|What tools are used to find the layers of the
earth if scientists can't go to the core?
|Question Date: 2018-05-10|
The outer layers are pretty easy -
go look at and pick up rocks from different
of the crust to study their composition,
structure, and order as depth below the very
surface increases. The inner layers are more
difficult to study and much of our knowledge
relies on indirect measurements based on
waves, most commonly those caused by
travel through the earth. Essentially, by
measuring how waves take to travel through a
certain distance of Earth's insides, scientists
can work out details of the material through which
the waves are traveling. This is discussed
extensively in the
ScienceLine responses to this
which was asked multiple times
in multiple forms
already. We can also infer that there must be a
liquid (at least partially anyway) core made of
metal because Earth has a
Even though we can only take samples from the
outer layer of the Earth, valuable information can
be gleaned. The prevalence of certain elements
(e.g. iron) in the rest of the solar system
relative to that in the crust, in combination with
the average density of the earth vs. that of the
crust, leads scientists to conclude that
core must be iron.
also has a very good article on this topic.
This is a great question!
There are lots of ways that geologists know what
is inside of the earth and how it is structured.
The most basic structure can be determined using
seismic waves. When an earthquake happens, the
earthquake produces three main types of waves:
P waves, S waves, and surface waves.
figured this out during the middle of the 20th
century, as nuclear bombs can produce the same
types of waves. Geophysicists (scientists who
study the physics of the earth) set up
all around the globe, and whenever an earthquake
happens, it gets recorded on these detectors.
Because S and P waves have different behavior (S
waves are slower, and P waves can't travel through
liquids) , geophysicists can map out the inside of
the earth as the waves travel through the
different parts. I've included a really great
diagram that shows this below, and have also found
a short video explaining how waves move through
seismic wave paths.
Since P-waves don't travel through
P-wave shadow zone indicates that there must be a
liquid part of the interior of the earth. By
mapping where the waves end up, we can see that
the outer core is liquid, and we can track
waves refract (bend) through the different layers
(e.g., crust, upper mantle, lower mantle, etc.) to
determine their densities and what they are likely
made out of. Even with all of that, though, there
is still a lot that we don't know about the inside
of the earth , like its composition (we know
generally what the chemistry of the earth's
interior is like, but scientists argue a LOT about
specifics). I hope this has answered your
question, and that you have more great questions
about Earth Science!
Look at this diagram.
Sound waves travel at different speeds depending
on the density of the material they are traveling
through. If waves are traveling through
that is changing the speed at which waves move,
then the waves will change direction as they cross
this boundary. Earthquakes make noise, and
geologists build machines that can detect these
sound waves as they travel through the earth. By
comparing the results from many different
listening devices at different locations, we can
reconstruct what layers the earth consists of.
We use physical simulations to try to guess at
what the layers with their densities are. So, for
example, we know that the outer core has about the
same density as a mixture of molten iron and
nickel, and the inner core has a density of solid
iron-nickel alloy. These, combined with
observations about how common what elements are in
the solar system and in the universe, lead us to
surmise that these elements are probably what the
core is made of.
To understand different rock layers near where
we live on the surface of the Earth we have
drilled to about 10 km deep. Doing this
helps us understand shallower rocks like those we
would find oil or natural gas in. Sometimes
mountain building can expose layers that we
usually don't see. That's why there are some rocks
in California at the surface that used to be in
the Earth's mantle, the layer bellow the crust.
To look at deeper layers we usually have to use
different types of signals coming from layers in
the Earth. For example, when earthquakes happen
deeper in the Earth they vibrate the ground. We
call the vibrations seismic waves. The
waves move faster through some rocks than others,
but we can then find the layers because layers
have different speeds for the seismic waves.
There's also another way to look at layers
through the way they carry electricity,
which we can use to tell the difference between
layers based on how well they carry electricity.
When scientists can't directly measure
something, like digging up rocks from beneath the
earth's surface to directly see what is there,
they use indirect measurements. Geologists
and Geophysicists are the scientists who study
rocks, and properties of the earth. The tools they
use for studying the layers of the earth are
called "seismic waves". They have tools to
emit seismic waves (like sound waves) through the
earth, and measuring how fast or slow they go.
How fast the wave goes depends on the material
the wave moves through. Think of when you
yell into air, compared to when you yell under
water -- the sound waves from your mouth travel
differently and sound very different. Sound waves
also travel differently through different types of
rock. Geologists and geophysicists measure how
the seismic waves travel through different rock
layers to identify what the layers are.
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