Chapter Five Weathering The Breakdown of Rocks CHAPTER

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.

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 (CO 2+H 2 O----HCO 3) i. CALCIUM CYCLE ii. ACID RAIN

p. H Scale

Climate weathering

3. OXIDATION (REACTION OF CERTAIN CHEMICALS WITH O 2) 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