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Weathering, Soil Formation, and Erosion


Weathering, Soil Formation, and Erosion Chapters 7-8 Glacial Deposition Moraines Ridges of glacial till Till is the mix of debris carried by the glacier Glaciers can ... – PowerPoint PPT presentation

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Title: Weathering, Soil Formation, and Erosion

Weathering, Soil Formation, and Erosion
  • Chapters 7-8

  • Breaking down of rocks and other materials on
  • the Earths surface
  • it is a slow, continuous process
  • effects arent easily observed
  • Two Types
  • Mechanical rocks are broken into smaller pieces
    but their chemical makeup does not change
  • Chemical chemical makeup of rocks is changed as
    rocks are broken down

Mechanical Weathering
  • What causes Mechanical Weathering?
  • Temperature
  • Rocks expand/contract (cycle).
  • Causes exfoliation (flaking)
  • Frost wedging
  • Water seeps into small cracks, freezes and
    expands, which enlarges cracks. (cracks in
    sidewalk, potholes in road)
  • Organic Activity (caused by living things)
  • Plant roots can loosen rock, make cracks larger
    (grass in sidewalk)?called root-pry
  • Abrasion wearing away of rocks by particles
    carried by wind, water, etc.
  • Rough edges of particles scrape off parts of
    rocks. Rocks in a riverbed are scraped by moving
    objects in the water?they become smooth

Chemical Weathering
  • What causes Chemical Weathering?
  • Water
  • Dissolves minerals that hold rocks together
  • Forms acids when mixed with gases in atmosphere
    (Acid precipitation)
  • Acid precipitation has a ph of less than 5.6
  • Combines with mineral to make new mineral
    (combine w/ feldspar to make clay)
  • Oxidation
  • Oxygen chemically reacts with something else.
    (Iron Oxygen rust) Inner rock will be diff.
    color than outer rock.

Chemical Weathering
Acid Precipitation and Fish
Chemical Weathering, cont.
  • Carbonation
  • Carbon dioxide and water form a weak acid. It
    dissolves certain rocks (limestone) but not
    strong enough to harm plants/animals
  • Sulfuric Acid
  • caused by pollutants-they dissolve in rain to
    form acid rain. It corrodes (wears away) rocks,
    metals, etc. quickly.
  • What do you think it does to monuments and
  • Plant Acids
  • Plants produce weak acids that dissolve certain
    minerals (mosses) and break rocks into smaller

What affects the rate of Weathering?
  • Rock composition (different rocks weather
  • Stable rock resists chemical weathering
  • Climate
  • Ex limestone OK in warm/dry climate when
    wet-weak acids weather
  • Time exposed on surface
  • Old unexposed rocks-no big changes
  • New exposed rocks-weather quickly

Climate and Chemical Weathering
What affects the rate of Weathering?
  • Surface area
  • When rock is in small pieces, more surface area
    is available for weathering
  • Topography, other variables
  • Materials on slopes are more likely to move due
    to gravity.
  • This exposes underlying rock, providing more
    opportunities for weathering.

There is a higher rate of mechanical and chemical
weathering in Asheville than in Phoenix.
  • Erosion
  • Process by which weathered rock and soil
    particles are moved form one place to another
  • Carries away products of weathering
  • Deposition
  • Process by which sediments are laid down in new
  • Final stage in the erosion process
  • Erosion moves materials deposition builds new

Agents of Erosion
  • Gravity
  • Wind
  • Running water
  • Glaciers
  • Waves
  • These are all forces that move materials from
    place to place

  • Pulls rocks and soil down slopes
  • Agent of mass movements
  • Landslides
  • Mudflows
  • Avalanches

  • Major agent of erosion in hot, dry climate or
    places with little vegetation
  • When wind erodes soil to depth that water is
    present, shrubs/grasses can grow?called oasis
    (happens in desert)
  • Wind barriers (windbreaks) are used to reduce
    effects of wind erosion
  • Trees, plants planted perpendicular to winds
  • Reduces soil erosion, can trap blowing snow,
    protect crops, etc.

