What physical processes contribute to the weathering of rock and why might you expect mafic igneous rocks to weather at a faster rate than felsic igneous rocks?
Weathering is the physical disintegration and chemical decomposition of rocks into soil, loose clasts, dissolved chemical components (ions), and solid chemical residues. Rocks are most stable at their formation conditions: the temperature, pressure, and other environmental conditions (water and air) that were present at the time the rock crystallized or lithified. Rocks that formed at the surface are at equilibrium with surface conditions, and are more resistant to weathering. Rocks that formed far underground are not at equilibrium with surface conditions, and are more prone to weathering.
Physical weathering is a “slow, unspectacular process”(Prothero & Schwab, Sedimentary Geology) resulting in changing solid, large rock into smaller, movable unconsolidated debris. These processes are driven by changes in temperature, changes in pressure, and organic activity.
When the ambient temperature oscillates near 0°C, water will repeatedly freeze into ice, melt, and re-freeze. As ice has a larger volume than water, water in joints or fractures will expand as it freezes, wedging the fractures into larger cracks. During melting, water will fill the new, larger crack, forcing it even larger during the next freeze. This freeze-thaw weathering is most effective in rocks with lots of fractures, in moist climates hovering near 0°C. The timescale of the temperature variation (hours, days, weeks, or months) will impact the rate of oscillation between freeze and thaw, with faster oscillations producing faster weathering. In hotter climates, salt crystallization of evaporating brine can produce similar effects.
When the ambient temperature shows significant daily variation (±20-30°C), the thermal contraction and expansion of the component minerals can break the rocks apart. Rocks subject to repeated hydration (thus swelling) and desiccation (thus contracting) will be subject to similar stresses. This insolation weathering produces identical weathering products to freeze-thaw weathering.
As a rock is unburied, the removal of overburden drops the confining pressure on the rock. This stress release results in expansion of the rock, producing expansion cracks or joints roughly parallel to the ground surface. Over time, these develop into curved, subparallel cracks between onion-like sheets on the rocks. As the curved slabs are spalled off, the exfoliation produces roughly spheroidal cores.
Finally, lifeforms that eek out a living on or in rocks can pry apart cracks, ingest mineral components, or borrow through material.
Physical weathering is fastest areas of high relief, in regions with temperature variation around the freeze-thaw temperature (0°C), or in regions with extreme daily temperature variation. Rocks with lots of existing fissures or joints weather more rapidly as various physical processes expand existing cracks.
Chemical weathering is the process of dissolving rocks completely, or dissolving some components resulting in new, altered minerals.
Simple solution is when a solid mineral is exposed to water or an acid, dissolving some ions into solution. Halite exposed to water will dissolve completely into sodium and chlorine ions. Calcite exposed to acid will dissolve, resulting in surface pitting or even entire cave networks (karst terrain). Hydrolysis is when a mineral with mobile cations is exposed to hydrogen ions (H+), resulted in a dissolved or altered rock. Calcite exposed to acid is a form of hydrolysis where no solid residue remains; other examples include feldspars altering to kaolinite clays or olivine dissolving into magnesium ions and dissolved silica.
Hydration of rocks occurs when a mineral is exposed to water and altered into a hydrated variant of the mineral. Dehydration is the inverse process, drying out hydrated minerals into dehydrated equivalents. Examples include gypsum dehydrating into anhydrite, the iron oxide hematite hydrating into the corrosive rust limonite, or kaolinite clay hydrating into gibbsite clay and dissolved ions.
Finally, oxidation and reduction are the paired processes of oxygen losing and gaining electrons when a rock is exposed to the atmosphere. During oxidation, rusted materials are produced: pyroxene exposed to air and water results in limonite and dissolved silica, pyrite exposed to air produces hematite and dissolved sulfur.
All these processes can be linked together into changes: pyrite oxidizing into hematite, which in turn hydrates into limonite, or feldspar decomposing into first kaolinite and later gibbsite clay with exposure to water and hydrogen ions. Chemical reactions are also reversible in favourable conditions providing all the components are still present; it is only once the reaction products are transported away that the rock can be considered permanently altered.
Chemical weathering is fastest in warm, wet climates (like the tropics). Hot, dry climates lack the water necessary for hydrolysis, simple solution, or hydration, while water is frozen into ice in colder climates. The abundance of hydrogen ions (acids) increases hydrolysis: more acidic waters occur in waterlogged soils, bogs, mine water, and rain, while more alkaline waters occur in saline water (oceans), groundwater, or streams. Areas with good drainage remove reaction products faster, preventing re-reaction, while areas of high relief may remove debris through physical weathering before much chemical decomposition has occurred. Minerals with finer-grained textures or lots of fissures have more grain surface area per unit volume, increasing the area for chemical reactions to take place.
Areas with high relief, high elevation, or at high latitudes are more likely to be subject to faster rates of physical weathering than chemical weathering, while areas of low relief, low elevation, and low latitudes are more likely to be subject to faster chemical weathering than physical weathering. Physical weathering will dominate in the arctic, in mountain environments, or in deserts, while chemical weathering will dominate in swamps, the tropics, and depositional basins.
Reaction Rates for Igneous Rocks
In terms of chemical composition, mafic rocks will have a higher concentration of heavier magnesium- and iron-rich minerals: pyroxene, olivine, amphibole, biotite mica, and plagioclase feldspar. Felsic rocks have a higher concentration of lighter silica- and oxygen-rich minerals: quartz, muscovite mica, and orthoclase feldspar. Because rocks are most stable under their formation conditions, volcanic igneous rocks that formed at the surface will be the most resistant to weathering than their plutonic equivalents. Likewise, minerals with higher solidification temperatures are more prone to chemical weathering than minerals with lower solidification temperatures. Olivine and pyroxene solidify at high temperatures, so mafic minerals decompose through chemical weathering more rapidly than felsic minerals.