Ultrabasic rock

Japanese: 超塩基性岩 - ちょうえんきせいがん（英語表記）ultrabasic rock

Ultramafic rock is also called ultramafic rock. A general term for igneous rocks containing less than 45% silicon dioxide by weight. They are composed mainly of mafic minerals and usually have a color index of 70 or more. Peridotite and picrite are typical examples of this type of rock, and are usually found at the bottom of relatively large intrusive rocks such as sills or lava flows, and transition to mafic rocks toward the top of the rock body. These rocks are thought to have been formed when mafic minerals that crystallized early during the solidification of magma settled and accumulated under the influence of gravity. Peridotite and serpentine, which are found in orogenic belts, are often thought to have intruded in a solid or semi-solid state. Some are the result of pre-existing mafic rocks undergoing metasomatic metamorphism accompanied by desilicification.

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

Japanese:

超苦鉄質岩 ultramafic rockともいう。塩基性岩のうち二酸化ケイ素の含有量が重量百分率で 45%以下である火成岩の総称。主として苦鉄質鉱物から成り，色指数は普通 70以上である。橄欖岩やピクライトなどはその代表的岩石で，通常比較的大規模な岩床などの迸入岩体あるいは溶岩流の底部に見出され，岩体の上部に向って塩基性岩に移化する。これらは，マグマの固結するとき，早期に晶出した苦鉄質鉱物が重力の場で沈降集積してできた岩石と考えられている。造山帯に産出する橄欖岩や蛇紋岩などは固態ないし半固態の状態で貫入したものが多いと考えられている。既存の塩基性岩が脱ケイ酸作用を伴う交代変成を受けた結果生成したものもある。

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