Topic 9 Weather and Erosion

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Topic 9Weather and Erosion

• Weathering the chemical and physical breakdown
  of rocks at or near Earths surface.
• Two types of weathering
• 1) Chemical weathering breakdown of rock
  through a change in mineral or chemical
  composition.
• ex when iron combines with oxygen to make
  rust (oxidation)
• ex the effect of water on minerals (water is a
  universal solvent)
• 2) Physical weathering breakdown of rock into
  smaller pieces without a chemical change.
• ex frost action water going through
  temperature changes, breaks the rock apart
• ex abrasion when rocks grind against each
  other

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Factors that Affect the Rate and Type of
Weathering

• Exposure generally the closer a rock is to the
  surface, the faster it will weather
• Particle size when rock particles are smaller,
  the total surface area per unit volume exposed to
  weathering is greater.

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Factors that Affect the Rate and Type of
Weathering Cont.

• Mineral composition different minerals have
  different physical and chemical properties
• Climate chemical weathering mostly occurs in
  warm, moist climates. In cold climates, frost
  action is most common (physical weathering).

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Soil Formation

• Physical and chemical weathering processes are
  important in the formation of soil. Soil is the
  mixture of rock particles and organic matter on
  Earths surface that supports rooted plants.
• Soils develop horizontal layers with distinctive
  profiles
• Soils can be transported from other areas by
  wind, moving water, or glaciers.

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Erosionweathering of rock particles that are
transported as sediments

• Erosion, shapes and lowers Earths surface
• Involves a transporting system which includes
• 1) an agent of erosion, such as a stream,
  glacier, wave, current, wind or human activity
• 2) the sediments being moved
• 3) a driving force (gravity)
• Energy from the Sun plays an important role
  Suns energy drives the water cycle, which
  produces precipitation and running water

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Gravity Erosion

• Gravity is the driving force behind erosion
• Gravity can pull rocks and sediments downhill,
  this is called mass movements

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Running Water Erosion and Streams

• Running water is the most common agent of
  erosion
• Running water erosion that is confined to a
  channel is called streams
• A smaller stream that flows into a larger stream
  is called a tributary
• Streams carry sediments in different ways
• 1) dissolved minerals are carried in solution
• 2) solid sediments are carried in suspension
• 3) larger sediments are carried by rolling,
  sliding or bouncing on the stream bottom
  (results in rounding of sediments by stream
  abrasion)

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Running Water Erosion and Streams Continued

• Over time, streams carve deeper channels and a
• V-shaped valley forms
• The area of land drained by any one stream is
  called its watershed or drainage area.

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Stream Velocity

• Three factors that determine average velocity
• 1) gradient (slope of the stream)
• 2) discharge (volume of water in the stream)
• 3) stream channel shape (the shape of the
  bedrock or loose materials that confine the
  stream)
• If the stream has a wide, flat stream channel,
  then there is a large surface in contact with
  moving water (more friction)
• If the stream has a semicircular stream channel,
  the velocity will be greater
• Velocity changes depending on the path of the
  stream
• -if the stream is straight, the greatest
  velocity exists in the center of the stream
• -if the stream curves, the greatest velocity
  exists on the outside of the curves
• A stream with greater velocity can carry larger
  sediment particles

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Stream Velocity

• Sediment Load changes depending the Streams
  velocity

• Velocity changes depending on the Shape of the
  Stream

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Evolution of a Stream

• Streams slowly change their characteristics over
  time.
• In its uppermost part, the stream, starts as a
  finger lake.
• In early stages there are many abrupt changes
  resulting in waterfalls and rapids
• Farther down the stream, more water is added to
  the stream (from tributaries and ground water) to
  increase velocity
• As the stream grows larger, it begins to shift
  and meander (bend or curve)
• If there is flooding, a lake may form from a
  meander and/or floodplain
• Near the end or mouth of a stream there is often
  a fan-shaped delta.

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Wind Erosion

• Two main points
• 1) deflation
• 2) sandblasting (abrasion)
• Deflation takes place in areas with small, loose
  sediments exposed to the atmosphere. Winds blow
  away the loose sediments, lowering the land
  surface.
• Sandblasting occurs when winds blow sand or silt
  grains and other objects. The pelting by the
  grains can erode, or abrade rocks.

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Glacial Erosion

• A glacier is a naturally formed mass of ice and
  snow that moves downhill on land under the
  influence of gravity
• Mountain glaciers are formed in high mountain
  valleys
• Ice-sheet or continental glaciers cover huge
  landmass regions
• The glacier acts like a fluid, it moves fastest
  in the center than on the sides
• If more snow and ice accumulates than wastes
  away, the glacier advances
• If more snow and ice wastes away then
  accumulates, the glacier retreats.

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Features of Glacial Erosion

• -When a glacier moves over land, rocks and other
  materials beneath in freeze into the ice and are
  dragged along
• -These sediments that are dragged along, scratch
  the surface beneath them
• -This action creates glacial grooves and glacial
  parallel scratches
• -The direction of exposed glacial grooves and
  scratches shows the direction of former glacial
  movement
• - The wide, thick ice of a glacier erodes its
  valley walls to form a U-shaped valley

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Wave and Current Erosion

• Waves and currents act as agents of erosion
• When waves enter shallow water near the shore,
  they drag against the bottom. The waves then
  become unstable and water rushes toward the shore
  as breaking waves or surf.
• These waves continue to pound against the shore
  line usually at an angle.
• Due to this angle, the water flows in one
  direction along the shore. This is called a
  longshore current.
• Over time, parts of the shore erodes away
• In the zone of breaking waves, deposited
  sediments can create a series of mounds called
  sandbars.

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Wind Erosion
Glacial Flow
Longshore current
Glacial Grooves - Striations