Faulted metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; Rt. 169 roadcut just east of Ely, Minnesota, USA) 2 : 無料・フリー素材/写真
Faulted metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; Rt. 169 roadcut just east of Ely, Minnesota, USA) 2 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Faulted metamorphosed pillow basalt in the Precambrian of Minnesota, USA.This outcrop is a glaciated surface of 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 rock 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.The outcrop was eroded and smoothed by glaciers during the Pleistocene Ice Age.The left-to-right fracture in the middle of the photo is a small fault that has noticeably offset the pillow margins.Geologic unit & age: upper member of the Ely Greenstone, Neoarchean, ~2.722 GaLocality: outcrop on the northern side of Rt. 169, 1.3 miles east of the Rt. 1-Rt. 169 intersection, just east of the town of Ely, northeastern St. Louis County, northeastern Minnesota, USA (47° 54' 41.26" North, 91° 48' 58.70" 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. |
| 撮影日 | 2015-06-10 08:09:14 |
| 撮影者 | James St. John |
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