[♪ INTRO] In 2003, the journal Nature published a paper describing a rather unusual proposal. The author suggested that scientists use a
nuke to crack open the Earth’s crust and then toss in a vibrating, grapefruit-sized recorder filled with scientific
instruments. The whole mess would sink through molten rock
and metal until it reached the Earth’s core a couple
of weeks later. Now, this article was not actually a supervillain
announcing their dastardly plans. It was more of a tongue-in-cheek thing. The goal was to illustrate just how hard it
is to study the inside of the Earth. I mean, we can see billions of light-years
through space. But when it comes to understanding what’s
beneath our feet? That is actually much harder. After all, space is see-through. Rock is not. Still, today we know a lot about how the Earth’s
interior is organized. There are distinct layers, for instance — the
crust, the molten mantle, a liquid outer core, and
a solid metallic inner core — with even more transitions
and subdivisions in between. But figuring it all out has involved some
inventive thinking across multiple scientific disciplines, and
it’s taken scientists to some surprising places. So, here are seven ways we have peered inside
our planet. Let’s start with the obvious one: just digging
a big hole. No one has been able to dig down to the mantle
yet, but that doesn’t mean people haven’t tried. In 2005, for example, an international group
of researchers called the Integrated Ocean Drilling Program
set out to reach the crust-mantle boundary. To do this, they targeted a thin point of
crust located on the floor of the North Atlantic ocean. In the end, they weren’t able to get there. After drilling down for more than 1.4 kilometers, they missed the thin patch by only about 300
meters. Even if they haven’t successfully made it
through, these types of projects have helped scientists
learn more about the crust and seafloor, such as the
unique microbial communities we can find down there. And we haven’t given up drilling yet. A team of Japanese researchers, for instance,
is looking at trying their hand somewhere near Hawai’i
in the future. A simpler idea is to study rocks and geological
activity right here on the surface. Studying volcanoes and fault lines, for example,
can teach us more about plate tectonics, where hotspots
in the mantle might be, and how magma reservoirs form underneath volcanoes. There’s a lot we can learn just from looking
at rocks -- especially older ones. Xenoliths are parts of Earth’s mantle brought
to the surface trapped inside other bits of volcanic rock. The most informative -- and spectacular -- type
of xenolith are diamonds. Diamonds form only under very specific conditions
at depths of 150 kilometers or more in the upper mantle. So anything trapped in the diamond — or
anything that comes up alongside it — must’ve come from at least
that far down. Scientists can also examine the chemical makeup
of ancient rocks. That’s revealed that, among other things,
some of what spews out from volcanoes isn’t fresh mantle material,
as you might expect -- but rather elements from old, recycled crust. In 2016, researchers measured the ratio of
magnesium isotopes in solidified lava from the French island
of Martinique. Then they compared that to the ratios previously
seen in other crust and mantle material. They found that the magnesium seen in the
Martinique lava looked like crust stuff. The team thinks that certain elements might
get squeezed out of the rocks along with water to travel towards
the surface as bits of old crust sink. In other words, bits of surface stuff seem
to sink deeper into the Earth, then get brought back up. Understanding how these fluids travel could
help us better understand how volcanoes and earthquakes
work. Digging, looking at rocks, and studying volcanoes
is all pretty hands-on stuff. But there are also ways to look inside the
Earth without heading into the field. Such as using seismic waves. These are the vibrations created not by us,
but by earthquakes that spread through the Earth like ripples
across a pond. Different densities of rock bend or reflect
those waves. By analyzing the pattern, researchers can
make inferences about the shape of things underground. This approach was key to one of our first
big breakthroughs in understanding Earth’s interior. In 1929, Danish seismologist Inge Lehmann
was examining seismic waves. At the time, scientists knew that Earth had
some solid and some liquid layers -- but they thought
the core was molten. If that was true, the waves from an earthquake
should spread out smoothly from its epicenter. But Lehmann noticed that some of the vibrations
seemed to “bounce” back towards the surface. The only explanation, she figured, would be
that they were reflecting off something big and rigid at
the center of the Earth. We now know that thing is our planet’s solid
inner core. Today, we’re still using seismic waves to
learn more about Earth’s interior. In 2019, for example, scientists found a kind
of iron “snow” falling from the outer core towards the inner
one using seismic wave data. We can also learn a lot from noticing when
things get weird. Irregularities in the planet’s properties
happen for a reason, and sometimes that points towards a cause that
we can’t actually see. For instance, there is a line known as the
Brunswick Magnetic Anomaly that runs through Alabama and Georgia where
