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
Mechanical Weathering
Mechanical weathering-contd.
Mechanical weathering-contd.
Surface Area & Weathering
Frost Wedging
CHEMICAL WEATHERING (LARGELY CONTROLLED BY CLIMATE)
*ROLE OF WATER
*DISSOLUTION
• WATER DISSOLVES HALITE AND GYPSUM
• CARBONIC ACID DISSOLVES LIMESTONE (CO2+H2O----HCO3)
i.
ii.
CALCIUM CYCLE
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
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
III SOILS AND SOIL FORMATION:
•
•
•
•
1. PARENT MATERIAL
a) MINERAL CONTENT DETERMINES SOIL NUTRIENTS AND RATES
OF FORMATION
b) DIFFERENT SOILS
2. CLIMATE
•
•
•
3. TOPOGRAPHY
i) AVAILABILITY OF WATER
ii) RATE OF SOIL ACCUMULATION
•
•
•
4. VEGETATION
i) ORGANIC MATTER
ii) O2 AND CO2
iii) H+ IONS
5. TIME
C. TYPICAL SOIL STRUCTURE
Effect of landscape on soil development
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
IV. WEATHERING IN EXTRA TERRESTRIAL ENVIRONMENTS
i) MOON: MECHANICAL WEATHERING CAUSED BY IMPACT OF
ANCIENT METEORITES – NO CHEMICAL WEATHERING (NO O2
OR CO2)
ii) VENUS: MECHANICAL WEATHERING CAUSED BY THERMAL
EXPANSION AND CONTRACTION AND EXFOLIATION AND HIGH
WINDS
iii) MARS: ANCIENT CHEMICAL WEATHERING BY OXIDATION
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