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Natural Selections: New mountains, old rocks

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The Adirondacks may be "new," but the rock is a billion years old, pushed up through the newer rock of the Champlain region. Mixed in, the remains of even older rock can be found in pockets and veins--blue calcite laid down by warm oceans before the evolution of coral. Martha Foley and Dr. Curt Stager get geological.

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The Adirondack Mountains are relatively young and are still going up. However, the rock on top of them is very old. This contributes to the region’s unique geography.

“It’s actually even kind of weirder to think about what might have been here before those really old rocks, the types of granite-type things and this rock called anarthacite, the gray stuff you find all over the high peaks,” said Dr. Curt Stager of Paul Smith’s College. This is the very old rock, and it’s a little over a billion years old. It is currently pushing up through half-billion year old rock in the Champlain Valley.

Anarthacite was formed at huge depths in the earth’s crust and isn’t a common type of rock to find. It doesn’t have very much quartz in it and can be reproduced in the laboratory only with huge pressures and very high temperatures. According to Stager, these conditions are only naturally found near the earth’s mantle, far below the crust.

“Just having this anarthacite stuff here tells geologists that there must have been something that pushed the crust way, way, like fifteen miles, down into the mantle. And the only thing that could have done that is a collision with another continent, where one rode up over on top of us. And there is good evidence that happened a little over a billion years ago.”

This is currently happening with the Himalayas as India pushes into Asia. The rocks in the Adirondacks were pushed far down, and a special type of lava created the rocks found in the mountains today. “Just by saying that this stuff was intruded into the crust means the crust was already there, which means that was older than the stuff being intruded into it. So the rocks the high peaks are made of are younger than the stuff they were squirted into.”

In places like Cascade Notch, just east of Lake Placid, people can find blue calcite. This is a carbonate left over from an ocean that was in that area before the Adirondack rocks formed. “This is the stuff that the lava was squired into,” said Stager. This rock is almost a billion and a half years old, and for it to have been deposited, it would have had to be in a warm, shallow ocean like the one currently around the Florida Keys.

“This is before coral existed, so there wouldn’t even have been coral reefs like you’d find around Florida. This is just lime coming out of the water,” said Stager. “So this stuff made limey rocks, limestone, without coral fossils in it or anything. And then when the continental collision happened, that stuff got smushed down into the great depths and cooked and it mingled with the other rocks and made this calcite.”

It was then pushed up as the Adirondacks started to rise, and has been eroding ever since. However, small pockets of marble and other rocks can be found on the high peaks of the mountains.

 

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