Precambrian Greenstone

Precambrian Ely Greenstone, Lake Michigan's Left Coast,, Elizabeth G Fagan

Precambrian Greenstone

The Canadian Shield constitutes the largest mass of exposed Precambrian rock on the face of the Earth. Extending into Minnesota and Wisconsin, the geological formation is a collage of rock categories. One type is teal-hued Ely Greenstone, named for its source in northeastern Minnesota. It is one of the oldest rock types on Earth.

Ely Greenstone dates from the Precambrian Age some 2.722 billion years ago. Yes that’s billion. It is a type of of Banded Iron Formation (BIF) that formed when metamorphosed basalt oozed onto the ocean floor into bulbous forms or “pillows.”

Iron-rich Ely Greenstone has BIF’s characteristic “banding,” in most cases rusty red or pearly white. It is fine-grained and heavy. The slab shown here weighs 19.8 pounds (8.981 kg). Its origin as “pillow lava” gives this piece an odd, serpentine shape.

Great Lakes rock hounds are particularly grateful for the glaciers that dumped myriad rocks and fossils in their wake. On the beach at Lake Michigan’s West Coast, the gorgeous deep teal of wet Greenstone stands out among the millions of plain limestone rocks.

We can’t wrap our minds around the deep time 2.722 billion years represents. Holding in your hands something so old is a transcendent, mystical experience. It reminds us that humans are a only a smudge on Earth’s timeline.

Elizabeth G Fagan,, Lake Michigan's Left Coast
Elizabeth G Fagan,, Lake Michigan’s Left Coast

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