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This Gorgeous Rock Was Made by a Supercontinent
This Gorgeous Rock Was Made by a Supercontinent
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0:00
There’s a legend told by the Inuit, of a brave warrior who was walking along
0:04
the icy shore, when he suddenly spotted lights, trapped within a dull gray stone.
0:10
He struck the stone with his spear, shattering the rock to release them,
0:14
and in doing so created the Northern lights.
0:17
Not all of the lights were released though, and some of them
0:20
stayed stuck in the rock, where they became known as ‘frozen fire’.
0:25
Today, we call these stones labradorite,
0:28
named after Labrador in northern Canada where they were first found.
0:32
But it can also be found in places like Finland and Madagascar.
0:36
Chances are, if you smash it like the Inuit warrior did,
0:39
the color will indeed disappear, since there’s actually no
0:43
blue or green pigment in these rocks.
0:46
Instead, that frozen fire is a product of a unique formation process
0:51
that spanned millions of years … and required a supercontinent to make it happen.
0:57
[♪ INTRO]
1:01
Our story begins in the middle of the ancient supercontinent of Gondwana,
1:06
in what one day would be called Madagascar.
1:09
Today, Madagascar is an island surrounded by tropical ocean on all sides.
1:14
But more than six hundred million years ago,
1:16
it was a sliver of land sandwiched between fragments of Africa and India,
1:21
with no ocean to be seen.
1:23
Long before the more famous Pangea,
1:25
the supercontinent of Gondwana contained the smooshed together landmasses
1:30
of modern-day Africa, India, Australia, Antarctica and South America.
1:36
It was by far the largest piece of land on Earth at the time,
1:40
covering up to a fifth of the Earth’s surface.
1:43
And, in addition to being big in area, Gondwana was thick.
1:48
Like continental crust today, it would have been about 40 kilometers thick
1:52
on average, extending even deeper up to 75 kilometers in the mountain roots
1:58
where continental chunks had crashed into one another.
2:01
This solid chunk of thick continental crust acted as
2:04
an impenetrable lid for anything trying to come up from underneath.
2:09
So whenever mantle convection or plumes of material came flowing up
2:14
beneath the supercontinent, the magma just got stuck instead of
2:18
punching through and creating volcanoes.
2:20
These plumes created magma chambers called plutons,
2:24
trapped within the heart of the continental crust, tens of kilometers down.
2:28
With no way out of the deep hot crust,
2:31
the molten magma stayed put and stayed hot, often for millions of years.
2:36
And when you leave a bubble of magma sitting on its own
2:39
for that long, something curious happens…
2:42
…it sorts itself out.
2:44
As the magma cools down super slowly,
2:47
the minerals that make up the rock begin to crystallize out.
2:51
But each mineral has a slightly different melting point,
2:54
so they will solidify in a very particular order as the temperature drops.
2:59
First to form are the dark-colored, iron-rich minerals, like olivine and pyroxene.
3:05
But these are also quite dense minerals too.
3:07
They’re denser than the rest of the magma,
3:10
and so once they form they sink to the bottom of the magma chamber.
3:14
In contrast, the lighter-colored minerals like quartz and feldspar are less dense,
3:19
and so they essentially ‘float’ to the top of the mix.
3:22
This process is known as fractional crystallization,
3:25
and it makes for some surprisingly well-sorted,
3:28
and sometimes even pure, mineral mixes, simply from being left for a long time.
3:34
This was happening underneath Madagascar 600 million years ago,
3:39
as the first crucial step to making labradorite and its frozen fire.
3:44
In the middle of Gondwana, a bunch of plutons created
3:47
a rock called anorthosite, made almost entirely from feldspar.
3:52
There are a lot of different kinds of feldspars,
3:54
and the concentration of metals in them determine which kind they are.
3:59
Feldspars that are rich in sodium are called albites,
4:02
those rich in calcium are called anorthites,
4:05
and ones rich in potassium are called orthoclases.
4:09
Labradorite is the name for a feldspar that’s between 50% and 70%
4:14
calcium-rich anorthite, with the rest made up of sodium-rich albite.
4:20
And in that ancient Madagascan pluton,
4:23
the rock that formed had just the right chemistry to make labradorite.
