Metamorphic Rocks Amphibolite

Amphibolite is a type of metamorphic rock that is rich in minerals of the amphibole group, of which the most common mineral is hornblende. Identification of this type of rock helps earth scientists to reconstruct the region’s geologic history based on  the conditions under which amphibolite forms.

Whether originally igneous or sedimentary in origin, any rock will undergo changes to the minerals present and the relationships between mineral grains during long exposure to extreme environments. Geologists call these changes “metamorphism,” and classify the changed rocks as one of three major division of rocks, the metamorphic rocks. Even a metamorphic rock can be re-metamorphosed, so the “parent rock” of a metamorphic rock can have been any kind of rock.

Metamorphism of rocks always involves heat, either from burial deep in the Earth’s crust or from a nearby magma (melted rock) body. Rocks that have been altered by a nearby heat source without significant pressure are termed contact metamorphic rocks. The most common metamorphic rocks, however, are those altered by regional metamorphism under extremes of heat and pressure. As “regional” suggests, this activity takes place over large areas. The most common sites of regional metamorphism are along zones of collision between tectonic plates. Such collisions create immense compressional forces in rocks buried so deep beneath the surface that they are exposed to the heat of the mantle.

Regional metamorphism’s combination of pressure and heat causes slow changes in the rocks. While the heat does not reach temperatures high enough for melting, it is enough to increase chemical activity in the minerals. Over millions of years, the atoms in some minerals recombine to form new minerals, and in others, existing minerals enlarge or recrystallize. During metamorphism, newly-created and altered mineral grains tend to align themselves perpendicular to the direction of the pressure. The resulting rocks have a recognizable texture; layers of mineral grains that are all oriented in the same direction. Finding this texture, which is called foliation, indicates that the rock sample in question has been created by regional metamorphism.

Regionally metamorphosed foliated rocks are assigned names based on the size of the mineral grains. From finest to coarsest, the rocks are termed phyllite, slate, schist, and gneiss. Because the parent rock, also called “protolith,” may have had almost any texture and mineralogy, those names do not provide much information about the conditions of metamorphism. Scientists also examine the minerals that have formed in during metamorphism to determine the temperature and pressure under which it occurred. As the level of metamorphism becomes more severe, new minerals form that can be used to determine the range of pressure and temperature at the rock’s formation. This approach has allowed scientists to identify metamorphic facies, based on mineralogy, that have been formed in certain conditions. One set of regional metamorphic facies include greenschist, blueschist, amphibolite, and granulite.

Amphibolites form under high temperature (500-700 degrees C) and high pressure (3-8 kilobars, or about 45,000 to 115,000 pounds per square inch). The dominant minerals present in amphibolites are plagioclase (calcium-sodium) feldspar and amphiboles, mostly hornblende but may include actinolite, tremolite, cummingtonite and crocidolite. Although regionally metamorphosed rocks are generally foliated, the foliation of amphibolites is often weak. Despite foliation that is difficult to distinguish, the unusual mineralogy – almost entirely amphiboles and plagioclase, with minor quartz – is diagnostic of amphibolite.

The original definition of the metamorphic facies that include amphibolite was based on study of metamorphosed basalts and other iron- and magnesium-rich protoliths, mainly in oceanic crust. The facies has also been observed, however, to result from metamorphism of mixed sediments such as limy shales and shaly limestones. Geologists typically use a different set of metamorphic facies for rocks whose protoliths are shaly (pelitic). These facies are defined by the presence of  index minerals including (from lowest to highest metamorphic grade) chlorite, biotite, garnet, staurolite, kyanite and sillimanite. Amphibolites correspond, in general, to pelitic rocks metamorphosed in the range of garnet through sillimanite, including schists and gneisses.

Amphibolites are found worldwide, generally in the cores of mountain ranges created at collision zones involving oceanic crust. In North America, amphibolite deposits are known in both the Appalachian Mountain chain, the west coast and Ontario. Amphibolite has historically been a prime source of asbestos, which comes from fibrous amphiboles including tremolite, crocidolite and actinolite. Amphibole asbestos minerals are responsible for mesothelioma, a particularly virulent form of lung cancer.

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