nat sci 3 earth science and astronomy chapter iii earth processes
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NAT. SCI. 3 – Earth Science and Astronomy CHAPTER III: Earth Processes. RONALDO T. BIGSANG, M.Sc. SURFACE PROCESSES: Weathering, Soil and Erosion. OBJECTIVES. Describe weathering as an important force in the rock cycle; Identify the types of weathering and the sub-processes under each;

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objectives
OBJECTIVES
  • Describe weathering as an important force in the rock cycle;
  • Identify the types of weathering and the sub-processes under each;
  • Discuss the components and horizons of soil;
  • Name some soil-forming factors; and
  • Explain the agents of erosion.
weathering
WEATHERING
  • surface process that breaks or decompose rocks into smaller pieces (i.e. sand, gravel, soil, etc)
  • involves little or no movement of decomposed rocks and minerals
  • 2 types: mechanical and chemical
mechanical weathering
MECHANICAL WEATHERING
  • also called PHYSICAL weathering
  • DOES NOT alter the chemical composition of weathered rocks
  • includes:
    • pressure-release fracturing
    • frost or ice wedging
    • abrasion
    • organic activity
    • thermal expansion and contraction
mechanical weathering1
MECHANICAL WEATHERING
  • pressure-release fracturing
    • tectonic force lift up buried rocks to the surface
    • erosion of the overlying rocks and sediments remove the pressure
    • buried rocks expand and fracture
mechanical weathering2
MECHANICAL WEATHERING
  • frost or ice wedging
    • water seeps into cracks in rocks
    • when temperature is very low, water freezes
    • ice expands and wedges the rock apart
mechanical weathering3
MECHANICAL WEATHERING
  • abrasion
    • weathering of rocks by friction and impact
mechanical weathering4
MECHANICAL WEATHERING
  • organic activity
    • cracks in rocks are expanded by tree or plant roots
mechanical weathering5
MECHANICAL WEATHERING
  • thermal expansion and contraction
    • when temperature changes rapidly (heating and cooling processes)
    • surface heats up (expands) or cools down (contracts) faster than the interior
chemical weathering
CHEMICAL WEATHERING
  • ALTERS the chemical composition of the weathered rock
  • rapid in tropical regions --- moist and warm
    • contrast to desert and polar regions?
  • includes:
    • dissolution
    • hydrolysis
    • oxidation

Table 1. Rate of chemical weathering in different climatic conditions.

chemical weathering1
CHEMICAL WEATHERING
  • dissolution
    • rock or mineral dissolves forming a solution
    • ACIDIC pH (lower pH than pure water)
      • CO2 + water = carbonic acid
    • BASIC pH (higher pH than pure water)
chemical weathering3
CHEMICAL WEATHERING
  • hydrolysis
    • mineral reacts with water and form a new mineral with water as part of its crystal structure
    • i.e: feldspar  kaolinite
    • 2KAlSi3O8 + H2O + 2H+ = Al2Si2O5(OH)4+ 2K+ + 4SiO2
chemical weathering4
CHEMICAL WEATHERING
  • oxidation
    • mineral decomposes when it reacts with oxygen
weathering1
WEATHERING
  • salt cracking
    • saltwater seeps into rock cracks
    • water evaporates, salt crystallizes
    • salt crystals expands and widen cracks in the rocks
weathering2
WEATHERING
  • exfoliation
    • concentric plates or shells split away from a main rock mass
    • pressure-release fracturing VS hydrolysis-expansion weathering
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