English: Weathered greenstone with pillow lava structures. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

The moderately large, irregularly-rounded areas in this rock are "pillows", which occur in volcanic lava flows that erupted underwater (usually the ocean). Pillow lavas have been directly observed in modern times, forming on the shallow seafloor off Hawaii.

The rock itself is weathered greenstone, a low- to intermediate-grade metamorphic rock formed by cooking basalt or gabbro. Greenstones usually have a finely to coarsely-crystalline texture, but can be weakly foliated. Typical greenstones are composed of the minerals plagioclase feldspar, chlorite, and epidote.

The provenance of this specimen is uncertain, but may have come from the Upper Peninsula of Michigan. If so, it is almost certainly from the Mona Formation, a very thick, very old, Neoarchean-aged, volcanics-dominated unit. It outcrops in the Marquette-Ishpeming area of northern Michigan. The Lower Member of the Mona Formation is well known to have metamorphosed pillow basalts - the morphology of the pillows is well-preserved.

(www.flickr.com/photos/jsjgeology/sets/72157632271616114) (www.flickr.com/photos/jsjgeology/albums/72157632829506131)

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