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Weathering and Soil Formation


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Title: Weathering and Soil Formation

Weathering and Soil Formation
Ayers Rock Another View
Coach Holmes indicator for, How far out in the
boonies are we? Distance from civilization.
Approximately how far away from Ayers Rock is the
closest McDonalds restaurant?
465 km (289 miles). The closest McDonalds is in
Alice Springs. 22 km to Yulara then another 443
km to Alice Springs
Essential Question
  • NCSCOS EEn.2.1.3
  • Explain how natural actions such as weathering,
    erosion(wind, water and gravity), and soil
    formation affect the Earths surface.
  • E.Q. - What are the mechanisms of weathering

Objectives and Goals
  • Recall that soil is the result of weathering of
    rocks and includes weathered particles sand,
    silt and clay.
  • Explain differences in chemical and physical
    weathering how weathering rates are affected by a
    variety of factors including climate, topography
    and rock composition.

  • In your journals
  • Give your definition of weathering.

Part I 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.

Types of Weathering
  • Rocks on the Earths surface are broken down by
    two types of weathering
  • mechanical and chemical.

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.

Causes of Mechanical Weathering
  • Must involve motion or movement
  • Temperature / Exfoliation
  • Frost action / Ice wedging
  • Organic activity / Root Pry
  • Gravity / Rolling down hill
  • Abrasion / Sand blasting
  • Glacier / Till

  • Rocks can be broken apart by changes in
    temperature. As rocks are heat up in the sun
    during the day, the outside of the rock expands.
    The inside of the rocks remain cool and do not
    expand. When the air temperature drops at night,
    the outside of the rock cools and contracts.
    This continuing cycle causes particles to break
    off. This is called exfoliation.

Rock Exfoliates
  Study the figure and answer the questions below
in your notebook.
a Are the changes in temperature gradual or
rapid? b In which regions do these fluctuating
temperatures often occur? c Where are the points
of weakness in the rock?
Frost Action
  • Unlike most liquids, water expands when it
    freezes. The repeated freezing and melting of
    water, called frost action, is another cause of
    mechanical weathering. When water freezes in
    cracks in the rocks, it expands, making the crack
    larger.In time, this causes the rock to break
    into pieces.

Ice Wedging / Frost Action
. Study figure and answer the questions below in
your notebook
  • a Where is the rainwater going?
  • b What happens when temperatures fall below 0C?
  • c Why has the crack widened?
  • d What effect will this have on the rock?
  • e Which process does this flow chart illustrate?

Organic Activity
  • Plants and animals can cause mechanical
    weathering. 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

(No Transcript)
Study the figure and answer the questions below
in your notebook.
  • a What happens to the crack as the roots get
  • b What kind of weathering is this?
  • c What kind of weathering does this process
  • d The decaying tree will provide

  • 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, know as MASS MOVEMENT. 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.

  • 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 or at the beach, 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.

Sandblasting Art http//
Chemical Weathering
  • When the chemical makeup of the rocks is changed
    it is called chemical weathering. During
    chemical weathering, changes occur in the mineral
    composition of rocks. Minerals can be added,
    removed or broken down (decomposed). Many
    substances react chemically with rocks to break
    them down.

Types of Chemical Weathering
  • The chemical composition must be changed
  • Water /dissolving
  • Oxidation / Rusting
  • Carbonation / Acid Rain weak
  • Sulfuric acid / Acid Rain strong
  • Plant acids / Mosses and lichens

  • Most chemical weathering is caused by water and
    carbon dioxide. Water can dissolve most of the
    mineral that hold rocks together. Rocks that
    dissolve in water are said to be soluble. Water
    can form acids when it mixes with certain gases
    in the atmosphere to speed up the decomposition
    of rocks. Water can also combine with a mineral
    to form a new mineral.

Our State (Magazine)
  • Linville Caverns Video
  • https//

  • Chemical weathering is also caused by oxidation.
    Oxidation is the process in which oxygen
    chemically combines with another substance. The
    result of oxidation is the formation of an
    entirely different substance. Iron in rocks
    combines with oxygen in the air to form iron
    oxide, or rust.

  • When carbon dioxide dissolves in water, a weak
    acid called carbonic acid is formed. When
    carbonic acid reacts chemically with other
    substance, the process of carbonation occurs. In
    nature, carbonic acid is formed when carbon
    dioxide in the air dissolves in rain. This acid
    rain falls to the ground and sinks into the soil.
    It decomposes feldspar and limestone.

