Metamorphism - Metamorphism

Japanese: 変成作用 - へんせいさよう

The reorganization of rock texture and mineral composition within the Earth's crust to suit the physicochemical conditions at that location. The physicochemical conditions include temperature, pressure, pressure of water and carbon dioxide in the rock, and deformational forces acting on the rock, with temperature having the greatest effect. Sedimentary rocks are usually reorganized at higher temperatures than when they were formed, and igneous rocks at lower temperatures.

[Mitsuo Hashimoto]

Conditions and classification of metamorphism

There are various geological conditions that cause metamorphism. One is when the temperature of the surrounding rocks rises due to the intrusion of igneous magma, and metamorphism occurs as a result. This is called contact metamorphism. In this case, the dominant factor is the rise in temperature, so it is also called thermal metamorphism. On the other hand, when the temperature rises over a wide area in the crust, regardless of the intrusion of igneous magma, rocks over a wide area are metamorphosed and become metamorphic rocks. In this case, it is called regional metamorphism. Regional metamorphism generally occurs with crustal movement, and is often accompanied not only by a rise in temperature but also by deformation. Furthermore, as sedimentation progresses, when the strata and the volcanic rocks between them are buried underground, they may be reconstructed under the influence of a general geothermal gradient. In this case, it is called burial metamorphism. In burial metamorphism, the temperature does not rise very high, and there is no significant deformation.

(1) Contact metamorphism As mentioned above, when magma intrudes into a part of the earth's crust, the heat from the intrusion raises the temperature of the surrounding rocks, causing reactions between existing rock-forming minerals, resulting in the formation of mineral combinations that are stable at high temperatures. This is usually not accompanied by deformation, and the resulting rocks with new mineral composition do not have special structures such as schistosity, but rather show a structureless texture. This type of metamorphic rock is called hornfels. During contact metamorphism, the pressure is not high compared to the temperature rise, so the rock-forming minerals in hornfels do not contain high-pressure minerals. Furthermore, when surrounding rocks are absorbed into the magma and placed under extremely high-temperature conditions, some of them melt and minerals that are stable at high temperatures are formed. This is called high-temperature metamorphism or pyrometamorphism.

(2) Regional metamorphism This is a phenomenon in which metamorphic rocks are formed over a vast area spanning hundreds of kilometers, and is one of the important elements of mountain building. In this case, the rocks are often deformed, and the regional metamorphic rocks that are formed become crystalline schist or gneiss with a pronounced directional texture, i.e., foliation or banded structure. Regional metamorphism is classified into several types, such as high-pressure/low-temperature type, medium-pressure type, and low-pressure/high-temperature type, depending on the ratio of temperature and pressure. In high-pressure/low-temperature regional metamorphism, high-pressure minerals such as glaucophanite, lawsonite, and jadeite are formed. This type of metamorphism is also said to occur in plate subduction zones.

(3) Burial metamorphism This is a metamorphic process that occurs when layers of rocks or rocks are simply buried, without any significant geological changes such as the intrusion of igneous magma or mountain building. Because the temperature and pressure are not very high and no deformation occurs, the reconstitution of the rocks is not complete, and the original structure and minerals often remain. The minerals that are formed are stable at low temperatures, such as zeolite and prehnite.

When various metamorphic processes occur, the chemical composition of the rock as a whole does not usually change significantly. However, water ( _H2O ) tends to be more abundant when the temperature of metamorphism is low and less abundant when the temperature is high, and the amount of water generally changes with metamorphism. Sometimes the chemical composition of the rock as a whole changes significantly, which in turn affects the mineral composition. This is called metasomatism. Metasomatism is often seen in limestone layers that have been intruded by magma, and can also result in the formation of various mineral deposits, that is, aggregates of useful ore minerals.

The reorganization of rocks by metamorphism generally occurs in a solid state. In other words, the reactions that occur between rock-forming minerals during metamorphism are solid-state reactions. This is inferred from the fact that metamorphic rocks often inherit and preserve the texture of the original sedimentary or igneous rocks. In other words, metamorphic rocks retain the texture of sedimentary or igneous rocks, and only the minerals have changed.

[Mitsuo Hashimoto]

Metamorphic facies

Metamorphism occurs in a temperature range higher than the formation temperature of sedimentary rocks and lower than the temperature of magma. This range is from about 100°C to about 700°C. Meanwhile, pressures can range from almost normal pressure to more than 10,000 atmospheres. This wide range of temperature and pressure is divided and classified into several regions, each of which is called a metamorphic facies (or simply phase). The metamorphic facies are named after the characteristic rocks or minerals that form under those conditions, as follows:

(1) Zeolite facies: The lowest temperature and pressure conditions. Conditions for burial metamorphism.

