Lecture 1a Soil From Rocks

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Lecture 1aSoil From Rocks

• Where does Soil Come From?
• Soil is everywhere!
• But how does it develop?
• What causes one soil to be productive and another
  to be poor?
• What are the rocks doing in this soil?

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Soil is

• The soil profile
• And the soil we pick up to feel consisting of
  sand, silt, clay particles and decomposed organic
  matter.

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Soil Texture

• Soil texture is determined by separating the
  amount of sand, silt and clay in a soil and
  determining the of each. We will learn about
  this in Week 2.
• This mineral part of the soil or the Sand, Silt
  and Clay particles are from the weathering of
  Rocks and Minerals.
• Where have you been aware of the rocks that were
  near the surface of the earth?

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Rocks Weather to Soil

• Weathering is the process by which all rocks at
  the earth's surface get broken down.
• Weathering occurs by both chemical
  (decomposition) and mechanical processes
  (disintegration).

epilithic
endolithic
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Rock Weathering

• Rock weathering Takes place in-situ (in place)
• It differs from erosion which involves removal of
  material away from a site.

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Physical Weathering

• Physical weathering - Rocks get broken into
  pieces but its chemical composition remains
  unchanged.
• Processes of Phys.Weathering
• 1) Freeze / thaw weathering - occurs when
  temperature freezes at night and rises during the
  day. Water expands when frozen which forces rocks
  open.
• 2) Biological weathering _ Roots of plants grow
  into cracks and force cracks open.

chasmolithic
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Physical Weathering Processes (cont.)

• 3) Exfoliation or Unloading - when rock at
  earth's surface is worn away.
• After a rock that has formed deep in the earth is
  exposed at the surface it expands and gradually
  breaks into sheets.

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Chemical Weathering

• Chemical weathering -Rock broken down by chemical
  change - water always plays a part. For Example
• Carbon dioxide dissolves in rain water forming
  carbonic acid which dissolves limestone rock
  which is carried away in solution as calcium
  hydrogen carbonate.
• Chemical weathering is faster for limestone than
  sandstone and is speeded up by heat.

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Chemical Weathering

• Chemical weathering occurs fastest at the sharp
  edges of rocks as they have a large surface and
  less volume so the chemical reactions are faster.
  
• Gradually the sharp edges become rounded.

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Chemical Weathering

• Chemical weathering produces clays on which
  vegetation can grow.
• A mixture of dead vegetation, clay, rock
  fragments of sand and silt size particles
  produces soil.

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A residual soil from limestone

• This soil is weathering from limestone bedrock.
• Limestone is CaCO3
• The 18 inches of soil present here are from the
  impurities in the rock and the additions of soil
  particles by wind to the surface of the soil.
• Prairie grass roots gave the soil a dark color.

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Chemical Weathering

• Common chemical weathering processes are
  hydrolysis, dissolution, and oxidation.
• Chemical weathering tends to weaken rock, thereby
  making it easier to break.

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Weathering

• Physical and chemical weathering occur together.
• Physical breaks rocks into pieces so more surface
  is exposed to chemical weathering which breaks it
  down further.

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Weathering

• Weathering is controlled largely by climate. The
  more water available, the more likely that
  chemical processes can proceed.
• Additionally, in warm temperatures chemical
  weathering can proceed even faster.

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Weathering

• In arid climates weathering processes occur very
  slowly because of the lack of water.
• Mechanical weathering will be the dominant
  process in arid climates
• however, because physical weathering relies on
  chemical weathering, it will also be quite slow

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Weathering of rocks produces soil particles

• By looking at the sand grains, we can determine
  the kind of rocks that physically weathered to
  make the sand.
• 
  

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Sands from around the world

• Italy
• Hawaii
• Madeline Is. Wisconsin
• St. Peter Sandstone, St. Paul, MN

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Silt Grains- The intermediate size soil particle

• In this sediment sample, the grains are a little
  smaller than in the sand photos. They look
  bigger due to magnification.
• Silt-sized particles have diameters between 0.05
  mm to 0.002 mm
• Most Silt grains are quartz because the less
  resistant minerals have been completely broken
  down.
• Silt feels very smooth.

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Silt in Soap?

• Glacier Silt Scrubbie
• A moderately abrasive bar made with silt from the
  Mendenhall Glacier, Designed to remove dirt, help
  cut grease, and leave hands smooth.

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Glacial Flour

• After the glacier grinds up the rock into silt
  size particles, the wind can pick up the glacial
  flour (silt) and blow it around.
• The resulting deposit of silt is called LOESS
  (sounds like us)

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LOESS

• Loess in the U.S. is derived from glacial
  outwash. The loess was blown directly
  from glacial deposits, and also
  carried by rivers to be blown off the flood
  plains.
• The loess in W. Wi. E. MN is thick and is
  absent in E.Wi. cent. Mn partly
  because the loess
  blanket
  naturally tapers, but also because most areas
  without loess were still ice-covered at the time,
  so loess was
  intermingled with other materials.

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Silt in Water

• Silt in water will remain suspended until the
  water is moving very slowly.
• Silt causes water turbidity and can harm fish and
  stream quality.

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Loess in Mississippi

• The loess deposits at Vicksburg Mississippi are
  believed to be 10,000 to possibly 100,000 years
  old.
• Fossils of vertebrate animals have been found
  buried in the loess and include mastodon, horse,
  tiger, bear, deer, and bison.

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Road Cuts

• Describe to your neighbor the road cut you saw
  this weekend and on your dyad sheet.

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Loess in China is indirectly derived from glacial
outwash, which is carried into the desert
lowlands by streams. Wind transports loess from
the deserts, and loess is in turn redeposited by
the rivers.
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Clay Particles The smallest soil particle

• In this sample, the flakey nature of clay-sized
  sediments is evident.
• Clay is the last of the sediments to be deposited
  by a stream due to its small grain size.
• Clay size particles are made by a recombination
  of minerals or synthesis from elements, not from
  grinding up of silt particles.
• Clays have diameters that are smaller than 0.002
  mm.

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Clay Soils

• Soils that are high in clay have unique
  properties
• Very sticky when wet and very hard when dry.

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• What are the rocks doing in this soil?
• The rocks are waiting to be broken down into sand
  grains, later into silt grains, and maybe
  eventually chemically weathered into clay
  particles if the weathering process has enough
  time and the right environment.

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The End