Over 3,000 mi below the surface, right
at the center of the Earth, lies a solid
spheroid.
Primarily made of iron and nearly as big
as the moon. Its temperature rivals that
of the sun.
This is Earth's inner core. It spins
independently at the very heart of our
planet. And while we can't see it for
ourselves, it's immense heat triggers
processes that impact us all. Plate
tectonics that manifest in some of the
most or inspiring and deadly sights on
the surface. Our planet's invisible
magnetic field which makes life
possible.
It's no wonder that Earth's core has
captured the imaginations of generations
of scientists who have developed ever
more sophisticated instruments to
investigate its fiery depths. And the
latest research efforts have yielded
discoveries that could have come
straight from a Hollywood film.
Our inner core appears to not only have
stopped spinning, but has in fact
started moving backwards.
Why has this happened? And what does it
mean for us topside?
I'm James Stewart and you're watching
Astramm Earth. Join me as we quite
literally explore our planet's deepest,
darkest secrets.
We'll investigate its inner workings,
assess the implications of our cause
change in motion, and ask the question,
how worried should we be?
In order to get to our planet's core, we
need to go down. Call it my version of a
journey to the center of the Earth. Each
layer has its own role in creating,
sustaining, and sometimes destroying the
world above. And all of them are
influenced [music] by the solid iron
heart. Here at the surface, we walk upon
the crust, a mere brittle shell. Though
it feels pretty solid to us, it is in
fact fractured into giant tectonic
plates that drift, collide, and grind
[music] past each other, causing violent
earthquakes and devastating volcanic
eruptions. How it moves is all thanks to
the layer below, the mantle. This layer
also appears solid, but is actually
flowing very, very slowly at the rate of
a few cm a year.
This tiny but inexurable movement
contributes to the plate tectonics that
impact our surface world. Dig deeper
still and you reach the outer core, a
sea of iron and nickel with its own heat
driven currents. Its motion generates
the Earth's magnetic field which
protects life on our planet from deadly
solar particles.
Finally, suspended in this outer core is
that primarily iron inner core. Here,
the temperature reaches 6,000°
C. Due to the immense pressures, the
metal isn't liquid, but solid, forming a
spheroid shape, like a slightly squashed
sphere. This is our planet's boiler
room. Its heat triggers the processes in
the layers above, driving those
allimportant convection currents in the
outer core and ultimately contributing
to the plate tectonics that shift the
Earth's crust.
It's easy to take for granted that we
understand our planet structure, but in
reality that knowledge was incredibly
hard one. It's one thing studying,
cataloging, and theorizing about the
world we can see all around us, but
quite another to investigate our
planet's hidden interior. So, how do we
know what's down there?
In Frankfurt, September 1896,
darkness was falling as a 34year-old
Emil Vett waited for his chance to speak
at a meeting of German scientists and
physicists. Clutched in his hands was
his latest research which proposed a
solution to a problem that had plagued
the scientific community for more than a
century. You see, the average density of
planet Earth had been calculated at
around 5.5 g per cm cubed. And yet,
surface rocks have a density of around
half that. So, where was that missing
mass?
One theory was that the Earth gets
progressively denser nearer the center.
But that wasn't good enough for Ehhat,
who reasoned that molecules in a solid
are already pretty densely packed and
even the compressional effects of high
pressure wouldn't be enough to achieve
the density required. And so when he
finally got the chance to speak, he
proposed something else. His theory was
that the density difference must be due
to there being a different, much denser
material hidden deep in Earth's
interior.
As to what that material could be, well,
the inspiration for his theories came
from the heavens in the form of iron
meteorites.
These little clumps of metal have a
density higher than the average of
Earth. And Vert reason that our planet's
interior must therefore contain a large
iron core, dense enough to account for
the difference between lighter surface
rocks and the higher planetary average.
Now, this wasn't an entirely new idea,
but not only had Vert deployed
mathematical reasoning rather than the
purely theoretical work of his
forebears, he'd also proposed it at just
the right time.
A new technology was emerging that would
finally allow the interior of the Earth
to be studied.
Earth's core is so deep and the
pressures and temperatures so intense
that you can't simply drill down there
and take a sample. Although that would
be nice. Even today, our deepest
man-made hole is the Cola super deep
ball hole in Russia. And at more than 12
km deep, that is still only a fraction
of a percent, 0.2 to be exact, of the
way there. And so in order to study the
core, scientists rely on measurements
taken here on the surface. Now
thankfully there's one way we can get
particularly insightful information and
that is from earthquakes. By the 19th
century, increased interest in seismic
activity had prompted the creation of a
device to measure them scientifically.
Yeah, this was the modern seismograph
and it looks something like this. For
the first time, the analysis of seismic
events weren't based on eyewitness
observation and hearsay, but on
quantifiable data, which we love here on
the channel. But while the device itself
was unquestionably a scientific
breakthrough, the real magic happened
when the invention spread across the
globe. As seismographic networks
expanded, scientists found they could
detect earthquakes happening as far away
as the other side of the planet. In
1897, the so-called great earthquake
erupted at Shillong plateau in India.
