Core Physical Rocks &weathering 3.2 Weathering and rocks

Weathering and Rocks

• Weathering is the decomposition and disintegration of rocks in their original location.
• Decomposition is chemical weathering that alters rock substances (e.g., granite to kaolinite).
• Disintegration is mechanical weathering that produces smaller fragments of the same rock.
• Biological weathering involves plants and animals altering and breaking rocks.
• The three types of weathering (decomposition, disintegration and biological) are interrelated.
• Weathering breaks down rock, facilitating erosion and transport, and is central to landscape evolution.
• Many minerals were formed under high pressure and temperature, then they become more stable as they cool
• Weathering causes irreversible changes in rocks, such as changing solid rock to fragmented or pliable states.
• Weathering can alter a rock's volume, density, grain size, surface area, permeability, consolidation, and strength.
• Weathering can form new minerals and solutions.
• Some minerals, like quartz, resist weathering.
• Minerals and salts can be removed, transported, concentrated, or consolidated by weathering.
• Weathering prepares rocks for later erosion and transport and new landforms and features are produced.

Physical/Mechanical Weathering

• Four main types: freeze-thaw, salt crystal growth, disintegration, and pressure release.
• Operates at or near the Earth's surface, where temperature changes are frequent.
• Freeze-thaw occurs when water in joints/cracks freezes, expanding by ~10%, exerting pressure, with a max of 2100 kg/cm² at -22°C (average is 14 kg/cm²).
• Most effective where moisture is plentiful and temps fluctuate above/below freezing (periglacial and alpine regions).
• Freeze-thaw is faster when combined with pressure release and salt crystallization.
• Salt crystallization decomposes rock using salt solutions, with two main types.
• In areas with temperatures around 26-28°C, sodium sulfate and sodium carbonate expand by ~300%, creating pressure, which causes cracks.
• When water evaporates, salt crystals are left behind.
• As the temperature rises, the salts expand and exert pressure on rock.
• Both are frequent in hot deserts with low rainfall/high temperatures, and polar areas with snowflakes.
• The most effective salts are sodium sulphate, magnesium sulphate and calcium chloride.
• Chalk decomposes the fastest, followed by limestone, sandstone, and shale.
• The rate of disintegration of rocks is closely related to porosity and permeability.
• Surface texture and grain size control the rate of rock breakdown.
• Salt crystallization is more effective than insolation weathering, hydration, or freeze-thaw
• A combination of freeze-thaw and salt crystallization produces the highest rates of breakdown.
• Heating and cooling may cause disintegration in hot deserts with large diurnal temperature ranges.
• Rocks heat up by day and contract by night; stress occurs only in outer layers due to poor heat conduction, causing peeling or exfoliation.
• Moisture is essential for this to happen; otherwise, temperature change alone will not cause breakdown
• The expansion of salts like sodium, calcium, potassium, and magnesium has been linked with exfoliation.
• Pressure release occurs when overlying rocks are removed by erosion, causing underlying rocks to expand and fracture parallel to the surface.
• This unloading of pressure causes cracks or joints to form at right-angles to the surface
• Vegetation roots may also physically break down rocks.

Chemical Weathering

• Water is the key medium for chemical weathering.
• Unlike mechanical weathering, chemical weathering is most effective sub-surface since percolating water has gained organic acids from the soil and vegetation
• Acidic water helps to break down rocks such as chalk, limestone and granite.
• Weathering takes place above the water table, since weathered material accumulates in the water and saturates it
• Three main types: carbonation-solution, hydrolysis, and hydration.
• Carbonation-solution occurs on rocks with calcium carbonate.
• Rainfall mixes with dissolved carbon dioxide or organic acid to form carbonic acid (CO2 + H2O → H2CO3).
• Calcium carbonate reacts with carbonic acid to form soluble calcium bicarbonate (CaCO3 + H2CO3 → Ca(HCO3)2).
• Solution depends on pH; for example, iron is highly soluble when pH is 4.5 or less.
• Hydrolysis occurs on rocks with orthoclase feldspar (e.g., granite).
• Feldspar reacts with acid water, forming kaolin, silicic acid, and potassium hydroxyl.
• Acid and hydroxyl are removed in the solution, leaving kaolin behind as the end product.
• Quartz and mica remain in the kaolin.
• Hydrolysis also involves solution as the potassium hydroxyl is carbonated and removed in solution.
• Hydration absorbs water, causing expansion and change (e.g., anhydrite to gypsum).
• Mechanical stresses accompany hydration
• Shales and mudstones can increase in volume up to 1600% when clay minerals absorb water.

Controls of Weathering

• The type and rate of weathering relies on climate, even though it's hard to find a direct relationship
• Peltier's diagrams (1950) illustrate weathering's relation to moisture and average temperature.
• Frost-shattering increases with more freeze-thaw cycles, while chemical weathering strengthens with increased moisture and heat.
• The rate of chemical weathering increases 2-3 times for every increase in temperature of 10 °C (up to a max of 60 °C) - called Van't Hoff's Law
• The efficiency of freeze-thaw, salt crystallisation and insolation weathering is influenced by critical temperature changes, frequency of cycles, diurnal and seasonal variations in temperature.
• Rock type influences weathering rate and type via chemical composition, cement nature, and joints/bedding planes.
• Limestone (calcium carbonate) is susceptible to carbonation-solution. Granite (feldspar) is prone to hydrolysis.
• Iron-oxide cements are prone to oxidation, whereas quartz cements are very resistant.
• Rock structure (folding, faulting, joint patterns, grain size) influences water movement and differential resistance.
• Coarse-grained rocks weather quickly owing to a large void space and high permeability.
• Fine-grained rocks offer a greater surface area for weathering and may be highly susceptible to weathering.
• Goldich (1938) emphasized the importance of individual minerals.
• Rocks formed of resistant minerals (quartz, muscovite, feldspar) resist weathering; weaker minerals weather rapidly.

Vegetation

• Vegetation's influence is linked to climate and soil, where moisture content, root depth, and humus acidity influence weathering rate/nature.
• Vegetation weathers rocks by secreting organic acids (chemical) and physically breaking them with root growth.
• Soil depth affects weathering; soils can protect rocks or increase breakdown due to vegetation support.

Relief

• Weathered material must be removed for weathering to continue.
• Intermediate slope angles may produce most weathering.
• Aspect is important if temperature differences are around a critical temperature (e.g., 0°C for freeze-thaw).