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Chapter 8: Weathering and Soil Formation
Section 1: Rocks and Weathering
1. Weathering and Erosion
a. Weathering: the process that breaks down rock and other substances at Earth’s surface
i. Heat, cold, water, and ice
ii. Oxygen and carbon dioxide in the air
iii. Repeated freezing and thawing can crack rock apart
iv. Rainwater can dissolve minerals
b. Erosion: the removal of rock particles by wind, water, ice, or gravity
i. Weathering and erosion work together to wear down and carry away rocks at Earth’s surface
c. Uniformitarianism: the principle that states the same processes that operate today operated in the past
i. The same forces that shape earth today also shaped Earth millions of years ago
d. There are two kinds of weathering: Mechanical Weathering and Chemical Weathering
i. Mechanical Weathering: the type of weathering in which rock is physically broken into smaller
pieces
1. Works slowly over time
2. Over long periods of time, mechanical weathering can wear away entire mountains
3. Causes of mechanical weathering: freezing and thawing, release of pressure, plant growth,
actions of animals, and abrasion
a. Abrasion: the grinding away of rock by rock particles carried by water, ice, wind, or
gravity
b. Freezing and thawing is the most important force of mechanical weathering in cool
climates
i. Water seeps into cracks and freezes—the water expands, acting like a
wedge, and forces things apart
ii. Ice Wedging: the wedging of ice in rocks that widen and deepen cracks
iii. When the ice melts, water seeps deeper into the cracks
ii. Chemical Weathering: the type of weathering that breaks down rock through chemical process
1. Causes of chemical weathering: action of water, oxygen, carbon dioxide, living organisms,
and acid rain
2. Can produce new minerals as it breaks down rock
a. Ex: granite is made of minerals like feldspar, quartz, and mica—through chemical
weathering, granite changes feldspar minerals into clay minerals
3. Chemical weathering creates holes and soft spots in rocks
4. Chemical and mechanical weathering work together
a. As mechanical weathering breaks rocks into pieces, more surface area becomes
exposed to chemical weathering
5. Water
a. Most important cause of chemical weathering
b. Dissolves rock
6. Oxygen
a. Oxidation: iron combines with oxygen in the presence of water to produce rust
b. Rocks that contain iron can oxidize
i. Makes rock soft and crumbly with a red or brown color
7. Carbon Dioxide
a. Gas found in the air that contributes to chemical weathering
b. Dissolves in water resulting in a weak acid called carbonic acid
i. Carbonic acid easily weathers rocks such as marble and limestone
8. Living Organisms
i. A seed can land on a rock and sprout
1. The roots push into cracks on the rock
2. The roots produce weak acids that slowly dissolve rock
ii. Lichens—plantlike organisms that grow on rocks—produce acids too
9. Acid Rain
i. Burning fuels such as coal, oil, and gas can pollute air with sulfur, carbon,
and nitrogen
1. When they react with water vapor in clouds, acid rain is formed
ii. Acid rain causes very rapid chemical weathering
iii. Rate of Weathering
1. Some kinds of rocks weather more rapidly than others
a. The most important factors that determine the rate at which weathering occurs are
the type of rock and the climate
2. Type of Rock
a. Minerals that make up a rock determine how fast it weathers
i. Minerals that do not dissolve easily in water weather slowly
b. Permeable rocks weather chemically at a fast rate
i. Permeable: a material that is full of tiny, connected air spaces that allow
water to seep through it
ii. Water seeps into the spaces and dissolves the materials
3. Climate—the average weather conditions in an area
a. Both chemical and mechanical weathering occur faster in wet climates
i. Rainfall provides water needed for chemical changes and freezing/thawing
b. Chemical reactions occur faster at higher temperatures
i. Weathering occurs more quickly where the climate is hot and wet
Section 2: How Soil Forms
1. What is Soil?
a. Soil: the loose, weathered material on Earth’s surface in which plants can grow
i. One of the main ingredients of soil comes from Bedrock
b. Bedrock: the solid layer of rock beneath the soil
i. Gradually is weathered into smaller and smaller particles that are the basic material of soil
c. Soil Composition
i. Soil is a mixture of rock particles, minerals, decayed organic material, water, and air
ii. Sand, silt, and clay make up the portion of soil that comes from weathered rock
d. Humus: dark-colored substance that forms as plant and animal remains decay
i. Helps create spaces in soil for the air and water that plants need
ii. Contains nutrients, including nitrogen, sulfur, phosphorus, and potassium
e. Fertility: a measure of how well the soil supports plant growth
i. Soil that is rich in humus has high fertility
ii. Sandy soil with little humus has low fertility
2. Soil Textures
a. Soil texture depends on the size of individual soil particles
i. The largest soil particles are gravel
ii. The smallest soil particles are clay—smaller than the period at the end of a sentence.
