Amphibolite_(Archean,_3.1-3.2_Ga;_Norris_South_roadcut,_Madison_County,_Montana,_USA)_1_(45574881922).jpg
Summary
| Description |
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) |
| Date | |
| 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. |