Posted on Woodsperson blog Aug. 17, 2013
Reprinted in part with permission.
NORTHERN WISCONSIN -- In our previous post on the presence of asbestiform minerals in the Penokee range, we promised to "follow the rumor" that the mineral had been found at the mine site. Lots of phone calls to good people like Tom Evans and Esther Stewart of the Wisconsin Geological and Natural History Survey (WGNHS) plus interviews with concerned geologists, mining engineers and attorneys completed the story.*
We’ll come back to the rumor later. But first, I found that a lot of people, including myself, did not fully understand the geological reasons behind the occurrence of asbestos in the Penokee rock. So, briefly, here’s how it got there.
A Little Geology
The Gogebic Range resulted when tremendous forces broke the miles-thick crust of the earth and thrust our part up at a 60 degree angle. On Lake Road near Mellen, we can see a cross-section of this standing-on-end earth crust starting on the south with a small outcrop of some of the oldest rock on earth -- the first stable crust -- some 2.5 billion years old. Going north we see layers of rock that previously had accumulated at the bottom of a shallow sea -- the Palms formation. At a very sharply defined point the rock turns magnetic -- the Ironwood formation. A story, which could be another complete post, is that if you look at this slow accumulation of silt as kind of a clock of the life history of the earth, this instant where non-magnetic rock becomes magnetic rock is the exact time that modern life forms began on earth.
In this photo the dark lines are "mafic" (lava -- probably gabro) intrusions into the lighter colored, fractured host material, a sedimentary rock of some kind -- not iron rich in this case. The arrowhead shaped feature at the end of the squiggly intrusion into a crack has some larger crystals which could be some asbestos-like material. (Photo © and courtesy Woodsperson blog)
The awful forces, strong enough to rend the crust of the earth, not only lifted the base to a 60 degree angle but, as you might imagine, shattered the layers of archaen crust and their overlaying layers of sedimentary (silty) rock. Cracks and crevices opened up and the underlying molten rock from lower in the crust oozed up in between the layers of shattered surface rock. This incredibly hot lava reacted with the iron and silicon and other elements in the rock to create metamorphosed new minerals. Some of those minerals cooled slow enough to form long or spiral or snowflake crystals of very high strength and high bulk density -- asbestos.
The geology lesson ends here. The result of this cataclysm is a random, chaotic set of asbestiform minerals scattered throughout the Gogebic range, being more prolific near the east and west ends of the range. Keep the chaos and randomness in mind as you read further. ...
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* For Part I of this series of articles on asbestos in the Penokees, see "They're doing it again!" -- posted on the Woodsperson blog Aug. 4, 2013.