File:Metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; large glacial erratic along Main Street, Ely, Minnesota, USA) 27.jpg

English: Metamorphosed pillow basalt in the Precambrian of Minnesota, USA.

The boulder shown above is a large glacial erratic 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 specimen 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.

This boulder was eroded from Ely Greenstone bedrock by glaciers during the Pleistocene Ice Age.

Geologic unit & age: upper member of the Ely Greenstone, Neoarchean, ~2.722 Ga

Locality: glacial erratic on display on the northern side of Main Street in the northern part of the town of Ely, northeastern St. Louis County, northeastern Minnesota, USA (47° 54' 25.64" North, 91° 50' 49.29" West)

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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.