Bedrock Base Map Location 1

The rocks seen here are what remains from ancient oceanic crust that formed from lava flows erupted on the sea floor of present-day Marquette, Michigan approximately 2.7 billion years ago. The curved pillow-like structures formed from erupted lava rapidly being cooled by sea water. Their green color indicates that the rocks, which were originally basalt, were subjected to elevated pressure and temperature which in turn caused the formation of green minerals such as chlorite, actinolite, and epidote. The rocks are typically folded into linear structures called greenstone belts which are known for hosting silver, copper, and zinc. Older greenstone belts from the Archean (2.5 to 4 billion years old) may also host rich gold deposits.

Greenstone Metabasalt

Bedrock near Marquette, Michigan, Upper Peninsula | Neoarchean | 2.7 billion years old