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[♪ INTRO]
Every once in a while, Earth’s magnetic field weakens to almost nothing,
flips north for south, and then bulks back up again.
In fact, this happens somewhat regularly.
You may have heard that we’re due for a flip-flop,
with catastrophic consequences, which isn’t really true.
But you may also have heard that nobody knows why they happen.
That is a little truer.
Fortunately, geophysicists have made huge steps in understanding
why our normally calm magnetic field sometimes goes haywire.
Let’s start by going over the basics:
Beneath our feet is the Earth’s crust.
Below that is the mantle, then the outer core, then finally the inner core.
Most of our magnetic field comes from iron in the outer core
moving as the planet spins: As electric charges in the iron move,
they create a magnetic field around them, making Earth into
a big bar magnet with magnetic south on top and magnetic north on the bottom.
And yes, that means right now it’s the opposite of what you’d expect,
though it doesn’t actually matter in the scheme of things.
This is known as the dynamo theory,
and it’s really the only game in town for explaining Earth’s magnetic field.
But it’s hard to study the outer core directly,
because it’s obscured by crust and mantle.
And changes down there can take millions of years,
which is time we just don’t have to work with.
So scientists have to get clever.
Measuring the current field really carefully can give us
some hints about what’s going on down there.
And while scientists have found that the field has
gotten weaker in the last couple of centuries, don’t get nervous just yet.
Because they can also look at ancient volcanic rocks.
As lava solidifies, bits of iron in the lava
line up with the magnetic field around them like tiny little compasses.
So volcanic rocks lock in the field that existed when they first froze.
Scientists started systematically looking at these magnetic fossils in the 1950s,
and right away they noticed that, magnetically, Earth’s crust is striped like a tiger.
In some eras, the north magnetic pole pointed south like it does today.
In others, it pointed north.
Line up the ages of those rocks, and it shows that over our long history,
the magnetic field has reversed hundreds of times.
There’s still debate over how long it takes for the planet to undergo a full reversal,
but it seems that over a few thousand years, the field will get weaker, then briefly
go away almost entirely, and then get stronger again in the opposite direction.
On average, this happens every few hundred thousand years or so,
but there’s a lot of variation around that average.
The field might flip a bunch of times within a few million years,
or it might be stable for tens of millions of years.
Which means it doesn’t really come “due” to flip, given the amount of variation.
Measurements show that the field today is about as strong as it was in
the middle of one of those really long gaps with no reversals.
There’s no reason to expect one coming soon to an Earth near you.
But measurements alone can’t tell us why the field flips in the first place.
For that, geophysicists turn to computers.
One of the going hypotheses is that reversals are tied to plate tectonics.
Specifically, when one plate subducts, or gets shoved under another,
and then drifts down into the depths.
The idea is that when the remains of an ancient plate reach the boundary
between the core and the mantle, it changes how heat transfers
between the layers, which in turn affects how iron moves in the core.
A problem with this hypothesis has always been that nobody knows
how long it takes for a plate to descend through three thousand kilometers
of the mantle after subducting up here on the surface.
Some calculations lead to estimates of around thirty million years;
others say it’s closer to three hundred million.
And since we don’t know, it’s hard to look for patterns
between field-flipping and subduction throughout Earth’s history.
But in 2018, a team of scientists approached the problem
from the opposite direction.
Instead of calculating the delay first, they started by looking at subduction
and magnetic field reversals throughout geologic time.
They were looking for a consistent pattern where
there was a lot of subduction, then a certain delay, then a lot of field flips.
And they found that lots of reversals tend to happen
around 120 million years after periods with a lot of subduction.
And those really long periods without reversals
tended to be about 120 million years after periods with very little subduction.
Simulations have shown that once the remains of a plate
reach the bottom of the mantle, it can change how heat escapes from the core,
which changes how iron moves around the outer core.
And research has also shown that when iron in the outer core is disrupted,
small variations in the behavior of iron between the core and the mantle
can have a big effect on whether or not a reversal happens.
So it makes sense that even in periods with lots of flips,
it’s not like one happens every half-million years.
There’s some randomness in what happens with the iron at the boundary.
Measurements have also shown that even between field reversals,
periods with lots of them tend to have weaker fields on average.
Which, again, makes sense if heat flow between
the core and the mantle is a key factor.
If there’s not enough time to build up a strong field before heat flow
gets disrupted again, it’s easier to break down the field and lead to a flip.
Once, scientists wondered whether something as big as an asteroid
was necessary to set off a magnetic field reversal.
But the science is increasingly clear: Earth’s tummy can do flips all on its own.
And while Earth is in the habit of flipping its magnetic field,
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[♪ OUTRO]