(2) Prehnite-pumpellyite facies: Slightly higher temperature than the zeolite facies.

(3) Greenschist facies: When temperatures are even higher, this is the minimum temperature condition for regional metamorphism.

(4) Amphibolite facies The most important of the metamorphic facies. The conditions under which amphibolite and gneiss are formed.

(5) Glaucophane Schist facies: When the pressure is high relative to the temperature, high-pressure minerals are produced.

(6) Epidote amphibolite facies: intermediate conditions between (3), (4) and (5).

(7) Granulite facies: The highest temperature conditions for regional metamorphism. Close to the temperature of magma.

(8) Eclogite facies: Extremely high temperature and pressure conditions, such as those found in the lower crust and upper mantle.

(9) Pyroxene-hornfels phase: When the temperature is high relative to the pressure. This is the high-temperature condition of contact metamorphism.

(10) Sanidinite phase: A condition where the pressure is close to normal but the temperature is extremely high, such as in xenoliths in lava. Part of the rock may melt.

[Mitsuo Hashimoto]

"Metamorphic Rocks and Metamorphic Belts" by Miyakonojo Akiho (1966, Iwanami Shoten)" ▽ "Metamorphic Rocks of Japan" by Hashimoto Mitsuo (1987, Iwanami Shoten)" ▽ "Illustrated Earth Sciences" edited by Sugimura Arata, Nakamura Yasuo, and Ida Yoshiaki (1988, Iwanami Shoten)" ▽ "Metamorphism" by Miyakonojo Akiho (1994, Iwanami Shoten)" ▽ "Iwanami Lecture Series on Earth and Planetary Sciences 9: Evolution of the Crust" by Taira Asahiko, Joken, Shikazono Naotake, Hiroi Yoshikuni, and Kimura Manabu (1997, Iwanami Shoten)" ▽ "Dynamics of Rock Formation" by Sakano Shohei, Toriumi Mitsuhiro, Obata Masaaki, and Nishiyama Tadao (2000, University of Tokyo Press)" ▽ "An Introduction to Petrology, Volumes 1 and 2, by Shuto Kenji and Oyamauchi Yasuto (2002, Kyoritsu Shuppan)"

Temperature and pressure conditions of various metamorphic facies
A diagram of estimated temperature and pressure conditions for various metamorphic facies (slightly modified from F. J. Turner's diagram). Outside the dashed lines are areas where metamorphism does not actually occur. Each metamorphic facies is surrounded by curved lines and separated to show that the range of conditions is not definitive but is merely estimated. ©Shogakukan ">

Temperature and pressure conditions of various metamorphic facies

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地殻の内部で、岩石の組織と鉱物組成とが、その場所での物理化学的条件に適合するように再構成されること。ここで物理化学的条件とは、温度、圧力、岩石中の水や炭酸ガスなどの圧力、岩石に働く変形作用などで、なかでも温度の効果が大きい。堆積(たいせき)岩はそれが生成したときよりも高い温度条件で、火成岩は低い温度で再構成されるのが普通である。

［橋本光男］

変成作用の条件と分類

変成作用がおこる地質学的条件にはいろいろな場合がある。一つは、火成岩マグマの貫入に伴って、その周辺の岩石の温度が上昇し、そのために変成作用がおこる場合で、それを接触変成作用という。この場合、支配的要因はもっぱら温度の上昇であるため、熱変成作用ということもある。一方、火成岩マグマの貫入とは直接関係なく、地殻内で広域にわたって温度が上昇すると、広い範囲の岩石が変成作用を受け変成岩になる。このような場合は広域変成作用とよばれる。広域変成作用は地殻変動に伴っておこるのが一般で、温度上昇のみならず、変形作用を伴うことが多い。さらに、堆積作用の進行に伴って、地層やその間に挟まれる火山岩が地下に埋没されていくと、それらは一般的な地温勾配(こうばい)の影響の下に再構成されることがある。このような場合は埋没変成作用という。埋没変成作用では温度はあまり高くならず、変形作用も著しくない。

(1)接触変成作用　前述のように、地殻の一部にマグマが貫入すると、その熱のために周囲の岩石の温度が上昇し、既存の造岩鉱物の間に反応がおこり、その結果、高い温度で安定な鉱物の組合せが生ずる。この際、変形作用は伴わないことが普通で、そのため生成する新しい鉱物組成の岩石は、片理のような特殊な構造をもつことなく、無構造ともいうべき組織を示す。このような変成岩をホルンフェルスという。接触変成作用のときには、温度上昇に比して圧力は高くないので、ホルンフェルスの造岩鉱物には、高圧鉱物は含まれない。さらに、マグマの中に周囲の岩石が取り込まれ、著しい高温条件に置かれると、一部が溶けたり高温で安定な鉱物ができる。このような場合は高温変成作用またはパイロ変成作用という。

