dropping Sediments
Agents of erosion drop the sediments they are carrying as they lose energy. This is called deposition.
When sediments are eroded, they are not lost from Earth—they are just relocated.

19. Mass Movement
Rocks and other materials, especially on steep slopes, are pulled toward the center of Earth by gravity.
A mass movement is any type of erosion that happens as gravity moves materials downslope.
Mudflows, rock slides, rockfalls, creep, and slump are most likely to occur on steep slopes, and they all depend on gravity to make them happen.

20. Slump
When a mass of material slips down along a curved surface, the mass movement is called a slump.
A curved scar is left where the slumped materials originally rested.

21. Creep
Leaning trees and human-built structures show another mass movement called creep.
Creep occurs when sediments slowly shift their positions downhill.
Creep is common in areas of frequent freezing and thawing.

22. How Glaciers Form and Move
A large mass of ice and snow moving on land under its own weight is a glacier.
Glaciers form in regions where snow accumulates. When snow doesn’t melt, it piles up.
As it accumulates slowly, the increasing weight of the snow becomes great enough to compress the lower layers into ice.

23. Glaciers
Continental glaciers are huge masses of ice and snow. Today, continental glaciers cover ten percent of Earth, mostly near the poles in Antarctica and Greenland.
Valley glaciers erode special landforms like bowl-shaped basins, called cirques, long ridges called an arêtes, and sharpened peaks called horns.

24. Plucking
The process called plucking, results in boulders, gravel, and sand being added to
the bottom and sides of a glacier.

25. Ice Depositing Sediment
When glaciers begin to melt, sediment drops or is deposited, on the land.
When a glacier melts and begins to shrink back, it is said to retreat.
As it retreats, a jumble of boulders, sand, clay, and silt is left behind.
This mixture of different-sized sediments is called till.

26. Moraine Deposits
Till is also deposited at the end of glacier when it is not moving forward.
Rocks and soil are moved to the end of the glacier, much like items on a grocery store conveyor belt.
Because of this, a big ridge of material piles up called a moraine.

27. Outwash Deposits
Material deposited by the meltwater from a glacier, most often beyond the end of the glacier, is called outwash.
Meltwater carries sediments and deposits them in layers.

28. Deflation Wind erodes Earth’s surface by deflation and abrasion.
Deflation and abrasion happen to all land surfaces but occur mostly in deserts, beaches, and plowed fields.
These areas have fewer plants to hold the sediments in place.
When wind erodes by deflation, it blows across loose sediment, removing small particles such as silt and sand.

29. Abrasion
When windblown sediment strikes rock, the surface of the rock gets scraped and worn away by a process called abrasion.
These sand grains strike against rock and break off small fragments.
The rocks become pitted and are worn down gradually.

30. Deposition by Wind—Loess
Wind deposits of fine-grained sediments are known as loess.
Loess is as fine as talcum powder.
Strong winds that blew across glacial outwash areas carried the sediments and deposited them.
The sediments settled on hilltops and in valleys.

31. Dunes A dune is a mound of sediments drifted by the wind.
A sand dune has two sides. The side facing the wind has a gentler slope. The side away from the wind is steeper.