Further Reading

Cefrey, Holly. Metamorphic Rocks. New York: The Rosen

Publishing Group, 2003. Kornprobst, Jacques. Metamorphic Rocks and Their Geo-dynamic Significance: A Petrological Handbook. New York: Springer, 2002. Moores, Eldridge M., and Robert Twiss. Tectonics. New

York: W. H. Freeman, 1995. skinner, B. J., and B. J. Porter. The Dynamic Earth: An Introduction to Physical Geology. 5th ed. New York: John Wiley & Sons, 2004. Stille, Darlene R., Lynn S. Fichter, Terrence E. Young, Jr., and Rosemary G. Palmer. Metamorphic Rocks: Recycled Rock. New York; Compass Point Books, 2008.

metasomatic The metamorphic process of changing a rock's composition or mineralogy by the gradual replacement of one component by another through the movement and reaction of fluids and gases in the pore spaces of a rock is called metasomatism, and these processes are metasomatic. Meta-somatic processes are responsible for the formation of many ore deposits, which have extraordinarily concentrated abundances of some elements. They may also play a role in the replacement of some limestones by silica, and the formation of dolostones.

Many metasomatic rocks and ore deposits are formed in hydrothermal circulation systems that are set up around igneous intrusions. When magmas, particularly large batholiths, intrude country rocks, they set up a large thermal gradient between the hot magma and the cool country rock. Any water above the pluton gets heated and rises toward the surface, and water from the sides of the pluton moves in to replace that water. A hydrothermal circulation system is thus set up, and the continuous movement of hot waters in such systems often leach elements from some rocks and from the pluton and deposit them in other places, in metasomatic processes.

See also metamorphism and metamorphic rocks.

meteor, meteorite Meteors are rocky objects from space, such as asteroids or smaller rocky objects called meteoroids, that enter the Earth's atmosphere. When meteors pass through Earth's atmosphere, they get heated and their surfaces become ionized, causing them to glow brightly, forming a streak moving across the atmosphere known as a shooting star or fireball. Most meteors burn up before they hit the surface of the Earth, but those that do make it to the surface are then called meteorites.

At certain times of the year, the Earth passes through parts of our solar system that are rich in meteors, and the night skies become filled with shooting stars and fireballs, sometimes as frequently as several per minute. These high-frequency meteor encounters, known as meteor showers, include the Perseid showers that appear around August 11 and the Leonid showers that appear about November 14 (both occur annually).

Many small meteorites have hit the Earth throughout time. Eyewitness accounts describe many meteors streaking across the sky with some landing on the surface. Fragments of meteorites are regularly recovered from the Antarctic ice sheets, where rocky objects on the surface could have come only from space. Although meteors may appear as flaming objects moving across the night skies, they are generally cold icy bodies when they land on Earth, and only their outermost layers get heated from the deep freeze of space during their short transit through the atmosphere.

Meteorites consist of several different main types. Stony meteorites include chondrites, which are very primitive, ancient meteorites made of silicate minerals, like those common in the Earth's crust and mantle, but chondrites contain small spherical object known as chondrules. These chondrules contain frozen droplets of material thought to be remnants of

Iron meteorite from Odessa, Ukraine (Astrid & HannsFrieder Michler/Photo Researchers, Inc.)

the early solar nebula from which the Earth and other planets initially condensed. Achondrites are similar to chondrites in mineralogy, except they do not contain the chondritic spheres. Iron meteorites are made of an iron-nickel alloy with textures that suggest they formed from slow crystallization inside a large asteroid or small planet that has since been broken into billions of small pieces, probably by an impact with another object. Stony-irons are meteorites that contain mixtures of stony and iron components and probably formed near the core-mantle boundary of the broken planet or asteroid. Almost all meteorites found on Earth are stony varieties.

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