Running Water
  • Water has more power than wind to move particles
    (exceptions are hurricane and tornado winds)
  • When water moves faster, erosion is greater
  • Erosion by running water in small channels on
    side of slope is rill erosion
  • When channels become deep it evolves into gully

Gully Erosion vs. Rill Erosion
  • Produce large-scale, dramatic effects
  • Have capacity to carry HUGE rocks, piles of
    debris over great distances
  • Scratch/grind the surface
  • Can polish others
  • Famous for their deposition

Plants, Animals, Humans
  • Materials get moved from place to place
  • Digging tunnels underground (animals)
  • Excavation
  • Planting gardens, etc.
  • Building roads, buildings, etc.

Mass Movements
  • Downslope movement of loose sediments and
    weathered rock caused by gravity
  • A form of erosion
  • Only occur on slopes
  • Also Known As Mass Wasting

Factors that affect MassMovements
  • Weight of material
  • Resistance to sliding/flowing
  • Triggers such as earthquakes
  • Amount of friction between material and slope
  • Amount of erosion that has taken place at the
    bottom of the slope
  • Mass movement occurs when the forces pulling
    material down a slope are more than the
    resistance of the materials

Types of Mass Movements
  • Creep
  • Slow, steady downhill flow of loose, weathered
    materials such as soil
  • Can be just a few centimeters per year
  • Flow
  • Weathered materials can flow like a liquid
  • Mudflows
  • Swift moving mixture of mud and water
  • Triggered by EQ, common in volcanic areas, sloped
    areas with short periods of intense rain (CA)

Types of Mass Movements
  • Slides
  • Rapid, downslope movement that occurs when a
    block of loose soil, rock, debris separates from
    the bedrock underneath
  • Landslide
  • Little internal mixing of materials
  • Very fast movement, some up to 200km/hr (124 mph)
  • Rockslide
  • When rock sheets move downhil
  • Often caused by EQ

Rockslide on Westbound I-90
Landslide El Salvador 2000 (Earthquake)
Types of Mass Movements
  • Slumps
  • Happens when material in a landslide rotates
    while it slides downhill
  • Common after rain, leave a crescent scar on the
  • Happen in areas with thick soil and medium-steep
  • Avalanche
  • Landslide with snow
  • Early in winter, warm ground melts snow (it
    refreezes, turns to ice). Vibrations from EQ, etc
    can trigger snow on top of this ice sliding down
    the mountain

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Types of Mass Movements
  • Rock Falls
  • Occur at high elevations, steep road cuts, and
    rocky shorelines
  • Cliffs become weathered, eroded, and rocks fall
    into a cone-shaped pile
  • Not likely in humid areas (water promotes plant
    growth, which lessens likelihood of rock falls)

Wind Erosion
  • Occurs in places with little precipitation
  • Suspension
  • Particles are in the air for a long period of
  • Saltation
  • Particles bounce around
  • Deflation
  • Lowering of land surface due to removing surface
  • Abrasion
  • Rocks shaped by abrasion are called ventifacts

Wind Deposition
  • Wind carries particles like sand through the air
  • When something blocks their movement, the sand
    gets deposited
  • Over time, the pile of sand becomes a dune
  • There are different types of dunes
  • Dunes are classified by shape
  • Human activity in coastal regions has disrupted
    dune formation/damaged sand dunes

  • Fine, lightweight particles that have been
    carried and deposited by the wind

  • A moving mass of ice
  • Form at Earths poles and in high elevations
    (mountainous areas)
  • Valley Glaciers
  • Form in valleys of mountainous areas
  • Occurs when ice is too heavy, flows down the
    mountain like liquid
  • Continental Glaciers
  • Cover a continent-sized area
  • Thickest at center
  • Callled Ice Sheets

Glacial Erosion
  • Most powerful agent of erosion because of size,
    weight, density
  • Glaciers carve U-shaped valleys, put
    scratches/grooves on rocks

Glacial Deposition
  • Moraines
  • Ridges of glacial till
  • Till is the mix of debris carried by the glacier
  • Glaciers can melt and the ice recedes/water flows
    into the valley
  • The location where the sediments get deposited is
    the Outwash plain
  • Drumlin
  • Elongated landform caused by glacial movement
  • Esker
  • Winding ridges of layered sediment deposited by
    streams under a glacier-esker
  • Glacial Lake
  • Occurs when block of ice breaks away from a
    glacier, melts in a depression. Precipitation,
    surface water combine with the water to form a
    glacial lake

Soil Formation
  • Soil is an important natural resource
  • It is found almost everywhere on Earth
  • Exactly what is it?

