Weathering and Soil Formation

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Weathering and Soil Formation
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Weathering and Soil Formation

• Section 1
• Weathering

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Weathering

• The breaking down of rocks and other materials on
  the Earths surface is called weathering. A
  slow, continuous process, it affects all
  substances exposed to the atmosphere.

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Mechanical Weathering

• When the forces of weathering break rocks into
  smaller pieces but do not change the chemical
  makeup of the rocks, the process is called
  mechanical weathering. During mechanical
  weathering, rocks are broken into different
  shapes and smaller pieces. At the beginning the
  edges are jagged, as weathering continues, they
  become round.

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Mechanical Weathering

• Weathering is the mechanical breakdown of rock
  into sediment like sand, silt and clay.
• There are two types of weathering mechanical
  weathering and chemical weathering
• Mechanical weathering occurs when rocks are
  broken apart by physical processes and the
  overall chemical makeup of the rock does not
  change.
• Every fragment of rock has similar
  characteristics to the original rock.

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Causes of Mechanical Weathering

• Causes of mechanical weathering.
• Ice wedging
• Organic activity plant and animal activity
• Abrasion
• Gravity

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Organic Activity

• Plants and Animals
• The roots of plants sometimes loosens rock
  material.
• A plant growing in a crack can make the crack
  larger as the root spread out. This is known as
  root-pry.
• It is organic since this activity is caused by
  living things.
• animals that burrow, loosen sediment and push it
  to the surface where other weathering can occur.

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Gravity

• Gravity is another agent of mechanical
  weathering. Sometimes gravity pulls loosened
  rocks down mountain cliffs in a landslide. A
  landslide is a large movement of loose rocks and
  soil. As the rocks fall, they collide with one
  another and break into smaller pieces. Falling
  rocks usually occur in areas where a road has
  been cut through, leaving cliffs on both sides.

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Abrasion

• Wind-blown sand causes mechanical weathering .
  Abrasion is the wearing away of rocks by solid
  particles carried by wind, water or other forces.
  In desert regions, the wind easily picks up and
  moves sand. The sharp edges of the sand
  particles scrape off pieces of exposed rocks.
  Running water also carries loose rocks which
  scrape against each other and break.

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Chemical Weathering

• This occurs when chemical reactions dissolve the
  minerals in rocks or change them into different
  minerals. Here the chemical composition of the
  rock is changed and can weaken the rock.
• Natural acids like carbonic acid, forms when
  water reacts with carbon dioxide in the air, and
  cause rock to dissolve (limestone).
• Feldspar is broken down into a clay mineral
  called kaolinite when it comes into contact with
  a natural acid.
• Clay is the end product of weathering.

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Types of Chemical Weathering

• There are several causes of chemical weathering.
• Natural acids such as carbonic acid that dissolve
  the limestone
• Oxidation oxygen causes chemical weathering. Rust
  is caused by oxidation. Oxidation occurs when
  some materials are exposed to oxygen and water.
• Plant acids some roots and decaying plants
  produce acids dissolve the minerals and return
  nutrients back to the soil.

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Factors Affecting the Rate of Weathering

• 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.
• Chemical weathering is more rapid in warm, wet
  climates. Thus, chemical weathering occurs
  quickly in tropical areas
• Lack of moisture in deserts and low temperatures
  in polar regions slow down chemical weathering.

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Factors Affecting the Rate of Weathering

• Type of Rock
• Rock type also can affect the rate of weathering
  in a particular climate. In wet climates, for
  example, marble weathers more rapidly than
  granite,

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Weathering and Soil Formation

• Section 2
• Nature of Soil

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Formation of Soil

• Soil is a mixture of weathered rock, decayed
  organic matter, mineral fragments, water, and
  air.
• Soil takes thousands of years to form and can be
  60 meters thick or just a few centimeters.
• Factors affecting the type of soil
• Climate, slope, types of rock, types of
  vegetation, and the length of time that rock has
  been weathered, all affect what type of soil is
  formed.

