Weathering

Key Takeaways

• Physical weathering (also called mechanical weathering or disintegration) breaks intact rock into smaller fragments without changing the chemical composition of the minerals, through processes including frost wedging (ice forming in fractures expands by approximately 9 percent in volume upon freezing, exerting tensile stress on the fracture walls that progressively widens and extends fractures until the rock splits; this is the dominant weathering mechanism in cold climates with alternating freeze-thaw cycles, producing angular scree and talus slopes at the bases of mountain cliffs), thermal expansion and contraction (daily and seasonal temperature cycles cause the rock surface to expand and contract, generating fatigue stress that eventually causes spalling and exfoliation, particularly significant in desert environments with large diurnal temperature swings), salt weathering (evaporation of saline water in pore spaces and fractures allows salt crystals to precipitate and grow, generating the same wedging stresses as ice crystals and being particularly destructive to building stones and coastal outcrops in arid or semi-arid climates), and pressure release (removal of overlying rock by erosion reduces confining pressure on the underlying rock, allowing it to expand and develop sheeting joints parallel to the exposed surface, characteristic of granitic domes such as Half Dome in Yosemite and Stone Mountain in Georgia).
• Chemical weathering alters the mineral composition of rock through reactions with water, oxygen, carbon dioxide, and organic acids that convert the unstable primary silicate and carbonate minerals into more stable secondary minerals under surface conditions: hydrolysis (the reaction of silicate minerals with water to produce clay minerals and dissolved silica and cations) is the most important chemical weathering reaction for igneous rocks, with feldspar hydrolysis producing kaolinite clay plus dissolved potassium, sodium, or calcium, and pyroxene and olivine hydrolysis producing smectite clay plus dissolved magnesium and iron; oxidation (the reaction of iron-bearing minerals with atmospheric oxygen to form iron oxides (limonite, goethite, hematite)) produces the characteristic orange and red coloring of deeply weathered tropical soils (laterites and bauxites) and of iron-rich sedimentary rock weathering products; dissolution (the chemical reaction of carbonate minerals (limestone, dolomite, marble) with carbonic acid (H2CO3, formed by CO2 dissolving in rainwater) to produce soluble bicarbonate ions, creating the characteristic karst landscape of sinkholes, caves, and disappearing streams in carbonate-dominated regions); carbonation (CO2 dissolving in water and reacting with silicate minerals to form carbonate minerals, the reverse of the chemical reaction by which silicate weathering consumes atmospheric CO2 and represents one of the major long-term carbon sinks in the global carbon cycle).
• Seismic weathering correction (also called statics correction or elevation statics) compensates for the variable travel-time delays caused by the heterogeneous near-surface weathered layer before seismic data is processed for subsurface structure and amplitude interpretation: the static correction computes the time shift that must be applied to each seismic trace to move it to a common reference datum (the floating datum or sea level) at the velocity of the consolidated rock beneath the weathered layer, effectively removing the time delay accumulated in the low-velocity weathered layer and the topographic elevation variation between shot points and receiver locations; the static correction is computed from uphole surveys (direct measurement of velocity in the near-surface by recording the arrival time of a shot in a nearby shallow borehole from surface geophones), refraction statics (recording the first arrivals of refracted waves from the base of the weathered layer on wide-spread surface arrays and inverting for the weathered layer thickness and velocity), and surface-consistent statics (estimating the shot and receiver static components from the traveltime residuals on normal moveout-corrected common midpoint gathers); residual statics (small remaining time shifts after the long-wavelength statics have been removed) are corrected by surface-consistent residual statics algorithms that maximize the stack power by aligning the reflection events within each CMP gather.
• Weathering profiles in petroleum reservoir and source rock assessment provide evidence of past tectonic and climatic history that affects both rock quality and petroleum system parameters: a thick laterite or bauxite weathering profile at a major unconformity surface indicates a long period of tropical subaerial exposure (typically millions of years) with hot and humid climate that leached soluble minerals and concentrated insoluble residue, relevant because such profiles indicate significant missing section (the time represented by the laterite profile is not recorded in the sedimentary section), are associated with secondary porosity development in the underlying carbonate or siliciclastic rock (the same acidic weathering solutions that formed the laterite also dissolved carbonate cements and feldspar grains in the underlying rock), and may represent the paleogeographic high where reservoir sands were absent (the eroded section) while adjacent structural lows accumulated the maximum sedimentary thickness; regolith (the loose, chemically altered surface material produced by in-situ weathering without erosion and transport) may have sufficient porosity and permeability to constitute a reservoir rock in unconventional plays (basement weathering plays in West Africa, China, and India produce oil from fractured and weathered granite and metamorphic basement reservoirs).
• Biological weathering encompasses all weathering processes mediated by living organisms, including root wedging (plant roots penetrating fractures and exerting hydraulic pressure that widens and extends fractures mechanically, one of the most effective weathering agents in humid temperate climates with dense vegetation), organic acid production (roots and decomposing organic matter produce carbonic acid, humic acid, and other organic acids that accelerate mineral dissolution, particularly of carbonates and feldspars), lichen activity (lichens on rock surfaces produce oxalic acid and other compounds that etch and dissolve mineral surfaces, initiating chemical weathering in otherwise bare rock environments such as arctic or desert outcrops), and microbial weathering (bacteria and fungi in soil and rock pores produce acids and chelating compounds that dissolve minerals and release nutrients, playing a significant role in the long-term chemical breakdown of silicate rocks that the geological carbon cycle models must include to correctly predict atmospheric CO2 concentrations over millions of years).