IV. Weathering and soils

A. Definition

1. disintegration and decomposition of rock at or near the surface
2. breakdown of parent rock into sediment
3. weathering vs. erosion

B. Processes

1. Mechanical - physical breakdown of rock into smaller pieces with composition identical to source rock
   1. crack formation (joints)
      1. pressure release - unloading (removal of overlying rock)

• tectonic
• igneous - exfoliation, sheeting (fig. 5.5, 5.6)

1. temperature change

• thermal contraction (columnar joints)
• thermal expansion ( sun & fires )

1. factors which enlarge cracks
   1. frost action (Fig. 5.3)
   2. biological factors
2. processes which don't require cracks
   1. biological
   2. abrasion (overlaps with erosion)

1. Chemical - change minerals - old minerals replaced by new with some elements washed away in solution
   1. Processes:
      1. dissolution (more effective if water is slightly acidic)
      2. oxidation
      3. alteration by carbonic acid (hydrolysis)

      H₂0 + CO₂ -->H₂CO₃

   2. Effect on minerals (Table 5.1)

C. soils

1. Definition
   1. combination of sediments and decayed organic matter (humus) and empty pore space
   2. residual soil
   3. transported soil
2. Soil profile -
   1. processes
      1. water percolating down - removes soluable material, leaching
      2. evaporation causes minerals to precipitate
   2. Horizons (fig. 5.20)

• O & A- organic
• E - removal (leaching and eluviation)
• B - accumulation
• C - transition
• parent rock

1. Soil types (Table 5.2)
   1. pedalfer
   2. pedocal - arid - caliche & hardpan
   3. laterite - wet, hot tropics (Box 5.2) bauxite