4:28
The thing is, on their own, a lot of feldspars are
4:31
a pretty unremarkable greyish color.
4:33
It takes a second step, with just the right conditions and chemistry,
4:37
for those feldspars to become iridescent and freeze fire.
4:42
See, at high temperatures calcium-rich anorthite and sodium-rich albite
4:46
are perfectly happy coexisting, mixed together in the same crystal of labradorite.
4:52
At the lower temperatures you find at the Earth’s surface,
4:55
they’d much rather be their own, purer thing.
4:58
And if they’ve already solidified into that albite-anorthite mix,
5:02
it’s reasonable to think that they’re stuck that way.
5:05
And that’s true under most conditions.
5:08
When feldspars crystallize and cool quickly, the mixture is fixed in place.
5:13
But in supercontinent plutons, there’s a rare opportunity to do
5:17
a little bit more sorting and separation, even after solidification.
5:22
That’s because, as I mentioned before, things stay hot for a really long time.
5:27
Even after temperatures have dropped low enough for crystals of feldspar to form,
5:31
it’s still hot enough for the atoms inside to have a bit of wiggle room.
5:37
So the calcium and sodium metal ions can migrate through the solid,
5:42
separating into their own, pure, albite and anorthite
5:45
layers inside the feldspar crystal.
5:48
Now, even though atoms are very small, migrating through solids isn’t exactly
5:53
the easiest thing to do, so they don’t actually move that far.
5:57
The result is a sequence of super thin alternating albite and anorthite layers
6:03
that are just a few hundred nanometers thick.
6:06
These are called ‘exsolution lamellae’, and in labradorite you only get them in
6:11
a very specific range of compositions, when the feldspar has between
6:16
fifty and seventy percent anorthite,
6:19
and when the temperature has stayed hot enough
6:22
for long enough for the layers to form.
6:24
And it’s these lamellae that are responsible for labradorite’s colorful flashes -
6:29
a phenomenon known as labradorescence.
6:32
It works in a similar way to how you get colors reflected from the back of a DVD,
6:37
or from a blue morpho butterfly wing.
6:40
The blues and greens you see are a type of structural color,
6:44
not a pigment like blue paint.
6:46
This happens because the tiny anorthite and albite layers have slightly different
6:51
refractive indices, which mean that they bend the light
6:55
that passes through them by slightly different amounts.
6:58
Light that enters the translucent labradorite crystal becomes refracted
7:02
by the lamellae, and reflected from the boundaries between the lamellae,
7:08
bouncing back to our eyes.
7:10
This refracted and reflected light interferes with itself,
7:14
reinforcing some colors and cancelling out others,
7:18
creating the play of color that seems to come from just underneath the surface.
7:22
To make the characteristic electric blue flash from labradorite,
7:26
the albite-anorthite layers need to have a specific thickness
7:31
of between 128 and 252 nanometers.
7:37
Since the layers might not be consistent across the crystal or stone,
7:41
the frozen fire you end up seeing is unique to every piece.
7:45
So the beautiful colors of labradorite owe their existence to
7:49
processes on some of the largest and smallest scales on Earth.
7:54
It took millions of years, and over a thousand degrees,
7:58
in the heart of a giant supercontinent, to assemble just the right chemistry
8:03
for just the right atoms to move just nanometers apart,
8:08
to create the most surprising flash of blue in an otherwise abundant mineral.
8:13
All of that might sound incredibly unlikely,
8:16
but labradorite isn’t as rare as you might imagine.
8:19
These big, slow, and precise processes are common enough
8:23
in the Earth to have created plenty of the iridescent mineral.
8:27
So much so that it is mined in Madagascar and
8:30
elsewhere into huge decorative slabs and sculptures.
8:34
And this month, Rocks Box subscribers will be receiving
8:38
their own piece of Madagascar’s geological history,
8:42
since we’re sending you a polished piece of labradorite.
8:45
Every month, Rocks Box subscribers get
8:48
a premium mineral or fossil sent right to their door.
8:51
But the fun’s not just for subscribers.
8:53
We also sell some of our greatest hits for individual purchase,
8:57
along with some cool merch.
8:59
If you want to learn more or browse some of our other offerings,
9:02
head over to Complexly.store/rocks to check it out.
9:07
And thanks for watching!
9:09
[♪ OUTRO]
