Weathering

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Weathering

• Weathering physical and chemical breakdown of
  earth material (rocks and minerals) at or near
  the surface.
• Includes disaggregation of parent materials,
  erosion and transportation of resulting clasts
  and sediments, and leads to deposition and
  lithification of these materials
• Differential weathering uneven weathering of
  heterogenous materials that leads to unique
  geologic structures such as hoodoos and arches

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Mechanical Weathering

• Physical processes cause weathering several ways
• Frost action freezing and thawing in cracks and
  pores in rocks high altitudes and latitudes
• Pressure release expansion of uplifted and
  exposed intrusive rocks exfoliation domes such
  as Stone Mountain, GA, rock bursts in mines
• Thermal expansion/contraction differential
  reaction by rocks and minerals to daily or
  seasonal temperature changes
• Salt crystal growth occurs primarily in hot,
  arid regions
• Biological actions burrowing, root wedging

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Chemical Weathering

• Processes that chemically alter parent material
• Solution/dissolution occurs when ions separate
  in water due to the solvent properties of H20,
  example dissolution of calcite in limestones by
  groundwater
• Oxidation occurs when oxides are formed by
  mineral ions combining with oxygen, example Fe
  O hematite (rust)
• Hydrolysis free ions of H and OH- in water can
  replace ions in minerals, which are liberated to
  undergo oxidation, example orthoclase weathers
  to kaolinite by loss of Al ions

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Chemical Weathering

• Chemical weathering operates on the surface of
  rocks and minerals, and can be observed in the
  difference between old and fresh surfaces of
  broken rock, controlled by
• Fractures produce increased surface area in
  equal volume of rock
• Climate presence of water greatly accelerates
  weathering. Humid, warm climates have high
  weathering rates
• Composition stable minerals are resistant to
  weathering, example Quartz less stable minerals
  rapidly weather (Bowens reaction series)
• One unique form of weathering is spheroidal
  weathering rocks often weather into spherioids
  due to uneven weathering. 3X at points compared
  to flats

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Soil

• Regolith sediments that cover the Earths land
  surface
• Soil regolith composed of weathered minerals,
  air, water, and organic material supports
  vegetation
• all land-dwelling organisms dependent on soil
• good farming soil composed of sediments (sand,
  silt, and clay), air, water, and humus.
• Humus carbon derived by bacterial decay of
  organic material

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Soil profile

• Soil forms in characteristic layers
• Horizon O thin subaerial layer, predominately
  organic material, including humus
• Horizon A weathered unstable minerals that
  provide nutrients, remnants of stable minerals
  such as quartz grains, a zone of biological
  activity.
• Horizon B where minerals leached from above
  accumulate (zone of accumulation)
• Horizon C unaltered to altered parent material

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Sedimentary Rocks

• Composed of sediments from weathering, chemical
  precipitation, and clastic material including
  fossil fragments
• Sediments are produced by erosion or chemical
  means, then transported by physical processed
  such as wind, water, and gravity, and deposited
  as loose aggregates of particles.
• Sedimentary particles are classified according to
  size gt 2 mm gravel, 1/16-2 mm sand,
  1/256-1/16 mm silt, lt 1/256 clay

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Sedimentary Rocks

• Depositional environment describes the basin in
  which sediments are deposited, including stream
  beds, lake bottoms, river deltas, coastal
  beaches, continental shelves, and sand dunes
• Variations in transport duration and conditions
  produce characteristic sedimentary particles
  abrasion reduces particle size, rounding wears
  away sharp angles, and sorting occurs during
  deposition
• After deposition, sediments can be lithified by
  compaction and cementation to produce sedimentary
  rocks

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Depositional Environments
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Rounding of Sediments
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Sedimentary Rocks

• The most common cements in sedimentary rocks are
  the minerals calcite and silica, which are
  derived from weathering of rocks and transported
  by water
• There are two major types of sedimentary rocks
• 1) DETRITAL SEDIMENTARY ROCKS-composed of solid
  particles (detritus) derived from weathered
  parent rocks, classified according to size and
  include
• conglomerate composed of gravel-sized particles,
  breccia refers to a conglomerate with angular
  clasts
• sandstone composed of sand-sized clasts of any
  composition, most common clast is quartz
• mudrock general term for sedimentary rocks with
  silt to clay sized clasts, varieties include
  siltstones, mudstones, claystones, fissile shale

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Sedimentary Rocks

• 2) CHEMICAL SEDIMENTARY ROCKS have a chemical
  origin and include
• limestone the most common carbonate rock,
  composed of carbonate mud usually precipitated
  from shallow, warm marine waters, and often
  containing fossil fragments
• coquina type of limestone composed of fragmented
  seashells
• oolitic limestone contains small, spherical
  grains (ooids), concentric rings of calcite
  precipitated around a nucleus

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Chemical sedimentary rocks

• Other chemical sedimentary rocks are
• evaporites composed of dissolved substances
  precipitated from evaporated water, examples
  halite (NaCl) and gypsum (CaSO4.2H2O)
• chert microcrystalline quartz deposited as beds
  of microfossils (radiolarians and diatoms), often
  re-crystallized, includes flint and jasper

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Sedimentary Rocks

• Coal derived from compressed, altered land plant
  remains
• peat barely altered plant material
• lignite more fully altered, low grade coal (70
  carbon)
• bitumin or bituminous coal, higher grade (80),
  with little remaining evidence of plant material
• anthracite high grade, high carbon coal (up to
  98 carbon)

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Sedimentary structures

• These are textural features formed by variation
  in deposition, and include
• bedding/strata horizontal layers
• graded bedding vertical grain size change
• cross bedding bedding at angles to depositional
  surface
• ripple marks small dunes preserved on sand
  deposits, formed by moving fluids (air, water)
• mud cracks indicative of subaerial drying

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Fossils

• remains of ancient life, found in sedimentary
  rocks
• trace fossils tracks, prints
• body fossils altered or unaltered remains of
  body parts such as bones, teeth or shells
• microfossils microscopic body fossils
• biostratigraphy correlation of sedimentary rocks
  using fossils

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Resources in Sedimentary Rocks

• clay, sand, and gravel
• evaporite minerals salt, gypsum, fertilizers
• placer deposits gold, silver, uranium
• coal
• petroleum/natural gas
• iron (b.i.f.s)