Lecture 4: Weathering - The Breakdown of Rocks

Core Definitions and Concepts of Weathering

• Definition of Weathering:     * Weathering involves the group of destructive processes that break down and loosen the surface minerals of rock.     * It encompasses both physical disintegration (mechanical) and chemical decomposition.     * In-situ Process: A critical characteristic of weathering is that it involves no movement of material (unlike erosion).
• Weathering Environment: This takes place primarily at the Earth's surface and near-surface fractures through the interaction of multiple spheres:     * Lithosphere     * Atmosphere     * Hydrosphere     * Biosphere     * Cryosphere
• Comparison of Conditions (Subsurface vs. Surface):     * Subsurface Conditions:         * Relatively high temperatures but constant at which minerals reach equilibrium.         * High confining pressure (stress).         * Little or no water.         * Absence of oxygen.     * Surface Conditions:         * Relatively low temperatures and highly variable.         * Little or no confining pressure (stress).         * Abundant water.         * Abundant oxygen.

Weathering and the Rock Cycle

• The Rock Cycle Framework: Weathering is a fundamental geologic process in the rock cycle, which describes transitions between sedimentary, metamorphic, and igneous rocks over geologic time.
• Equilibrium and Transformation:     * Rocks are altered when forced out of their equilibrium conditions.     * Driving forces include plate tectonics and the water cycle.     * Example: An igneous rock like basalt may break down and dissolve when exposed to the atmosphere, or melt when subducted under a continent.
• Progression of Breakdown:     * Big rocks weather into smaller rocks.     * Smaller rocks weather into the constitutive minerals.     * Primary minerals weather to form secondary minerals.
• Geological Sequence: Rocks undergo weathering and erosion, followed by deposition, and finally lithification and diagenesis to become sedimentary rocks.

Mechanical (Physical) Weathering Processes

• Definition: The breakdown of rocks into smaller particles through physical processes without changing the chemical composition; minerals remain intact.
• Agents: Water, ice, acids, salts, plants, animals, and temperature fluctuations.
• Key Mechanical Processes:     * Frost Wedging: Water enters cracks and expands by $9\%$ upon freezing. This is most significant in environments where freeze-thaw cycles occur frequently.     * Mechanical Exfoliation (Sheeting): Also known as Pressure-Release. Caused by the unroofing of deep-seated rocks (like granite) via erosion. This results in the formation of sheet joints, joint sets, and exfoliation domes.     * Thermal Expansion and Contraction: Differential expansion and contraction caused by temperature changes lead to fracturing along existing joints due to thermal stress.     * Abrasion: The mechanical breaking of rocks or minerals by friction against other rocks or impacts during transport. Media include water, ice, and wind (e.g., wind charged with sand scouring rock faces or glacial abrasion producing striae).     * Salt Crystal Growth (Precipitation of Crystals): Salts precipitating from water in rock crevices/cracks exert force that widens opening.
• Surface Area Increase: Mechanical weathering increases the total surface area available for chemical attack.     * $1$ block ($2\,m \times 2\,m \times 2\,m$) has a surface area of $24\,m^2$.     * Breaking it into $8$ cubes ($1\,m$ per side) increases area to $48\,m^2$.     * Breaking it into $64$ cubes ($0.5\,m$ per side) increases area to $96\,m^2$.