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Composition of Soil

• In addition to the things we spoke of in soil
  already, most organic matter in soil comes from
  plants. The other organic matter comes from
  microorganisms.
• Decayed organic matter, such as plants, turns
  into a dark colored material called humus, which
  serves as a source of nutrients for plants.
• Humus gets mixed into the ground by worms,
  insects and rodents that burrow into the soil.

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Soil Profiles

• Soils have different layers called horizons.
  These horizons altogether form a soil profile.
• There are 3 soil horizons, A,B, and C.
• A horizon, also called topsoil, is the top layer
  of soil. Its usually covered with litter, which
  is made up of leaves, twigs, and other organic
  material that can be changed into humus by
  decomposing organisms.
• Topsoil has more humus than mineral and rock
  particles than other soil profiles.
• Humus gives the topsoil its dark color and
  provides nutrients for plant growth.

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Soil Profiles

• B Horizon lies underneath the A horizon and has
  less organic material. This makes it lighter in
  color than an A horizon, and there is less humus.
• The B horizon is less fertile, and contains
  material that has moved down from the A horizon
  by a process called leeching.
• Leeching is the removal of minerals that have
  been dissolved in water (coffeemaker).

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Soil Profiles

• C horizon consists of partially weathered rock
  and it is the bottom most and thickest horizon in
  a soil profile.
• It has hardly any organic matter, its not
  affected by leeching, and is lightest in color of
  all horizons.
• You find a rock in a C horizon. This rock will
  eventually become horizon A and B through erosion
  and weathering.
• We call the rock in C horizon parent material.

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Soil Types

• Not all soils are the same. Some are thick and
  red, while others could be brown.
• Climate can reflect soil type. Places with
  different climates such as deserts and forests
  will have different soil types.
• A desert will contain little organic material,
  and are also thinner than soils in a wetter
  environment.
• Prairies will have a thick, dark A horizon
  because of contributions of the growing grass.

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Other factors of soil

• Parent material has an effect on the type of soil
  that will develop. Clay soils often develop on
  rocks like basalt, because minerals in the rock
  weather to form clay.
• The type of rock also effects the type of
  vegetation that grows in the region, because
  different rocks provide different amounts of
  nutrients.
• Slope is another factor that effects soil. On
  steep slopes, soil is poorly developed, because
  the materials cannot be weathered much.

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Soil Erosion

• Section 3
• Soil Erosion

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Causes and Effects of Soil

• Soil erodes when it is moved from the place where
  it formed.
• Erosion occurs as water flows over Earths
  surface or when wind picks up and transports
  sediment.
• Generally, erosion is more severe on steep slopes
  than on gentle slopes. Its also more severe in
  areas where there is little vegetation.
• Under normal conditions, a balance between soil
  production and soil erosion often is maintained.
  This means that soil forms at about the same rate
  as it erodes.

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CAUSES OF EROSION

1. Removing ground cover. Ground cover is vegetation
   that covers the soil and protects it from
   erosion. When vegetation is cleared, as shown in
   Figure, soil erosion often increases.

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CAUSES OF EROSION

• Agricultural Cultivation
• Forest Harvesting
• Over grazing

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Preventing Erosion

• No-till farming provides cover for the soil year
  round, and reduces soil erosion.
• Farmers leave plant stalks in the field over the
  winter months. At the next planting, they seed
  crops with-out destroying these stalks and
  without plowing the soil.
• Contour farming is another way to reduce soil
  erosion. Contour farming is when you plant along
  the natural contours of the land. This slows the
  flow of water down the slope, and prevents
  erosion of soil.
• Terracing, On steep slopes, terracing is a method
  in which steep sided, level topped areas are
  built on the sides of steep hills so crops can be
  grown. This reduces the slope which reduces
  erosion.

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Contour Farming
No-till Farming