(2)広域変成作用　これは何百キロメートルにも及ぶ広大な地域に変成岩が生成する現象で、造山運動の重要な要素の一つである。この場合には、岩石に変形作用が働くことが多く、そのため、生成する広域変成岩は顕著な方向性組織、すなわち片理や縞状(しまじょう)構造をもつ結晶片岩や片麻(へんま)岩になる。広域変成作用では温度とともに圧力も重要な要因であって、温度と圧力の比によって高圧低温型、中圧型、および低圧高温型など、いくつかの場合に分けられる。高圧低温型広域変成作用では、藍閃(らんせん)石、ローソン石、ひすい輝石などの高圧鉱物が生成する。また、この種の変成作用は、プレートの沈み込み帯に生ずるといわれている。

(3)埋没変成作用　火成岩マグマの貫入や造山運動などのような著しい地変に伴うことなく、地層や岩石の単なる埋没によっておこる変成作用。温度も圧力もあまり高くならず、変形作用も働かないので、岩石の再構成は完全でなく、もとの組織や鉱物が残っていることが多い。生成する鉱物も、沸石類やぶどう石のように、低温で安定なものである。

　いろいろな変成作用を受けるとき、岩石全体の化学組成は著しく変化しないのが普通である。もっとも、水H₂Oは、変成作用の温度が低いときには多く、高ければ少なくなる傾向があり、一般に水の量は変成作用に伴って変化する。ときには岩石全体の化学組成も著しく変化し、そのため鉱物組成もその影響を受けることがある。そのような場合をとくに交代作用という。交代作用はマグマの貫入を受けた石灰岩層などによくみられ、またそれに伴っていろいろな鉱床、つまり有用鉱石鉱物の集合体のできることもある。

　変成作用による岩石の再構成は、一般に固体状態のままでおこる。すなわち、変成作用のときに造岩鉱物間でおこる反応は、固体反応である。そのことは、変成岩がしばしば、もとの堆積岩や火成岩の組織を受け継ぎ、保存していることから推察される。いいかえると、変成岩は堆積岩や火成岩の組織を保ち、鉱物だけが変化したものともいえる。

［橋本光男］

変成相

変成作用は、堆積岩の生成温度よりも高く、マグマの温度よりも低い温度範囲でおこる。それはほぼ100℃から700℃ぐらいにわたる。一方、圧力はほとんど常圧から1万気圧以上に及ぶこともある。このように広い温度圧力範囲は、いくつかに分割、分類され、それぞれは変成相（または単に相）とよばれている。変成相は、その条件下で生成する特徴的な岩石あるいは鉱物の名前をとって、次のように名づけられている。

(1)沸石相　温度も圧力ももっとも低い場合。埋没変成作用の条件。

(2)ぶどう石パンペリー石相　沸石相よりもやや高温。

(3)緑色片岩相　温度がさらに高い場合。広域変成作用の最低温度条件。

(4)角閃岩相　変成相のなかでももっとも重要なもの。角閃岩や片麻岩のできる条件。

(5)藍閃石片岩相　温度に対して圧力の高い場合。高圧鉱物を生ずる。

(6)緑簾(りょくれん)石角閃岩相　(3)(4)および(5)の中間的条件の場合。

(7)グラニュライト相　広域変成作用の最高温度条件。マグマの温度に近い。

(8)エクロジャイト相　温度も圧力もきわめて高い場合。地殻下部や上部マントルの条件。

(9)輝石ホルンフェルス相　圧力に比して温度の高い場合。接触変成作用の高温部の条件。

(10)サニディナイト相　溶岩中の捕獲岩のように、圧力は常圧に近いが、温度がきわめて高い条件。岩石の一部は溶融することがある。

［橋本光男］

『都城秋穂著『変成岩と変成帯』（1966・岩波書店）』▽『橋本光男著『日本の変成岩』（1987・岩波書店）』▽『杉村新・中村保夫・井田喜明編『図説地球科学』（1988・岩波書店）』▽『都城秋穂著『変成作用』（1994・岩波書店）』▽『平朝彦・徐垣・鹿園直建・広井美邦・木村学著『岩波講座地球惑星科学9　地殻の進化』（1997・岩波書店）』▽『坂野昇平・鳥海光弘・小畑正明・西山忠男著『岩石形成のダイナミクス』（2000・東京大学出版会）』▽『周藤賢治・小山内康人著『岩石学概論』上下（2002・共立出版）』