Thursday, July 28, 2011

Keweenawan Jewels

By Jack Parker (aka jprockdoctor)*

Guest author Jack Parker, mining engineer, calls this rock an "amygdaloidal beach cobble." See below to learn why. (Rock painting by Gustavo Bourdieu. Photo © 2011 and courtesy Jack Parker)

You think it’s hot here this month?

You should have been here a couple of billion years ago, when it was being built. The whole earth was in a state of upheaval. There were no trees, no grass, no lakes or beaches. She was rumbling and shaking, as if waking from a bad dream.

Volcanoes were showering bombs and hot ashes on Keweenaw County, cracks opened up and vast flows of molten rock oozed across the land. Fire and brimstone -- the brimstone being sulfur. The atmosphere was really toxic -- there being no EPA to regulate it.

Thirsty? Sorry, there is no fresh water. Hungry? You’re going to have to wait. There’s nothing to eat yet …

But there is so much going on -- it keeps the amateur geologist thinking and wondering and puzzled to this day. Always looking for clues.

Vesicles for example: As those lavas flowed across the landscape gas bubbles rose to the tops of the flows and many of them were preserved in the rocks as they cooled, bubbles large and small. So we can recognize a flowtop when we see it.

Then, again over a very long period of time, fluids passed through the rock and entered the bubbles, often precipitating new minerals in them. The filled bubbles are called amygdules (from the Latin word for almond), frequently white with calcite, glassy with quartz, yellowish- green with epidote, sometimes banded with agate, occasionally with native copper and rarely with native silver.

If there is enough value we mine those flowtops, calling them "Amygdaloids." Some folks search in them for semi-precious stones, including agates, amethysts, greenstones, and thompsonites.

Today folks scour the wasterock piles and gravel pits and the beaches to find specimens which please them -- tons of them. Michigan Tech has contributed to a lot of what is called "student erosion" of the UP, now scattered all over the world in showcases, basements and garages.

One very special collection can be seen at the "Tori Market" in Hancock on Wednesdays and Saturdays. You will find vegetables, honey, artwork, handicrafts and not a few tall stories, in the open air or under canvas.

The above photograph is of an amygdaloidal beach cobble, about three inches long -- the prettiest I have seen in eighty years of looking. It’s a happy rock. Just to look at it makes me smile.

* Editor's Note: Keweenaw Now guest author Jack Parker of Baltic is a semi-retired mining engineer / geologist, who specializes in practical rock mechanics.

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