Soil Formation
  • Soil
  • Loose covering of broken rock and humus on the
    earths surface
  • Humus is decaying organic matter (leaves, etc)
  • Soil formation takes a very long time

Soil Composition
  • Soil forms in layers during the process of its
  • The parent rock is the solid bedrock from which
    weathered pieces of rock first break off.
  • The smallest pieces of weathered rock, along with
    living and dead organisms, remain in the very top
  • Rainwater seeps through this top layer of
    materials, dissolves soluble minerals, and
    carries them into the lower layers of the soil.

Soil Composition
  • Residual soil is soil located above its parent
  • Transported soil is soil that has been moved to a
    location away from its parent bedrock by agents
    of erosion, such as running water, wind, and
  • The parent bedrock determines what kinds of
    minerals a soil contains.
  • The parent rock and climatic conditions of an
    area determine the length of time it takes for
    soil to form.

Soil Profiles
  • What is a soil profile?
  • A vertical sequence of soil layers
  • A soil horizon is a distinct layer, or
    zone, within a soil profile.
  • There are three major soil horizons
    A, B, A, B, and C.
  • Horizon A contains high concentrations of organic
    matter and humus.
  • Horizon B contains subsoils that are enriched
    with clay minerals.
  • Horizon C, below horizon B and directly above
    solid bedrock, contains weathered parent
  • The O horizon is organic material, that may or
    may not be present.

  • The topography of a region affects the thickness
    of developing soil.
  • Soils on slopes tend to be thin, coarse, and
  • Soils formed in lower areas, such as in valleys,
    are thick and fertile.

Soil Types
  • Polar Soils
  • Form at high latitudes and high elevations
  • Located in Greenland, Canada, Antarctica
  • No distinct horizons, very shallow soil
  • Just below the soil the ground is permanently
    frozen-known as permafrost

Permafrost in Denali
Soil Types
  • Temperate Soils
  • Lots of variation
  • Support forests, grasslands, prairies
  • Amount of rainfall determines what grows in this
    type of soil
  • Grasslands-lots of humus-soil is rich and fertile
  • Forests-soil is less deep/less fertile,
    containing clays and iron oxides (Eastern US)
  • Prairies-dry soil-lots of grasses and bushes

Soil Types
  • Desert Soils
  • Very little precipitation
  • High levels of salts
  • Limited vegetation
  • Little or no organic matter
  • Very thin topsoil (A horizon)
  • Light colored and coarse soil

Soil Types
  • Tropical Soils
  • High temperatures, heavy rainfall
  • Soil is intensely weathered, infertile
  • Source of ores (iron), but not great for growing

Soil Textures
  • Particles of soil are classified according to
    size as being clay, silt, or sand, with clay
    being the smallest and sand being the largest.
  • The relative proportions of these particle sizes
    determine a soils texture.
  • The texture of a soil affects its capacity to
    retain moisture and therefore its ability to
    support plant growth.

Soil Textures
  • To determine the texture of a soil sample, find
    its percent for sand, silt, and clay.
  • The texture of the soil will be where all three
    lines intersect.

Soil Fertility
  • A measure of how well a soil can support plant
  • Factors involved include
  • Availability of nutrients/minerals
  • Precipitation
  • Topography
  • Acidity
  • Number of microorganisms present

Soil Color
  • Factors that determine soil color include
  • Climate
  • Soils composition
  • Topsoil-usually dark (rich in humus)
  • Red/yellow soils-caused by iron minerals
  • Yellow soils are often poorly drained, associated
    with environmental problems
  • Gray/bluish soils-poorly drained, constantly wet,
    lacking in oxygen
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