Chapter Five
Weathering: The
Breakdown of Rocks
CHAPTER 5: WEATHERING: THE
BREAKDOWN OF ROCKS
A) WEATHERING: PROCESS BY WHICH ROCKS AND MINERALS
BREAK DOWN AT OR NEAR THE EARTH’S SURFACE
BENEFITS OF WEATHERING: PRODUCES SOILS (MINERALS AND
ELEMENTS)
DETRIMENT OF WEATHERING: DESTROYS STRUCTURES WE BUILD
B) EROSION: PROCESS BY WHICH MOVING WATER, WIND, OR ICE
CARRIES PIECES OF ROCKS AND DEPOSITS
C) SEDIMENT: LOOSE, FRAGMENTED SURFACE MATERIAL
II WEATHERING PROCESS:
A. MECHANICAL WEATHERING (BREAKS ROCKS INTO SMALLER
PIECES)
  FROST WEDGING (EXPANSION OF CRACKS IN ROCK AS WATER IN
THE CRACK FREEZES AND EXPANDS)
  SALT CRYSTAL GROWTH (FORCES CRACK’S WALLS FARTHER
APART)
  THERMAL EXPANSION AND CONTRACTION (ALTERNATE
ENLARGEMENT AND SHRINKING)
  MECHANICAL EXFOLIATION (FRACTURING AND REMOVAL
OF SUCCESSIVE ROCK LAYERS AS DEEP ROCKS EXPAND
U PWARD AFTER OVERLYING ROCKS HAVE ERODED AWAY)
OTHER MECHANICAL WEATHERING
PROCESSES
GROWTH OF PLANT ROOTS (EXPANDS EXISTING CRACKS
IN ROCKS)
o
BURROWING ANIMAL ACTIVITIES
o
ABRASION OF TRANSPORTED PARTICLES
o
Mechanical Weathering
• Frost action
– Mechanic effect of freezing (and expanding)
water on rocks
• Pressure release
– Removal of overlying rock allows expansion
and fracturing
• Plant growth
– Growing roots widen fractures
• Burrowing animals
• Thermal cycling
– Large temperature changes fracture rocks by
repeated expansion and contraction
Mechanical Weathering
Mechanical weathering-contd.
Mechanical weathering-contd.
Surface Area & Weathering
Frost Wedging
CHEMICAL WEATHERING (LARGELY CONTROLLED
BY CLIMATE)
*ROLE OF WATER
*DISSOLUTION
1. WATER DISSOLVES HALITE AND GYPSUM
2. CARBONIC ACID DISSOLVES LIMESTONE
(CO2+H2O----HCO3)
i. CALCIUM CYCLE
ii. ACID RAIN
pH Scale
Climate weathering
3.OXIDATION (REACTION OF CERTAIN CHEMICALS WITH O2)
i.
IRON OXIDES
ii.
COPPER OXIDES
4. HYDROLYSIS (REPLACEMENT OF MAJOR POSITIVE IONS
WITH PROTONS) OF POTASSIUM FELDSPAR INTO
i) CLAY: BECOMES PART OF SOIL
ii) SILICIC ACID: CEMENTS SEDIMENTS OR FORMS
ANIMAL SHELLS AND SKELETONS
iii) POTASSIUM IONS: PROVIDE PLANT NUTRIENTS
Spheroidal weathering
FACTORS THAT INFLUENCE CHEMICAL
WEATHERING
1.CLIMATE
i) MOISTURE
ii) HEAT
iii) VEGETATION
2. LIVING ORGANISMS
3. TIME
Mineral composition
MINERAL COMPOSITION: RELATIONSHIP BETWEEN A MINERAL’S
TEMPERATURE AND PRESSURE OF CRYSTALLIZATION AND ITS
SUSCEPTIBILITY TO WEATHERING (HIGH TEMP CRYSTALLIZATION----LESS STABLE AND EASILY WEATHERED-EXAMPLE: OLIVINE &
PYROXENE)
REGOLITH: LOOSE, FRAGMENTED MATERIAL THAT COVERS MUCH OF
THE EARTH’S SURFACE
SOIL: UPPERMOST ORGANIC-RICH PORTION OF THE REGOLITH
Rounded Boulder
D.SOME PRODUCTS OF CHEMICAL WEATHERING:
a) CLAY MINERAL
i) KAOLINITE
ii) SMECTITE
iii) PRACTICAL USES FOR CLAYS
b) METAL ORES
i)
FORMATION OF BAUXITE
ii)
OTHER ORES
Soil
• Soil - a layer of weathered, unconsolidated
material on top of bedrock
– Common soil constituents:
•
•
•
•
Clay minerals
Quartz
Water
Organic matter
• Soil horizons
– O horizon - uppermost layer; organic material
– A horizon - dark layer rich in humus, organic acids
– E horizon - zone of leaching; fine-grained components
removed by percolating water
– B horizon - zone of accumulation; clays and iron
oxides leached down from above
– A horizon - partially weathered bedrock
Bedrock composition on soil.
Soils and Climate
• Soil thickness and composition are
greatly affected by climate
– Wet climates:
• More chemical weathering and thicker soils
• Soils in moderately wet climates tend to have
significant clay-rich layers, which may be solid
enough to form a hardpan
– Arid climates:
• Less chemical weathering and thinner soils
• Subsurface evaporation leads to build-up of salts
• Calcite-rich accumulation zones may form,
cementing soil together into a hardpan
– Extremely wet climates (e.g., tropical rainforest)
• Highly leached and unproductive soils (laterites)
• Most nutrients come from thick O/A horizons
Vegetation and soil development
Typical Mature soil
CLASSIFYING SOILS
1. OLD CLASSIFICATION SYSTEM
  PEDALFERS
  PEDOCALS
  LATERITES
2. MODERN CLASSIFICATION SYSTEM
a) BASED ON MANY PHYSIAL & CHEMICAL CHARACTERISTICS
b) EXAMPLES OF SOIL TYPES
1) ENTISOL
2) VERTISOL
3) OXISOL
4) ULTISOL
3. PALEOSOLS (“OLD SOILS”)
Typical Mature Soil
SUMMARY – CHAPTER 5:
• DEFINITION OF WEATHERING AND
EROSION-DIFFERENCE BETWEEN THEM
• MECHANICAL WEATHERING
• WEATHERING BY THERMAL EXPANSION &
CONTRACTION
• MOST IMPORTANT FACTOR AFFECTING CHEMICAL
WEATHERING
• OXIDATION
• HYDROLYSIS
• MINERAL THAT IS MOST SUSCEPTIBLE TO
WEATHERING
• WET/WARM VS DRY/COLD
• MINERAL’S STABILITY FOR WEATHERING
• FORMATION OF CLAY MINERALS
• REGOLITH AND SOIL
• DIFFERENT SOIL HORIZONS