Weathering and Soil

6.1 Weathering

I. Evidence of WeatheringA. Weathering – the process that breaks down

rocks into smaller and smaller fragments. Rocks break into small pieces called sediment.

B. Sediments can form soil.II. Mechanical Weathering

A. Mechanical weathering – breaks rocks without changing the chemical composition. Breaks by physical process.

B. Mechanical weathering can be caused by growing plants, expanding ice, mineral crystal growth, lightning, and expansion and contraction when an area heats and cools.

C. Plantsa. Roots - wedge rocks apart.b. Lichen plants – expand and shrink with the

amount of available water.D. Ice Wedging

a. Ice Wedging – ice freezes and expands cracking rocks.

b. Noticeable in mountainsc. Can cause potholes.

III. Chemical WeatheringA. Chemical weathering – chemical composition of

the rock changes.B. Water

a. Can cause new substances to be formed.

C. Acidsa. Naturally forming Carbonic acid reacts with

minerals.b. Limestone and Carbonic acid – Figure 6-5c. Carbonic acid and granite – produces clayd. Plant roots have acids as well

D. Oxygena. Oxidation – occurs when iron is exposed to

oxygen and water. Causes rust.IV. Climate and Weathering

A. Climate – the pattern of weather that occurs in a particular area over many years.

B. Cold Climates – mechanical weathering happens rapidly.

C. Warm and Wet Climate – chemical weathering happens rapidly.

D. Warm Dry – slower chemical weathering.

E. Mechanical and Chemical weathering work together.

F. Mechanical weathering increases surface areaG. Chemical weathering increases as surface area

increases.

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Weathering and Soil6.2 Soil

I. Formation of SoilA. Soil – mixture of weathered rock, organic

matter, mineral fragments, water, and airB. Soil supports vegetation.C. Climate, types of rock, slope, amount of

moisture, and length of time rock has been weathering affect the formation.

D. Composition of Soila. As soil develops, organic materials decayb. Humus – dark colored matter

i. Provides nitrogen, phosphorous, potassium, and sulfur for plants.

ii. Promotes good soil structure, and helps soil hold water

c. Good quality surface soil is half broken down rock and half hums

d. Soil can take thousands of years to forme. Fertile soil often found near riversf. Solids have small spaces in them. g. These spaces fill up with air and water.

II. Soil ProfileA. Soil profile – different layers of soilB. Horizon – each layer in the soil profileC. Horizon A

a. Top Layerb. Litter – leaves, twigs, and other organic

materials that changes to humus when it is exposed to decomposing organisms

c. Also known as topsoild. Has more humus and smaller rock and mineral

particles than the other layers

D. Horizon Ba. Lighter in color than Horizon A and contains less

humusb. Leaching – removal of minerals that have been

dissolved in water.c. Elements get leached into Horizon B from

Horizon A.E. Horizon C

a. Consists mostly of partially weathered parent rock

b. Leaching from Horizon Bc. At the bottom is solid rock

F. Many places land is covered by material that was deposited by glaciers.

G. Glacial deposits made fertile soilH. Midwest farming soils were made this way.

http://www.enchantedlearning.com/geology/soil/

Soil Horizons

III. Types of SoilA. Texture depends on the amounts of sand, silt,

and clayB. Texture affects how water run through soilC. Climate affects soil profilesD. Deserts are dryE. Prairies are demi dryF. Temperate zones – moderate amount of rain,

moderate temperatureIV. Soil Types Reflect Climate

A. Chemical weathering is much slower in areas of little rainfall.

B. Deserts have little organic material

C. Soil horizons in drier areas are thinner than soils in wetter climates

D. Precipitation affects how much leaching occursa. Soils that have been leached are light in color

E. Time changes soil characteristicsa. Short time means parent rock determines soil

characteristicsb. As weathering continues soil resembles the

parent rock less and lessF. Slope affects as well

a. Steep slopes, soil horizons are poorly developedb. Bottomlands (lots of water) solids are often

thick, dark, and full of organic material. c. South facing slopes get more sunlight, so

different soil profile.G. Humus affects soil profiles.

Soil Profileshttp://soils.usda.gov/gallery/photos/profiles/

V. Soil – An Important ResourceA. Needed for food, livestock, paper production,

and others.B. Without plants

a. soil is more likely to be eroded.b. Soil development slows and sometimes stops.

C. Plowing and Soil Lossa. Increasing populations means more food is

needed.b. Farmers plow increasing amounts of soil.c. Plowing soil by machines is effective for crops,

but removes plant cover that holds soil in place. d. Causes erosion due to wind to speed up e. Under these conditions, soil is a nonrenewable

resource because it cannot be replaced quickly.

D. Soil Erosion in the Tropicsa. Tropical rains running down steep slopes erodes

soilb. Thousands of square kilometers of rain forest

are cleared every year.c. Only the first few cm of tropical soil have good

nutrientsd. Farming a particular spot can only last a few

years. e. This is leading to loss of the world’s rain forests.

E. Farmers work to Minimize Soil Lossa. Shelter belts for wind coverageb. Cover bare soil with decaying plantsc. In dry areas, famers graze rather than plowd. Crop rotation