Study the figure and answer the questions below
in your notebook.
a What kind of weathering is this? b How does
this kind of weathering affect the rock? c Which
climates speed up this kind of weathering?
Sulfuric Acid
  • The air in certain areas is polluted with sulfur
    oxides. Sulfur oxides are a byproduct of the
    burning of coal as a source of energy. These
    compounds dissolve in rainwater to form sulfuric
    acid. Rain that contains sulfuric acid is one
    type of acid rain. It is much stronger than
    carbonic acid. Sulfuric acid corrodes rocks,
    metals and other materials quickly.

Plant Acids
  • Plants produce weak acids that dissolve certain
    minerals in rocks. Mosses and lichens produce
    weak acids that dissolve some of the minerals in
    the rocks they grow on in areas of PERMAFROST /
    Polar Climates. Gradually the rocks break into
    smaller pieces. They are important in the
    formation of soil.

Rate of Weathering
  • The composition of the rock
  • The amount of time that the rock is exposed on
    the Earths surface
  • The amount of exposed surface on a rock
  • Climate
  • Relief / Topography (Height)

Rate of Getting Wet
  • Check your understanding of weathering rates.
  • With a partner list five different ways that your
    clothes become saturated with rain more and/or
    faster. Then compare that list with your notes on
    the rates of weathering

Rate of Weathering Graph
Composition of Rocks
  • Two different types of rocks in the same climate
    can weather differently, depending on the
    minerals that make up each rock type. If a rock
    resist weathering, the rock is called a STABLE
    ROCK. The stability of a rock can vary depending
    on the climate in which the rock is found.
    Limestone is stable in a dry climate but not in a
    wet climate.

Amount of Time of Exposure
  • The amount of time that rock is exposed on the
    Earths surface also affects its rate of
    weathering. A very old rock that has not been
    exposed to the forces of weathering can remain
    almost unchanged. If a newly formed rock is
    deposited on the Earths surface it will begin to
    weather right away.

Which Is Older????
The Amount of Exposed Surface
  • The amount of exposed surface area on a rock also
    affects its rate of weathering. As rocks are
    broken down into many small pieces, more rock
    surfaces are exposed and more weathering takes
    place. In rocks that contain many joints or
    cracks, various chemicals easily come into
    contact with the rock surfaces and break them

Which Dome is more exposed???
Chimney Tops G.S.M.N.P.
(No Transcript)
  • Cold and/or dry climates favor physical
    weathering. Arctic/Polar
  • Warm and wet climates favor chemical weathering.
  • Frost action works best in areas where the
    temperature fluctuates wildly. Temperate Climates

  • Part 2

Part II Erosion and Deposition
  • Essential Question
  • Compare erosion by water, wind, ice, and gravity
    and the effect on various landforms

What Caused This?
What is Erosion?
  • Erosion the moving of rock material from one
    place to a new location
  • For erosion to occur three processes must take
    place detachment of particles, lifting them, and
    transporting them
  • Many agents of erosion - flowing water, moving
    ice, waves, gravity, or wind
  • Sand, silt and clay consists of small pieces of
    rock that have been weathered from a parent rock,
    eroded, and deposited somewhere else

What Is Wind Erosion?
  • Wind - responsible for wearing away rocks and
    creating great deserts like the Sahara Desert and
  • Most effective in moving loose material
  • Two main effects (1) Wind causes small particles
    to be lifted and moved away. (2) Suspended
    particles may impact on solid objects causing
    weathering by abrasion (rubbing/blasting).
  • Occurs in areas where there is not enough
    rainfall to support vegetation

What Is Water Erosion?
  • Water - most influential force in erosion
  • Ability to move materials from one location to
    another over long distances
  • The faster water moves in streams the larger
    objects it can pick up and transport. Known as
  • Responsible for wearing away of rocks in rivers,
    lakes, and the oceans

What Is Wave Erosion?
  • Waves - relentless pounding
  • Erodes the softer, weaker parts of the rock
    first, leaving harder, more resistant rock behind
  • Can take over 100 years to erode a rock to sand
  • Energy of waves along with the chemical content
    of the water erodes the rock off the coastline

What Is Gravitational Erosion?
  • Mass movement - downward movement of rock and
    sediments, mainly due to the force of gravity.
  • Moves material from higher elevations to lower
    elevations where streams and glaciers can pick up
    the material and move it to lower elevations
  • Process is occurring continuously on all slopes,
    some act very slowly while others occur very
    suddenly until equilibrium is reached

What Is Glacial Erosion?
  • Ice - moves and carries rocks, grinding the rocks
    beneath the glacier
  • Weathering Abrasion cuts into the rock under the
    glacier, smoothing and polishing the rock surface
  • With the same glacial process material is
    transported to new locations.
  • HOMES ???

