2012年11月10日托福阅读真题解析_第3页

　　第三篇：WEATHERING OF ROCK

　　第三篇讲岩石风化。

　　段落很多所以就一起讲了好了。首先总起是列举了两种风化方式：化学上的和生物上的。文章主讲了化学上的，化学上的有三种，水侵蚀、二氧化碳侵蚀、氧气侵蚀（翻译很不专业就表嘲笑啦）。水侵蚀主要说的是水与岩石中物质接合使得其膨胀等等；二氧化碳侵蚀就是列举了类似钟乳石那种因潮湿环境融入二氧化碳而慢慢风化侵蚀的现象（化学的涉及到碳酸盐遇碳酸根生成可溶的碳酸氢盐，然后流失），干燥环境下不是很明显；氧气侵蚀主要发生在阳光照射强烈的地方，氧化后呈红色，多发生于热带。（这里有题大概是考推测热带的土多为红色的啥的）最后作者也说了些生物侵蚀的例子，强调的是它可能没有想象中的作用那么大，因为其中也包含着化学侵蚀部分。而且生物侵蚀有利于土壤的生成，本身是对植物好的。文章末尾讲了一个真菌，通过从岩石中提取矿物质，使岩石最终崩碎呈小块状以及另外一种方式（忘了）来进行生物侵蚀。（生物影响涉及到植物的根，不过这一点作用被过分关注了；讲了lichen，这种生物对石头的侵蚀既有生物作用又有化学作用）

　　Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind and gravity.

　　Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals （also known as biological weathering） in the breakdown of rocks, soils and minerals.

　　The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types （as in glacial, aeolian or alluvial sediments） often makes more fertile soil. In addition many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition.

　　【Chemical weathering】Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation and hydrolysis are most important.

　　The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca++ and other minerals into surface waters.[6]

　　Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

　　Some minerals, due to their natural solubility （e.g. evaporites）， oxidation potential （iron-rich minerals, such as pyrite）， or instability relative to surficial conditions （see Goldich dissolution series） will weather through dissolution naturally, even without acidic water.

　　One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weakcarbonic acid which reacts with calcium carbonate （the limestone） and forms calcium bicarbonate. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas.[citation needed] Carbonation is therefore a large feature of glacial weathering.

　　The reactions as follows:

　　CO2 + H2O => H2CO3

　　carbon dioxide + water => carbonic acid

　　H2CO3 + CaCO3 =>Ca（HCO3）2

　　carbonic acid + calcium carbonate => calcium bicarbonate

　　Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement. This process is most effective along the joints, widening and deepening them.

　　【水合作用】Hydration

　　Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.

　　Hydrolysis on silicates and carbonates：Hydrolysis is a chemical weathering process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals.

　　【氧化】Oxidation

　　Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ （iron） and combination with oxygen and water to form Fe3+ 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 CuFeS2 oxidizing to copper hydroxide and iron oxides.

　　【生物风化】Biological weathering

　　A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. moss on roofs is classed as weathering. Mineral weathering can also be initiated and/or accelerated by soil microorganisms.

　　【lichen例子】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.

　　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.

　　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.[8] It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients.[9] To date a large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces and/or to weather minerals, and for some of them a plant growth promoting effect was demonstrated.[10] 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.

　　Source：http://en.wikipedia.org/wiki/Weathering#Chemical_weathering

　　词汇题：

　　Tangible sequence subsequently prominent withstand mature modestly

　　life history and population dynamics.