Chemical Weathering Processes

• Definition: The decomposition of rocks where the internal structure of minerals is destroyed and new minerals are created to reach equilibrium with surface conditions.
• Variables Affecting Rates:     * Temperature.     * Surface Area (fragmentation increases rate).     * Availability of water or natural acids.
• The Role of Acids: Rainwater becomes acidic by picking up atmospheric and organic materials.     * Carbon Dioxide ($CO_2$) yields Carbonic Acid ($H_2CO_3$).     * Sulfur Oxides yield Sulfuric Acid.     * Nitrogen Oxides yield Nitric Acid.
• Acidity of Natural Waters (pH Scale):     * Neutral pH is $7$. Battery acid is pH $1$, while Lye is pH $14$.     * Natural rain has a pH around $5.5$; acid rain is below $5$.     * Most natural waters range from pH $4$ to $9$.
• Goldich Stability Series: This is the inverse of Bowen's Reaction Series.     * Olivine ($(Mg,Fe)_2SiO_4$) is the least stable and weathers very quickly.     * Quartz ($SiO_4$) is the most resistant and is a major constituent of mature sediments.
• Specific Chemical Reactions:     * Dissolution: Ionic bonds dissolve in water.         * Example: Halite ($NaCl$) and Gypsum ($CaSO_4 \cdot 2H_2O$).         * Limestone dissolution: $2(HCl) + CaCO_3 \rightarrow Ca^{2+} + 2Cl^- + H_2O + CO_2$.     * Carbonization: Reaction between carbonic acid and minerals (addition of $CO_2$).         * Reaction: $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$.         * Limestone Carbonization: $CaCO_3 + H_2CO_3 \rightarrow Ca^{2+} + 2(HCO_3^-)$.     * Oxidation: Chemical combination with oxygen (addition of $O_2$). Affects iron and sulfur most easily.         * Iron-oxide colors: Red, yellow, orange, brownish (e.g., limonite, hematite, goethite).         * Copper oxidation produces green and blue minerals.         * Magnetite to Hematite: $2Fe_3O_4 + 0.5 O_2 = 3Fe_2O_3$.         * Pyrite to Limonite: $FeS_2 + nO_2 + mH_2O \rightarrow Fe_2O_3 \cdot nH_2O$.     * Hydration: Absorption/addition of water to form new minerals.         * Anhydrite to Gypsum: $CaSO_4 + 2H_2O \rightarrow CaSO_4 \cdot 2H_2O$.         * Hematite to Limonite: $Fe_2O_3 + nH_2O \rightarrow Fe_2O_3 \cdot nH_2O$.     * Hydrolysis: Combined action of water and $CO_2$; replacement of major positive ions with protons ($H^+$).         * Feldspar weathering leaches $Na^+, Ca^{2+}, \text{ and } K^+$ ions, leaving behind clay minerals.         * Potassium Feldspar to Kaolinite: $2H^+ + H_2O + 2KAlSi_3O_8 \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_{2(aq)} + 2K^+$.

Biological Weathering

• Definition: The weakening and disintegration of rock by plants, animals, and microbes.
• Biological-Mechanical:     * Plant Roots: Grow into existing cracks and expand them.     * Burrowing Animals: Move rock fragments to the surface.
• Biological-Chemical:     * Lichens (Symbiosis of fungi and algae): Fungi release chemicals to break down minerals; algae consume them. This creates holes/gaps in rock.     * Microbial Activity: Bacteria alter chemical composition, making rocks more susceptible to other weathering agents.     * Organic Acids: Produced by roots or microorganisms to dissolve minerals.

Factors Influencing Weathering Rates

• Rock Composition: Minerals like Calcite weather quickly via dissolution; Quartz is highly resistant. Mafic rocks (ferromagnesian) weather more easily than felsic ones.
• Geological Structure: Distribution and frequency of joints. Closely spaced joints (centimeters apart) weather faster than massive rocks.
• Climate:     * Tropics (Warm/Wet): Chemical weathering is dominant and fast; mechanical weathering is less important.     * Arctic/Deserts (Cold/Dry): Mechanical weathering is prevalent; chemical weathering is slow.
• Topography/Relief: Steep relief aids physical weathering (gravity) but slows chemical weathering. Gentle slopes promote soil development and chemical weathering.
• Vegetation: Dense vegetation increases water retention and organic acid production, promoting weathering. However, if removed, rocks become vulnerable to abrasion.
• Tectonic Setting: Young, rising mountains weather rapidly, primarily via mechanical processes.
• Time: Exposure duration (longer exposure equals more weathering).

Products of Weathering

• Clastic Particles: Clay, silt, sand, gravel.     * Clays: Hydrous alumino-silicates (e.g., Kaolinite, Smectite, Montmorillonite, Illite).     * 1:1 Layered Clays: Kaolinite.     * 2:1 Layered Clays: Smectite/Montmorillonite, Illite, Vermiculite, Chlorite.
• Secondary Minerals:     * Oxides/Hydroxides: Limonite, Gibbsite, Hematite.     * Clay minerals formed by recombination of silica, alumina, and metal cations.
• Ions in Solution: $Na^+, K^+, Ca^{2+}, Mg^{2+}, \text{ and } HCO_3^-$; these are transported to groundwater, streams, and oceans.
• Metal Ores: Concentration of minerals like Bauxite or Gold.
• Geologic Layers:     * Regolith: The layer of loose, heterogeneous superficial material covering solid rock.     * Saprolite: Chemically weathered rock layering.     * Soil Profile: Includes horizons (O-horizon plant debris, A-horizon humus, E-horizon leaching, B-horizon accumulation of clays/oxides).
• Landforms:     * Spheroidal Weathering: Rounding of sharp edges into spherical shapes.     * Differential Weathering: Inconsistent weathering rates where resistant rocks (e.g., Sandstone) stand out against less resistant rocks (e.g., Shale).

Assignment Information (Assignment 4)

• Task: Identify and photograph weathering in the surroundings.
• Requirements: Describe the location and type (chemical, physical, biological) for at least 3 locations per type.
• Format: Teams of 2 permitted.
• Deadline: January 9, 2023, before midnight.