File:Greenstone (metamorphosed pillow basalts) (Mona Formation, Neoarchean, 2.7 Ga; Rt. 41 Rt. 28 roadcut west of Marquette, Michigan, USA) 2 (8281288751).jpg

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Greenstones (metamorphosed pillow basalts) in the Precambrian of Michigan, USA. (geology hammer for scale)

The outcrop shown above is part of the Mona Formation (a.k.a. Mona Schist), an extremely thick, metamorphosed, heterolithic unit of Neoarchean age in the Marquette-Ishpeming area of Michigan's Upper Peninsula. The original rocks making up the Mona are inferred to be, and in many cases are demonstrably, volcanic rocks.

This outcrop consists of greenstones of the Lower Member of the Mona Formation - these rocks were originally pillow basalts. Despite being multiply metamorphosed and subjected to several orogenies in geologic history, the geometry and morphology of the pillows are well preserved. Pillow structures in basalt lavas are diagnostic of subaqueous/seafloor eruptions.

Stratigraphy: Lower Member, Mona Formation, Neoarchean, ~2.7 Ga (= metamorphic age)

Locality: glaciated bedrock knob above roadcut on the northern side of Rt. 41/Rt. 28, just west of the Rt. 502 intersection, west of the town of Marquette & east of the town of Negaunee, Upper Peninsula of Michigan, USA
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Source Greenstone (metamorphosed pillow basalts) (Mona Formation, Neoarchean, 2.7 Ga; Rt. 41/Rt. 28 roadcut west of Marquette, Michigan, USA) 2
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@N05/8281288751 (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.

10 October 2019

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current00:15, 10 October 2019Thumbnail for version as of 00:15, 10 October 2019960 × 720 (189 KB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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