iii. Soil texture is important for plant growth
iv. Mostly clay soil has a dense, heavy texture
1. Holds a lot of water—plants may drown if they don’t get enough air
v. Sandy soil has a coarse texture
1. Water drains through it—plants may die for lack of water
vi. Loam: soil that is about half sandy soil and half clay soil
1. Crumbly texture—holds air and water
2. Best for growing most types of plants
3. The Process of Soil Formation
a. Soil forms as rock is broken down by weathering and mixes with other materials on the surface
i. Soil is constantly being formed wherever bedrock is exposed
ii. Soil gradually develops layers called horizons
1. Soil horizons: a layer of soil that differs in color and texture from the layers above or below
it
2. Scientists classify the soil into three horizons
a. The A horizon is made up of topsoil
i. Topsoil: a crumbly, dark brown soil that is a mixture of humus, clay, and
other minerals
b. The B horizon is made up of subsoil
i. Subsoil: consists of clay and other particles washed down from the A
horizon, but little humus
c. The C horizon contains only partly weathered rock
b. The rate at which soil forms depends on the climate and type of rock
i. Soil develops more quickly in areas of warm and wet climates
ii. Weathering and soil formation take place slowly in areas where the climate is cold and dry
iii. Limestone weathers faster than granite
1. So—soil forms more quickly from limestone than from granite
4. Soil Types
a. There are thousands of different types of soil in the US
b. Soil is classified into groups based on climate, plants, and soil composition
i. Fertile soil can form in regions with hot, wet climates, but ran may wash humus and minerals out of
the A horizon
ii. In cold, dry climates, the soil is often thin
iii. The thickest, most fertile soil forms in temperate climates with moderate rainfall
c. The common plants in a region are helpful in classifying the soil
i. Grassland soils are different than forest soils
d. Soil is also classified by its composition—rocky, sandy, or rich in clay, the type of bedrock, and the amount
of time it has been developing
e. Major soil types in North America include forest, prairie, desert, mountain, tundra, and tropical soil
5. Living Organisms in Soil
a. Some soil organisms make humus, the material that makes soil fertile
i. Other organisms mix the soil and make spaces in it for air and water
b. Forming Humus
i.
ii.
iii.
iv.
Plants contribute most of the organic remains that form humus
Litter: the remains of plant leaves and stems
Although plant remains are full of stored nutrients, they are not yet humus
Humus forms in a process called decomposition
1. Decomposition: the process when organisms turn dead material into humus
v. Decomposers: the organisms that break the remains of dead organisms into smaller pieces and
digest them with chemicals
1. Include fungi, bacteria, worms, and other organisms
2. Fungi—organisms like molds and mushrooms that grown on, and digest, plant remains
3. Bacteria—microscopic decomposers that cause decay, attack dead organisms and their
wastes in soil
4. Small animals such as mites and worms also decompose dead organic material
c. Mixing the Soil
i. Earthworms do most of the work of mixing humus with other materials in soil
1. They carry humus down to the subsoil and from the subsoil up to the surface
2. They also pass out the soil they eat as waste—adds nitrogen to soil
ii. Burrowing animals break up hard, compacted soil and mix humus through it
1. Also add nitrogen to soil through waste
2. Add organic material when they die and decay
iii. Both help aerate the soil; plant roots need the oxygen
Section 3: Soil Conservation
1. The Value of Soil
a. Natural resource: anything in the environment that humans use
i. Soil is one of Earth’s most valuable natural resources because everything that lives on land depends
on soil
ii. Plants depend directly on the soil to live and grow
iii. Humans and animals depend on plants—or other animals that depend on plants—for food
iv. Fertile soil is valuable because there is a limited supply—less than 1/8 of the land on Earth
v. Soil is also valuable because it takes hundreds of years to form
b. Soil Damage and Loss
i. The value of soil is reduced when soil loses its fertility and when topsoil is lost due to erosion
ii. Loss of fertility
1. Soil can be damaged when it loses fertility
2. Soil that has lost its fertility is said to be exhausted
3. In the south during the 1800’s, soils became exhausted when only cotton had been grown
a. George Washington Carver developed new crops and farming methods that helped
restore soil fertility
b. Peanuts were used—they are legumes
i. Legumes have small lumps their roots that contain nitrogen-fixing bacteria,
an important nutrient for plant growth
iii. Loss of Topsoil
1. Water and wind can quickly erode exposed soil
2. Plant cover can protect soil from erosion
a. They break the force of rain and hold soil together
3. Wind erosion is most likely to occur where farming methods are not suited to dry conditions
a. Contributed to the Dust Bowl on the Great Plains
iv. Soil Loss in the Dust Bowl
1. The soil of the Great Plains is fertile, but rainfall steadily decreases from east to west
2. Droughts dried up the land, quickly drying out the land which turned to dust
3. Plowing removed the grass and exposed the soil
4. The wind blew the soil east in great, black clouds that reached as far as New York City
2. Soil Conservation: the management of soil to prevent its destruction
a. Can be done through contour plowing, conservation plowing, and crop rotation
i. Contour plowing: farmers plow their fields along the curves of a slope
1. Helps slow the runoff of excess rainfall, preventing soil loss
ii. Conservation plowing: farmers disturb the soil and its plant cover as little as possible
1. Dead weeds and stalks are left in the ground to help return soil nutrients, retain moisture,
and hold soil in place
2. Also called low-till or no-till plowing
iii. Crop rotation: farmers plant different crops in a field each year
1. Different types of plants absorb different amounts of nutrients from the soil
a. Cotton and corn absorb large amounts of nutrients
b. The next year, plants like oats, barley, or rye that absorb few nutrients are planted
c. The year after, legumes-alfalfa or beans-are planted to restore nutrients