Weathering; Verwitterung

Japanese: 風化作用 - ふうかさよう（英語表記）Weathering; Verwitterung

The phenomenon in which the surface of the earth's crust is destroyed or deteriorated due to the action of water, air, etc. on the surface.
(1) Physical weathering: The phenomenon in which rocks on the surface of the crust are cracked or broken down by wind, water, etc. When water seeps in and expands and contracts repeatedly due to the temperature difference between day and night, onion-shaped cracks appear (onion weathering). In ice crystal growth, the crystallization pressure reaches a maximum (2t/ ^cm2 ) at -20°C, which is greater than the strength of the rock, so when groundwater freezes during ice ages, the rocks undergo freeze fracture. Near the surface (up to a depth of about 100m), horizontal cracks form due to frost heaving where the confining pressure is stronger than the strength of the rock, and below that, when the crystallization pressure is stronger than the strength of the rock, the rocks break into pieces several tens of centimeters in size and become lumps. In areas where glaciers have developed, frost cracks are not very noticeable, but in areas where glaciers have not developed, such as Japan, frost fracture of rocks is significant. When these reach the surface, they form rock-glacier fields (Felsenmeer) and rock-glacier flows (Felsenstrom).
(2) Chemical weathering: A process in which carbon dioxide ( _CO2 ) from the air dissolves in groundwater from the surface of the earth, causing rocks to react and change, and the entire groundwater-storing area reaches equilibrium. The surface of the earth is acidic (pH = 4.7), dissolving silica from silicate minerals, and metal ions such as potassium (K) and sodium (Na) are leached and move downward into the groundwater-storing area. The groundwater changes from neutral to slightly alkaline (pH = 8-9), and clay minerals and hydroxides are deposited according to the pH value. The altered area of the entire groundwater-storing area is called a weathering crust, and in granite, dacite, etc., the following altered mineral zones are formed from the surface to deep underground.

Surface	Red Masa	Gibbsite (Al ₂ O ₃ ~Al(OH) ₃ ), Hematite (Fe ₂ O ₃ ~Fe(OH ₃ ))
	White Masa	Kaolin, halloysite Al ₄ 〔(OH) ₈ · Si ₄ O ₁₀ 〕 · 4(H ₂ O) ₄
		Onita [(Fe,Mn)(OH) ₈ ]
	Yellow Masa	Montmorillonite (AL,Mg) ₂ _〔 (OH) _2.Si4O10 _〕 _.Na0.3 ( _H2O ) ₄
	Weak weathering zone	Clinoptilolite [(Na,K) ₂ Al ₂ Si ₇ O ₁₈・6(H ₂ O)]
		Opal ( _SiO2 )
Deep underground	Unweathered granite

Weathering reactions are progressive and irreversible, so if the temperature and _CO2 concentration are high, the reaction proceeds, but if the temperature and CO2 concentration are high, the reaction stops and the reaction remains metastable. The large-scale weathering crust (deep weathering crust) currently seen on the earth's surface, which is 200 m thick, was formed during the tropical rainforest climate 15 to 10 million years ago, and weathering reactions will not proceed unless the climatic potential exceeds this. For example, the thick Masa (masa) that develops in granite areas in Japan is a weathering crust that was formed in a hot and rainy climate 15 to 10 million years ago, and weathering will not proceed under current climatic conditions. If the temperature and _CO2 concentration become higher than when this weathering crust was formed and the weathering potential increases, weathering will proceed.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

地表の水，空気などの働きで，地殻表面が破壊・劣化する現象。
(1) 物理的風化作用　地殻表面の岩石が風，水などの働きで割れたり細粒化したりする現象。水がしみこみ，日中と夜間の温度差により膨張収縮を繰り返すとタマネギ状に割れ目が入る（タマネギ状風化）。氷の結晶成長では，－20℃で結晶圧が最大（2t/cm²）となり，岩石強度より大きくなるため，氷河期などに地下水が凍結すると岩石は凍結破砕を起こす。地表付近（深さ 100mくらいまで）で，封圧が岩石強度より強いところでは凍上現象により水平な割れ目ができ，それ以下で結晶圧が岩石強度より強い場合，岩石は数十cm大に割れ，塊状となる。氷河が発達した地域には凍結割れ目はあまり顕著ではないが，日本など氷河が発達しなかった地域では岩石の凍結破砕が著しい。これらが地表に出ると，岩塊原 Felsenmeerや岩塊流 Felsenstrom; rock-glacierができる。
(2) 化学的風化作用　地表から空気中の二酸化炭素（CO₂）が地下水に溶け，岩石が反応して変質し，地下水滞水域全体が平衡に達するプロセス。地表付近は酸性（pH＝4.7）で，ケイ酸塩鉱物からシリカを溶かし，カリウム（K），ナトリウム（Na）などの金属イオンが溶脱して地下水滞水域下方に移動する。地下水は中性から弱アルカリ性（pH＝8～9）に変化し，その pH値に応じた粘度鉱物・水酸化物が沈積する。地下水滞水域全体の変質した部分を風化殻 Weathering crustというが，花崗岩，デイサイトなどでは，地表から地下深部にかけておおよそ次のような変質鉱物帯ができる。

地表	赤マサ	ギブス石〔Al₂O₃～Al(0H)₃〕，赤鉄鉱〔Fe₂O₃～Fe(OH₃)〕
	白マサ	カオリン，ハロイサイト Al₄〔(OH)₈・Si₄O₁₀〕・4(H₂O)₄
		オニイタ〔(Fe,Mn)(OH)₈〕
	黄マサ	モンモリロナイト(AL,Mg)₂〔(OH)₂・Si₄O₁₀〕・Na_0.3(H₂O)₄
	弱風化帯	斜プチロル沸石〔(Na,K)₂Al₂Si₇O₁₈・6(H₂O)〕
		オパール(SiO₂)
地下深部	未風化花崗岩

風化の反応は増進的，非可逆的であるから，気温や CO₂濃度が高ければ反応は進むが，低温，乾燥の状態では反応は停止し，準安定 metastable状態にある。現在地表でみられる厚さ 200mもある大規模風化殻（深層風化殻）は，1500万～1000万年前の熱帯雨林気候当時にできたもので，気候的ポテンシャルがこれをこえないかぎり，風化の反応は進行しない。たとえば日本の花崗岩地帯に発達する厚いマサ（真砂）は，1500万～1000万年前の高温多雨気候下にできた風化殻で，現在の気候条件では風化は進行しない。この風化殻ができたときより気温や CO₂濃度が高くなり，風化のポテンシャルが上がれば，風化は進行することになる。

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