Shocks were felt in Kolkata and even as
far away as Burma. But seismographs
detected this earthquake much much
further away in Europe where the
seismoggrams they created were poured
over by British geologist Richard Dixon
Alden, leader of the Geological Survey
of India. Olden noticed that the
seismoggram showed three distinct wave
shapes. In his papers, he compared the
data found from this and several other
earthquakes and found that they too
showed the same three waves.
The reason he could see them was being
far from the epicenter, differing speeds
of the waves meant they arrived one at a
time. The first arrivals were later
known as primary waves, pressure waves
or P waves which are longitudinal. They
oscillate in the same direction as
they're traveling in. These were then
followed by secondary sheer or S waves
which are transverse. Their oscillations
are perpendicular to their direction of
travel. Finally, surface waves ripple
through the Earth's crust, and it is
these that tend to cause the most
destruction.
It had already been established by this
point that longitudinal waves can pass
through both solids and liquids, [music]
but transverse waves can only pass
through solids. And you can probably see
where this is going. Since the seismic
waves had traveled through the inside of
Earth to reach the seismoggrams, the
resulting seismographs could yield
information about what they passed
through, namely the structure of Earth
itself.
This realization would allow centuries
of speculation to finally be put to
rest.
Put simply, noticed that waves that had
traveled a long way to distant
seismographs seemed to travel much
slower than the 6 km/s it usually moved
through the mantle. To explain this, he
concluded that they had traversed a
central core composed of matter which
transmits them at a slower speed, 3 km/s
to be exact. He calculated the size of
this core to be 4/10 of the Earth's
radius, deduced that it bends earthquake
waves, and that it behaves fundamentally
differently to the rest of the Earth's
interior. But he did not speculate as to
what this core might be made of,
refusing to go beyond what he could
prove with data. And so by the early
20th century, you had a theory that the
Earth had an iron core and apparent
proof that some sort of core did in fact
exist. We were finally beginning to
crack the inner structure of our planet.
And yet, despite these great strides,
these pioneers missed something.
impact and see the results in less than 30 days as we head back to the video.
As seismographs continued to spread
around the world and more and more data
was gathered for scientists to pour
over, they noticed that S-waves didn't
register on some seismographs. Something
was stopping them. Now, remember I said
that S waves don't transmit through
liquids? Well, it was a small leap,
therefore, to go from that fact to the
idea that Earth's core must therefore be
liquid iron. It wasn't completely wrong,
but there was something they had all
missed, and it took a Danish scientist
to find it.
In 1929, Inger Layman was chief of the
seismological department at Denmark
[music] State Geodetic University.
Her responsibilities involved keeping
the instruments correctly adjusted,
interpreting the seismoggrams, and
publishing the bulletins of the seismic
stations. But her job did not extend to
original research. So when she decided
to embark on her own research project,
she had to do so alone without the
typical team of assistants to help her.
But it was precisely because she was
forced to troll through so much data by
herself that she noticed something odd
about the Pwaves she was studying. You
see, if the earth had a liquid core, P
waves should travel down from an
earthquake, reach that core, and be
refracted due to the liquid's
properties, like light entering a prism.
The expected result would be a shadow
zone where no P waves would appear
because they had all in fact been
refracted away. Yet, after an earthquake
in New Zealand, she discovered Pwaves in
the shadow zone, precisely where they
shouldn't be.
In a paper titled simply P, she
attempted to explain what she had seen
and reconcile it with existing
observations, concluding that the core
had two parts. An outer core that was
liquid and so stopped the S-waves, but
crucially also an inner core that was
solid so could transmit some P waves to
the shadow zone where she had seen them.
As a woman working in the maledominated
world of science, Ingga had to fight to
have her conclusions heard. But she
would turn out to be absolutely correct.
That solid inner core was the final
piece of the puzzle.
However, as is often the way with
scientific research, this discovery
simply paved the way for more questions.
Since the solid inner core is held
within a liquid outer core, does it
necessarily spin in sync with the rest
of the planet or could it be rotating at
its own speed? To find out, scientists
would require incredibly sophisticated
seismographs and they would soon have
access to such instruments. Although it
would not be thanks to pure science
alone, but to war. Well, kind of.
By the second half of the 20th century,
the world was in the midst of the Cold
War. But while the general public lived
with the spectre of nuclear Armageddon,
for seismologists, it was proving to be
a time of incredible excitement. As the
United States instigated project veiler
uniform, its aim was to develop a
suitable system for detecting
underground nuclear testing in the
Soviet Union. And what resulted was an
investment of over half a billion
dollars in today's money in
seismographic technology and networks.
Crucial to the project's success was the
ability to distinguish between the
seismic waves produced by nuclear tests
and those made by naturally occurring
earthquakes.
And so by the time the cold war ended,
researchers at Harvard University had
made breakthroughs using these
seismographs.
