Weathering, Soil Formation, and Erosion

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Weathering, Soil Formation, and Erosion

• Chapters 7-8

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Weathering

• 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

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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

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Chemical Weathering

• What causes Chemical Weathering?
• Water
• Dissolves minerals that hold rocks together
  (hydrolysis)
• 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.

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Chemical Weathering
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Acid Precipitation and Fish
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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
  buildings?
• Plant Acids
• Plants produce weak acids that dissolve certain
  minerals (mosses) and break rocks into smaller
  pieces.

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What affects the rate of Weathering?

• Rock composition (different rocks weather
  differently)
• 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

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Climate and Chemical Weathering
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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.

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There is a higher rate of mechanical and chemical
weathering in Asheville than in Phoenix.
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Erosion

• 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
  locations
• Final stage in the erosion process
• Erosion moves materials deposition builds new
  landforms

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Agents of Erosion

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

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Gravity

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

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Wind

• 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
  direction
• Reduces soil erosion, can trap blowing snow,
  protect crops, etc.

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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
  erosion

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Gully Erosion vs. Rill Erosion
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Glaciers

• 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

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Plants, Animals, Humans

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

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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

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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

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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)

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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

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Rockslide on Westbound I-90
Landslide El Salvador 2000 (Earthquake)
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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
  land
• Happen in areas with thick soil and medium-steep
  slope
• 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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Slumps
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(No Transcript)
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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)

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Wind Erosion

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

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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

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Loess

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

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Glaciers

• 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

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Glacial Erosion

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

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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

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Soil Formation

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

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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

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Soil Composition

• Soil forms in layers during the process of its
  development.
• 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
  layer.
• Rainwater seeps through this top layer of
  materials, dissolves soluble minerals, and
  carries them into the lower layers of the soil.

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Soil Composition

• Residual soil is soil located above its parent
  bedrock.
• 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
  glaciers.
• 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.

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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
  material.
• The O horizon is organic material, that may or
  may not be present.

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Topography

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

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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
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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

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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

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Soil Types

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

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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.

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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.

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Soil Fertility

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

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Soil Color

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