File:Metamorphosed pillow basalt (Ely Greenstone, Neoarchean, ~2.722 Ga; large loose block at Ely visitor center, Ely, Minnesota, USA) 8 (21453463175).jpg

Metamorphosed pillow basalt in the Precambrian of Minnesota, USA. (geology hammer for scale)

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.

The smoothed surface of individual pillows was produced by glacial erosion 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 grounds of the Ely visitor center, immediately southwest of the Rt. 1-Rt. 169 intersection in the eastern part of the town of Ely, northeastern St. Louis County, northeastern Minnesota, USA (47° 54' 10.61" North, 91° 50' 30.75" 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.