They showed that seismic waves traveling
along Earth's north south axis through
the inner core moved faster than those
undertaking the journey in an east west
direction. To explain this speed
difference, they theorize that the
Earth's core is anotropic,
meaning it has a crystalline structure
aligned with Earth's rotation roughly
along its magnetic field. Think of it
like the grain on a plank of wood. And
just as a plank of wood is easier to
mill in one direction than the other,
it's easier for earthquake waves to pass
through the core on the north south axis
than the east west.
Well, almost. It turns out that this
crystalline structure doesn't align
precisely with the rotation of the
Earth, but rather is tilted a few
degrees off axis.
This was crucial because it gave
scientists a potential way of
determining how fast the inner core
spins. If it spins at a different rate
to the rest of the planet, then that
would continually change the way those
crystals are aligned with the rest of
the Earth, which would in turn affect
the wave travel. This realization gave
birth to a method of assessing inner
core spin that is still used to this
day.
Now, working out whether Earth's core
was spinning at its own rate wasn't
easy. Scientists recognized they needed
data from multiple earthquakes that had
happened in nearly the same spot, but
far enough apart in time that the core
could have moved. Fortunately, they had
decades of earthquake data thanks to the
Cold War. And so, the hunt was on for
what they termed earthquake dublets.
Now, such dublets are incredibly rare.
Imagine hunting for identical twins when
they're sat in different parts of a
crowd at Wembley Stadium. Fortunately,
it was no longer the era of pen and
paper. And with the increased
digitization of seismic data, what would
have once been thousands of human hours
was now the work of algorithms.
By 1996, a team of researchers at Colia
University had found what they were
looking for. data from a series of
earthquakes that took place in close
proximity at the South Sandwich Islands
and also crucially a seismic station
that had reliably monitored them for 32
years at College Alaska. By comparing
the readings, they found that from 1967
to 1995,
these similar waves were taking a faster
and faster path through the Earth's
core. They had their answer. Earth's
core must have rotated at a different
speed to the rest of the planet.
It was the first observational evidence
of inner core rotation. And they'd
calculated that it was spinning faster
than its surroundings. And now with the
technique proven, well, the floodgates
opened.
In the last couple of decades, some
studies have been published that agree
the core rotates faster, while others
have concluded that it spins more
slowly. Further studies have suggested
that it fluctuates, sometimes spinning
faster and other times slower, maybe as
frequently as every couple of years. So,
what's actually going on? Who's right?
In 2023,
scientists at Peing University in
Beijing published a new research paper,
arguing that inner core rotation has
nearly paused and relative to the mantle
has actually been rotating slightly
backwards over the past decade. They
stated that such a change of rotation
also occurred in the early '7s,
ultimately concluding that the Earth's
inner core oscillates with a period of
about 70 years. The following year,
another study was produced, this time by
the University of Southern California,
which approached the same problem from a
subtly different angle.
Rather than looking for differences in
waves from the same earthquakes, it
looked for matching waveforms from
different times. reasoning that if
previous studies were correct and the
[music] inner core did go through a
cycle of faster and slower rotation,
then there would be times where relative
to the mantle, the inner core was back
in the same position. I think of this
like a car and a bus driving in the same
direction side by side. If the car
alternates between traveling slower and
faster than the bus, then there'll be
times when it's ahead of the bus,
there'll be times when it's behind, and
times when it's back aligned with the
bus again.
So the study examined P waves produced
from 121 earthquakes that took place
between 1991 and 2023 in the South
Sandwich Islands.
The waves passed through the Earth's
inner core and were detected by seismma
in Alaska and Canada. Using this
methodology, they confirmed the findings
of the previous year that the changes in
the inner cause rotation speed follow a
70ear cycle. And more than that,
according to their calculations, it's
just about due to speed up again.
As for what causes this change, well,
scientists aren't yet certain. Possibly
the magnetic field of the liquid outer
core could drag on the solid iron
within, speeding it up or slowing it
down, while gravity from dense regions
in the mantle could pull on variations
in the inner core affecting its
rotation. But what does that mean for
us? Do we need to worry about this?
Well, remember how I said earlier that
the inner core is like the boiler room
of our planet? Well, it turns out that
changes in its behavior do seem to
correlate with other phenomena on the
surface.
The authors of the 2023 study noticed
that 70-year oscillation coincides with
fluctuations in the length of a day and
changes in the magnetic field. And
beyond that, they even link them to
changes in global mean temperature and
sea level. But before you're too
alarmed, we're talking about tiny
changes here. a thousandth of a second
here, a fraction of a degree there.
Certainly not enough to explain global
warming. The long story short is its
early days. Scientists are continuing to
study the inner core and its impacts on
the world above. And with every passing
year yielding more and more seismic data
ripe for analysis, new insights are only
a matter of time. Perhaps Ingger Layman
put it best. A few years before her
death, at the ripe old age of 104, she
wrote, "The first results for the
properties of the inner core were
naturally approximate. Much has been
written about it, but the last word has
probably not yet been said."
Let me know in the comments what you
think of this. Is this something to
worry about? How do you think this will
affect life on Earth, if at all?
It appears that when it comes to the
deepest, darkest secrets our planet
holds, we really are still only scraping
the surface.