File:Metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; Rt. 169 roadcut just west of Ely, Minnesota, USA) 9 (21536183561).jpg
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Metamorphosed pillow basalt in the Precambrian of Minnesota, USA. This is an exposure of the Ely Greenstone, a distinctive and famous unit in northeastern Minnesota. Greenstone is a metamorphic rock typically dominated by the minerals plagioclase feldspar and chlorite, plus often actinolite and epidote. Before metamorphism, greenstone was basalt or gabbro - both are mafic igneous rocks. The above rocks used to be basalt - it was part of an underwater lava flow. The bulbous structures seen in this outcrop are pillows, which are diagnostic of underwater basaltic lava flow eruptions. Pillow basalts can be observed forming underwater in today's oceans at basaltic volcanic sites such as Hawaii. Geologic unit & age: upper member of the Ely Greenstone, Neoarchean, ~2.722 Ga Locality: roadcut on the northern side of Rt. 169, 1.8 miles west of the Rt. 1-Rt. 169 intersection, just west of the town of Ely, northeastern St. Louis County, northeastern Minnesota, USA (47° 54' 01.33" North, 91° 52' 42.23" West) References on the Ely Greenstone: Jahn & Murthy (1975) - Rb-Sr ages of the Archean rocks from the Vermilion District, northeastern Minnesota. Geochimica et Cosmochimica Acta 39: 1679-1689. Jirsa et al. (2010) - Geologic map of Minnesota, Precambrian bedrock geology, description of map units. Minnesota Geological Survey State Map Series S-22. |
| Date | |
| Source | Metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; Rt. 169 roadcut just west of Ely, Minnesota, USA) 9 |
| 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@N05/21536183561 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 00:16, 10 October 2019 | | 4,000 × 3,000 (4.21 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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