File:Amphibolite (Archean, 3.1-3.2 Ga; Norris South roadcut, Madison County, Montana, USA) 1 (45574881922).jpg

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Weathered amphibolite from the Precambrian of Montana, USA.

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Amphibolite is a dark-colored, amphibole-dominated metamorphic rock. It is usually weakly foliated but can be crystalline-textured. Amphibolites are principally composed of hornblende amphibole and plagioclase feldspar. Many amphibolites form by metamorphism of basalt or gabbro (= mafic igneous rocks).

Age: Archean, metamorphic date probably 3.1-3.2 Ga (= age of peak metamorphism during intrusion of Spanish Peaks area trondhjemites and tonalites)

Locality: roadcut on the eastern side of Route 287, Burnt Creek-Bradley Creek Valley, south of the town of Norris, northern Madison County, southwestern Montana, USA (45° 31' 29.67" North latitude, 111° 41' 50.54" West longitude)
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Source Amphibolite (Archean, 3.1-3.2 Ga; Norris South roadcut, Madison County, Montana, USA) 1
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/45574881922 (archive). It was reviewed on 6 November 2018 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

6 November 2018

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current04:41, 6 November 2018Thumbnail for version as of 04:41, 6 November 20184,000 × 2,489 (8.41 MB)Lu Brito (talk | contribs)Transferred from Flickr via #flickr2commons

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