Earth’s magnetic field seems unusually weak. Scientists can map it thanks to magnetometers, which measure the strength of a magnetic field. Magnetic anomalies can be caused by the composition
of rock in an area. A streak of magnetite ore, which contains
iron, may have an unusually strong magnetic field. On the flipside, a particularly weak field
might mean there’s a significant lack of magnetic material. Sedimentary rocks like sandstone often contain relatively little metal. These anomalies can also teach us about geologic
history. For example, a 2014 study suggested that the
Brunswick anomaly was due to rock left behind millions of years
ago as the supercontinent Pangea split up, separating North America from Africa. We can also look for anomalies in how well
the crust or upper mantle conduct electricity. Earth’s magnetic field varies naturally
over time, and changing magnetic fields create currents
of electricity. Scientists can measure that current by planting
electrodes in the ground. Then, by comparing how changes in the magnetic
field lead to changes in current, they can calculate
how well the rock below conducts electricity. Depending on how things are set up, this technique
can peer hundreds of kilometers below the surface,
revealing properties like the temperature and even composition
of the material down there. It can also help researchers calculate how
much water is trapped in the rock. In fact, one study found that there might
be as much water in the mantle as in all the oceans, locked
up in water-containing minerals like ringwoodite. So far, we’ve looked at real measurements
in nature. But figuring out what those mean often relies
on Scientists’ models and lab experiments. The pressures and temperatures deep within
Earth can be extreme, resulting in physics and chemistry that behave
differently than up here on the surface. One tool scientists use is the diamond anvil
cell, which consists of two small, flawless diamonds ground to
precision points and mounted on pistons. Since pressure is force divided by area — according
to the math — when the pistons apply their huge force to
the tiny points of the diamonds, the pressure can be ridiculous. In 2009, for instance, scientists reported
subjecting an iron alloy to two hundred billion Pascals of pressure
— more than half of what it would be inside the inner core! Amazingly, to find some of the planet’s
interior anomalies, we actually have to go to space. This is especially true for one particular
kind -- gravitational anomalies. From 2002 until 2017, NASA’s GRACE mission
used two spacecraft more than 500 kilometers above the Earth’s
surface to map out fluctuations in Earth’s gravitational
field. The result was maps like these that show where
gravity is oddly strong or weak. If you think back to high school physics,
you might remember that the more mass something has, the more
gravity it exerts. That means fluctuations in Earth’s gravitational
field can point to parts of the planet that are more or less
dense than others. And since the crust, mantle, and core are
made of stuff with different densities, scientists can translate
these variations into physical understanding. For example, if the crust in an area is known
to be less dense than the material in the mantle, weaker gravity
might point to a bit of crust getting sucked down into
the mantle. The precision of these measurements can get
even more specific than that, though. GRACE has also helped map the disappearance
of aquifers and measure the rate at which ice sheets are
melting. And this is wild - scientists have even used
ground-based gravity measurements to locate abandoned mineshafts
in England. Finally, not only can we examine the interior
of the Earth by going to space -- we can also let space
come to us. Meteorites can represent the solar system’s
building blocks -- the same stuff planets like Earth formed
out of billions of years ago. By studying them, scientists can learn about
the Earth’s starting conditions and how things have changed
over time. For instance, in 2005 a group narrowed down
the date that early Earth’s crust turned from a sea
of molten rock into an actual, solid surface. They did it by examining the ratio of a radioactive
isotope of the element lutetium, to the element it
decays into, hafnium, in samples collected from both a
meteorite and Earth’s oldest rocks. Both of these elements were present on Earth
way back when the planet’s surface was still molten. The material Earth formed from had a particular
ratio of one to the other, but they got split up unequally
as the crust separated from the mantle. As that happened, crystals of the mineral
zircon trapped bits of lutetium and hafnium inside, but in this
new, different ratio. All the stuff that didn’t form into Earth
stayed in space with the original mix. Billions of years later, bits and pieces arrived trapped inside meteorites. By comparing the lutetium-to-hafnium ratio
from Earth’s oldest rocks to these new samples from space, scientists
were able to work out when the crust must’ve formed. Their answer -- around four billion years
ago -- suggests the crust started solidifying less than a
hundred million years after the Earth itself formed. So, yeah, unfortunately, the Earth isn’t
see-through. And yet, thanks to a range of careful, often
clever observations, we can still picture to a remarkable degree
the complicated, roiling world beneath our feet. Thanks for watching this episode of SciShow, which was brought to you with the help of
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