What is Deposition?
  • Deposition - laying down of sediment that has
    been eroded/transported by a medium such as wind,
    water, waves, gravity or ice
  • Process of erosion stops when the moving
    particles fall out of the transporting medium and
    settle on a surface. This settling of eroded
    material into a new location is called
  • Obstacles, whether natural or man-made, will
    often decide where the deposition occurs and the
    nature of the feature formed.

Whats the Difference?
  • WEATHERING - think weather wearing rock down
  • EROSION - think of a road and traveling
  • DEPOSITION think of depositing money in a bank

Part III Soils
  • What is Soil ?

What is Soil?
  • Soil - Soil is a natural body comprised of solids
    (weathered minerals, rock and decaying organic
    matter called humus), liquid (water), and gases
    (air) that occurs on the land surface, occupies
    space, and is characterized by horizons, or
    layers, that are distinguishable from the initial
    material as a result of additions, losses,
    transfers, and transformations of energy and
    matter or the ability to support rooted plants in
    a natural environment.

What is Dirt???
  • Often referred to mistakenly as soil. Dirt is
    actually unclean matter, especially when it is in
    contact with a person's clothes, skin or
    possessions when they are said to become dirty.
    Common types of dirt include mud, dust and, yes,
    soil itself. Any unclean substance, such as mud,
    dust, excrement, etc. filth

Whats the difference between soil and dirt?
  • Dirt is what you find under your fingernails.
    Soil is what you find under your feet.
  • Think of soil as a thin living skin that covers
    the land. It goes down into the ground just a
    short way. Even the most fertile topsoil is only
    a foot or so deep. Soil is more than rock
    particles. It includes all the living things and
    the materials they make or change.

Soil Formation
  • The weathering of rocks on the Earths surface
    results in the formation of soil.
  • Soil is formed when rocks are continuously broken
    down by weathering.
  • As rocks weather, they break into smaller pieces.
  • These pieces are broken down into even smaller
    pieces to form soil.

Importance of Soil
  • The formation of soil is extremely important to
    most living organisms.
  • Plants depend on soil as source of food.
  • Soil supplies plants with minerals and water
    needed for growth.
  • Animals depend indirectly on soil since they eat
    plants and other animals that eat plants.

Residual Soil Lives at Home
  • Sometimes soil remains on top of its parent rock,
    or the rock from which it was formed. This is
    called residual soil. Residual soil has a
    composition similar to that of the parent rock it

Transported Soil Erosion
  • Some soil is removed from the parent rock by
    water, wind, glaciers and waves. Soil that is
    moved away from its place of origin is called
    transported soil. Transported soil can be very
    different in composition from the rock it covers.

  • The layer of rock beneath the soil is called
    bedrock. Also Known as Parent Rock

  • Certain bacteria in the soil cause the decay of
    dead plants and animals.
  • This decaying material is called humus.
  • Humus is a dark-colored material that is
    important for the growth of plants.
  • Some of the chemicals produced during the
    process of decay speed up the breakdown of rocks
    into soil. Acids

Living Things
  • Living things such as moles, earthworms, ants and
    beetles help to break apart large pieces of soil
    as they burrow through the ground. The burrows
    allow water to move rapidly through the soil.
    The water speeds up the weathering of the
    underlying rock.

Soil Composition
  • Pieces of weathered rock and organic material, or
    humus, are the two main ingredients of soil.
    Organic materials is material that was once
    living or was formed by the activity of living
    organisms. Rock particles form more than 80 of
    soil. Air and water are also present in soil.

Minerals in Soil
  • Clay and quartz are the most abundant minerals in
    soil. Because they are stable, they exist in the
    greatest quantities. Potassium, phosphorus and
    the nitrogen compounds called nitrates are
    important chemicals in soil. They are vital to
    plant growth.

Pore Spaces
  • Air and water fill the spaces between soil
    particles. These are called pore spaces.
    Plants and animals use the water and air in these
    spaces, as well as the minerals dissolved in
    water. Pore spaces provide needed oxygen for
    healthy plant root growth.

Different Compositions of Soil
  • The composition of soil varies from place to
    place. The type of rock broken down by
    weathering determines the kinds of minerals in
    the soil. The type of weathering also affects
    the composition of soil.
  • Mechanical weathering produces soil with a
    composition similar to the rock being weathered.
  • Chemical weathering produces soil with a
    different composition.

Soil Texture
  • The type of weathering also affects soil texture.
    Texture refers to the size of the individual
    soil particles. Soil particles vary from very
    small to large. Both mechanical and chemical
    weathering first breaks rocks into gravel
    (2-64mm) and then in sand (less than 2mm) and
    then into silt and finally clay.

