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EARTH SCIENCE CHAPTER SEVEN
WEATHERING, SOIL, AND EROSION
7.1 WEATHERING
Weathering is the breakup of a rock due to exposure to processes that occur at Earth’s surface
Two main types: Mechanical weathering (disintegration) occurs
when rocks are broken down into smaller pieces but do not change in composition.
Chemical weathering (decomposition) occurs when the rock’s minerals are changed into different substances
MECHANICAL WEATHERING
Several processes: Frost wedging Wetting and drying Abrasion by rock materials Plant and animal activity Exfoliation due to upward expansion
FROST WEDGING
Water occupies 10 % more space when it freezes than it does as a liquid.
When water that is trapped inside the cracks and pores rocks freezes, it can break the rock as it expands.
This is called frost wedging or ice wedging.
WETTING AND DRYING
Liquid water also plays a role in mechanical weathering.
Some rocks that are made heavily of clay and silt (like shale), swell when they are wet and shrink as they dry.
This repeated action can break the rock over time.
ABRASION
Occurs when moving sand, rocks, or pebbles grind and scrape against one another.
The sand we walk on at the beach was once rock that has been repeatedly broken down and carried by the waves.
PLANT AND ANIMAL ACTIVITY
Lichens, plants, and mosses grown on rocks and wedge into their pores—roots can eventually split the rocks.
Even trees have been known to grow through boulders.—pic pg 259
Many animals (ants, earthworms, rabbits, etc) make holes in the ground that expose rocks to agents like wind, ice, etc.
UPWARD EXPANSION
Upward expansion occurs when granite beneath the earth’s surface is pushed upward by forces below.
This extreme force causes rocks on top of the granite sheets to break apart and erode.
The granite sheets are then exposed and begin to break and erode also.
This is called exfoliation-- The peeling away of surface layers of exposed
bedrock
CHEMICAL WEATHERING
Can occur anywhere that water or water vapor is present
Can occur by acids Can occur by oxygen
WATER AND CHEMICAL WEATHERING
The reaction of water with other substances is called hydrolysis.
Water readily reacts with some minerals (like feldspar, hornblende, and augite) to dissolve into ions that slowly react with water to form different types of clay minerals.
ACIDS AND CHEMICAL WEATHERING
Carbonic acid, H2CO3, is an acid commonly found in water. It is formed when CO2 in the atmosphere dissolves in rainwater.
This acidic mixture of carbonic acid and water breaks down even more minerals to form clay.
Sulfur dioxide, nitrogen compounds, and carbon dioxide from pollution forms acid rain.
This acid rain can very quickly decompose many types of structures.
OXYGEN AND CHEMIMCAL WEATHERING
Oxidation is the chemical reaction of oxygen with other substances.
Oxidation is very effective in weathering rocks that contain iron. Oxidized iron is called rust.
RATES OF WEATHERING
Under average conditions, weathering is a slow process.
The process is influenced by three major factors: Surface exposure—all weathering happens at the
surface, so more surface area=faster process Composition of the rock—certain types of rock are more
susceptible to oxidation and acid reactions, thus they would weather faster than other rocks in the same area
Climate—in warmer, wetter climates—both mechanical and chemical weathering play about the same role. However, in dryer, colder climates, mechanical weathering plays a greater role than chemical weathering
7.2 EROSION and DEPOSTION
Erosion is the removal and transport of materials by natural agents such as wind and running water. Is affected by climate Is affected by the composition of the
rock The overall effect depends on how the
process of erosion compares with the ongoing process of uplift
Deposition is when the materials are dropped in another location.
AGENTS OF EROSION
Gravity Running Water Glaciers Wind Living things To be discussed more in Chapter 8
7.3 SOIL
Soil is made of loose, weathered rock and organic material in which plants with roots can grow
Without the weathering process we studies in Section 1, there would be no soil.
Without soil, what would we eat?
HOW SOIL FORMS—PARENT MATERIAL
The material from which soil forms is called the parent material. If the parent material is the bedrock
beneath the soil, the soil is called a residual soil.
If the parent material is not from the underlying bedrock but from weathered particles that were carried there by wind, waves, etc., the soil is called a transported soil.
HOW SOIL FORMS—THE PROCESS
Forms as parent material is weathered away.
Rate varies by location. Climate plays a role. Organic materials (decaying plant
and animal remains) are added.
SOIL PROFILE
A cross-section of the earth exposed by digging
Allows us to study the “layers of soil” Mature soils have three distinct
zones or soil horizons A Horizon B Horizon C Horizon
A-HORIZON
Called topsoil Generally gray to black in color Contains humus—organic material
that forms from decayed plant and animal materials
Tends to be sandy
B-HORIZON
Called subsoil Usually red or brown from oxides that
formed in topsoil and then got washed down into the subsoil
Often contains clay and soluble minerals like calcium and potassium
C-HORIZON
Made of slightly weathered parent material
Often has rock fragments Lies just above the bedrock or parent
material
SOIL COMPOSITION
Several factors affect soil composition: Parent material Time Plants and animals Topography Climate
SOIL AS A RESOURCE
The world’s population is increasing at an alarming rate.
All of these people need to eat!!!!!! However, less than 25 % of the Earth’s
land can be used to grow crops. Soil fertility is the ability of soil to grow
plants. Certain types of soil are best suited for certain
crops. Many things currently threaten soil fertility.
SOIL DEPLETION
Soil depletion occurs when the soil gradually becomes so lacking in nutrients that it can no longer be used to grow a usable crop.
Farmers can do some things to prevent this. Allow the soil to rest by rotating crops
SALINIZATION
Irrigation is often used to make soils fertile.
Irrigation water often brings in dissolved minerals.
These dissolved minerals are often salts.
Over time, the buildup of salt in the soil is called salinization. Often renders the soil completely
useless
SOIL CONSERVATION
Several methods exist: Windbreaks—also known as shelter belts—lines of trees
planted along the edges of fields Contour farming—farmers plant crops parallel to land
contours to prevent water from running rapidly down a slope
Terraces—flattens a slope into separate terraces to slow the speed of runoff
Strip cropping—alternate rows of crops to minimize soil erosion
No-till—method of farming where everything is done at once (plowing, planting, fertilizing, weed control) and the soil is left completely alone until harvest time