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[ intro ]
Geology really loves its layers.
Older stuff on the bottom, newer stuff on top.
That is, until this planet of ours cooks, flips,
and smushes those rocks every which way.
Sometimes super-ancient rock just sort of bubbles up
out of the ground to build an entire, brand-new mountain range.
Such as New York’s Adirondack Mountains.
See, the rock on the high peaks of the Adirondacks is pretty old and rare.
We’re talking a billion years here.
Yet the mountains themselves have only been around for five million years.
To put that into perspective, multicellular life
was only sort of a thing a billion years ago.
Whereas five million years ago,
humans were probably splitting off from chimpanzees.
Some of these particular rocks are also relatively uncommon,
at least here on Earth.
They’re called anorthosite, an igneous rock --
that is, one formed from magma --
that’s pretty common in the higher elevations of the Adirondacks,
and is also found in samples from the Moon.
Anorthosite forms deep underground,
where both the pressure and temperature are high.
And it requires some pretty extreme conditions -- like, say, two continental plates colliding.
Which is what happened to create the Grenville Orogeny --
a mountain-building event roughly a billion years ago that created a huge range
that was as high as the Himalayas and stretched from modern-day Canada
all the way to Mexico.
As part of that event, anorthosite got squeezed up from deep in the Earth’s mantle,
helping form these new mountains.
But as time marched on, the Grenvilles eroded, and the igneous material became buried in
sediment.
And around 400 to 500 million years ago,
the mountain range was replaced by a shallow ocean.
Over time, that sea floor accumulated new layers of sediment,
burying the remnants of the Grenvilles under more and more material.
But five million years ago, some thing happened in that corner of New York to lift up that
old rock, forming a series of dome-shaped mountains.
And geologists are only starting to understand how.
Because in this case, no new continental plates came together.
It’s more like upstate New York just decided to blow bubbles one afternoon.
In 2018, a study published in the journal Geophysical Research Letters
described two potential mechanisms for how the dome-shaped Adirondacks
might have bubbled up from below.
Around a hundred kilometers below the surface of the Adirondacks
is a layer of low-density, light rock.
Compared to the stuff around it, this rock is buoyant.
Researchers think some of this material may have flowed upwards
to fill cracks and expand, eventually accumulating enough
to push everything above it up and create a mountain.
Another possibility is that heat may have played a role in expanding the rock
and creating that same upward movement.
In fact, the researchers think both mechanisms may have played a role.
Regardless, the process is still happening today, with the Adirondacks rising
at a rate of two to three millimeters per year.
Which adds up!
That’s thirty centimeters in a century!
When the process started, the old igneous rock was still hidden under that newer sediment.
But igneous rock is much harder than the sediments deposited at the bottom of a sea, so, over
time, the newer stuff eroded away, revealing the original, billion-year-old rock.~
Which is apparently how you upcycle a new mountain range out of an old one.
The newer rock hasn’t eroded away completely,
and you can still find it around the edges of the mountains.
But the peaks are that crazy old anorthosite.
And that means the super-old rock of the Grenvilles gets to ascend to brand new heights.
These mountains don’t break the rules of geology as we know them
-- it just takes a little problem solving to figure out why.
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Brilliant’s course Joy of Problem Solving might convince you otherwise.
It’ll take you through some of their most delightful challenges.
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