Soil Horizons
  • As soil forms, it develops separate soil layers
    called horizons. Each soil horizon is different.
    A cross section of the soil horizons is called
    soil profile. A soil profile shows the different
    layers of soil.

Evolution of Soil Horizons
Topsoil Root Zone
Parent Rock
New Horizon Classification
Mature Soil
  • Soil that has developed three layers is called
    mature soil. It takes thousands of years and the
    proper conditions for soil to develop three
    layers. The uppermost layer of mature soil is
    called the A horizon. The A horizon is a
    dark-colored soil layer in which much activity by
    living organisms takes place. Bacteria,
    earthworms and beetles help the decay.

O (Organic) Horizon
  • This is the organic debris that has recently
    accumulated on the surface of the soil. It has
    begun to decompose.

A Horizon
  • The soil in the A horizon is called topsoil.
    Topsoil consists mostly of humus and other
    organic materials. Humus supplies minerals
    essential for plant growth. Humus is spongy and
    stores water. It also contains pore space for
    air and water. Topsoil is the most fertile part
    of the soil.

E (Eluviation) Horizon
  • This eluviation (leaching) layer is light in
    color this layer is beneath the A Horizon and
    above the B Horizon. It is made up mostly of sand
    and silt, having lost most of its minerals and
    clay as water drips through the soil (in the
    process of eluviation).

Virgin Soil
  • Soil that has little or no humus/organic mater
    incorporated. B horizon and below
  • Soil that is as yet undeveloped
  • Soil that has not been cultivated before
  • Virgin soil, when compact and of considerable
    depth, is a good material to build on, providing
    the structure is not an extremely lofty or heavy

B Horizon
  • Water that soaks into the ground washes some
    minerals from the A horizon into the second layer
    of soil, or the B horizon. This process is
    called leaching. The B horizon is just below the
    A horizon. The B horizon is also made of clay
    and some humus. The soil in the B horizon is
    called subsoil. Subsoil is formed very slowly.

C Horizon
  • The third layer of soil is called the C horizon.
    The C horizon consists of partly weathered parent
    rock. The C horizon extends down to the top of
    the un-weathered parent rock. The composition of
    the soil in the C horizon is similar to that of
    the parent rock.

What are the differences between eluviation and
  • Eluviation is the leaching down or movement of
    the particles (such as minerals and organic
    matter) into lower soil horizons. While,
    Illuviation is the accumulation of those
    particles in the lower soil horizons.

R(rock) Horizon
  • Solid un-weathered bedrock. Referred to Parent
    Rock. It is either
  • Permeable allowing most substance to pass or
    flow in and out
  • Semipermeable - allowing some substances to pass
  • Impermeable or Nonpermeable materials cannot
    pass through

Immature Soil
  • In some places, the upper layers of soil are
    removed and the rocks below the soil are exposed.
    The weathering process then forms new soil from
    the exposed rocks. This recently formed soil is
    immature because there has not been enough time
    for all three soil layers to form. The soil in
    the northern regions where glacial erosion has
    taken place, is immature soil.

Formation of Soil
  • There are several factors that determine whether
    three layers of soil will form.
  • Time
  • Climate
  • Type of rock / Parent Rock
  • Surface features of the region

  • Time is one of the most important factors in soil
    formation. The longer a rock is exposed to the
    forces of weathering, the more it is broken down.
    Mature soil is formed if all three layers have
    had time to develop.

  • Climate is another important factor in the
    formation of soil. In areas with heavy rainfall
    and warm temperatures, weathering takes place
    more rapidly. Heavy rainfall may wash much of the
    topsoil away. Since Organisms are more plentiful
    these areas, the soil is quickly replaced. They
    speed up the chemical and mechanical weathering
    of rocks.

Type of Rock
  • The type of rock in an area also affects soil
    formation. Some rocks do not weather as rapidly
    as other do. Rocks that do not break down easily
    do not form soil rapidly. In some climates it
    takes along time for granite to break down. So
    soil formation from granite is slow. But
    sandstone breaks easily and forms soil quickly.

Surface Features of Region
  • The surface features of the region also determine
    the speed at which soil is formed. On very steep
    slopes, rainwater running off the land erodes the
    soil and exposes rock to weathering.

Four Soil Formation Processes
  • These processes of soil genesis, operating under
    the influence of environmental factors, give us a
    logical framework for understanding the
    relationships between particular soils and the
    landscapes and ecosystems in which they function.
    This approach is known as internal process
    modeling. In analyzing these relationships for a
    given site, ask yourself

Ask yourself
  • What are the materials being added to this soil?
  • What transformations and translocations are
    taking place in this profile?
  • What materials are being removed?
  • How do climate, organisms, topography, and parent
    material affect these processes over time?