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Part 2

• ROCK JOINTS, SPECIFIC SURFACE AREA,
• and WEATHERING

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• All rock is jointed media
• This shows orthogonal jointing in layered
  sedimentary strata.
• Groundwater moves through the joints
• Joints often occur in semi-parallel clusters,
  which are more subject to weathering and
  disintegration enlarging flow conduits

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• Regional systematic joints perturb layered
  sedimentary rock strata, generally orthogonal to
  the bedding. Navajo sandstone above Colorado
  City, Arizona

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• Stepped cut in volcanic strata illustrating
  problems with raveling in upper portion of the
  weathered horizon. Note presence of trees, along
  mid-slope bench and shear zones.

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• Joints break the rock mass up into discrete
  pieces, creating greater specific surface area
  for chemical reaction and cation/anion exchange
  with oxygenated groundwater, or enzymes secreted
  by tree and plant roots

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• Plot showing asymptotic relationship between
  number of particles and specific surface area for
  one cubic meter of rock divided into increasingly
  smaller blocks
• Intensely jointed or fractured rock can be
  expected to undergo weathering much more quickly
  than less fractured, or lower permeability
  materials

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• Blocks within joint clusters can often weather
  out completely, becoming filled with porous and
  low cohesion grus, which is easily eroded, as
  shown here near Olmsted Point in Yosemite Natl
  Park.

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• At greater depths, minerals in percolating
  groundwater can foment cementation along joint
  systems, as shown here.
• Most common cementaceous agents are carbonate,
  silica, and iron derivatives
• Note maroon colored zones of iron oxide extending
  outward from orthogonal joint planes

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Precipitation of Iron Oxide

• Chemical reactions involving iron-rich minerals,
  such as pyroxene, in the presence of oxygen and
  water.
• Iron oxide can be one of the strongest binders

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• Iron cementation along orthogonal joints in
  precambrian granite in the western Grand Canyon
• Note mineralization halos radiating outward from
  the original joints, at right angles to one
  another.
• Note granite core stones weathering into grus
  between the ironstone halos