<P> Within the weathering environment chemical oxidation of a variety of metals occurs . The most commonly observed is the oxidation of Fe (iron) and combination with oxygen and water to form Fe hydroxides and oxides such as goethite, limonite, and hematite . This gives the affected rocks a reddish - brown coloration on the surface which crumbles easily and weakens the rock . This process is better known as' rusting', though it is distinct from the rusting of metallic iron . Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, such as chalcopyrites or CuFeS oxidizing to copper hydroxide and iron oxides . </P> <P> A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. the effect of moss growing on roofs is classed as weathering . Mineral weathering can also be initiated or accelerated by soil microorganisms . Lichens on rocks are thought to increase chemical weathering rates . For example, an experimental study on hornblende granite in New Jersey, USA, demonstrated a 3x--4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces . </P> <P> The most common forms of biological weathering are the release of chelating compounds (i.e. organic acids, siderophores) and of acidifying molecules (i.e. protons, organic acids) by plants so as to break down aluminium and iron containing compounds in the soils beneath them . Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering . Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils . </P> <P> The symbiotic mycorrhizal fungi associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition . It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients . To date a large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces or to weather minerals, and for some of them a plant growth promoting effect was demonstrated . The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as the production of weathering agents, such as protons, organic acids and chelating molecules . </P>

Which of the following is not a type of mechanical (physical) weathering