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A Closer Look: Mapping Earth's Interior
How do we know the nature of Earth's interior structure?
Much
of what we know about Earth's interior comes from seismic waves.
Seismic waves are waves of energy that can be caused by
earthquakes. The two main types of seismic waves are body waves
and surface waves. Body waves travel through the Earth’s
interior in all directions. Surface waves travel only along the
surface of the Earth, like ripples on water. It is the behavior
of body waves that gives us clues about the nature of Earth's
interior. There are two types of body waves: primary waves (P
waves) and secondary waves (S waves).
What are P and S waves?
P and S waves.
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P waves stand for “primary waves.” They’re
considered to be primary because they travel faster than S waves
and, after
any given earthquake, will reach a seismic recording station first.
In the video, two children simulate P waves by holding opposite
ends of a Slinky on the floor. One child pushes the end of the
Slinky towards the other child. As a wave moves down the Slinky,
the coils can be seen to push forward and compress, then pull back
and open up again. This simulates the action of P waves. P waves
are compressional waves that exert a force in the direction that
the wave travels. These waves push through rock in the same way
that sound waves push though air.
S waves stand for “secondary waves.” In the video,
two children hold opposite ends of a Slinky on the floor and one
child moves the Slinky from side to side. In this case, as the
wave moves down the Slinky, the coils can be seen to shake side
to side, elastically springing back. S waves are shear waves that
exert a force perpendicular to the direction that the wave travels.
What
are the differences between P and S waves?
Scientists have learned
about Earth’s internal structure
by studying how these waves travel through the Earth. The technique
is straightforward — it involves measuring the time it takes
for both types of waves to reach seismic stations from the epicenter
of an earthquake. Since P waves travel faster than S waves, they’re
always detected first. The farther away from the epicenter, the
larger the time interval between the arrival of P and S waves — and
if the Earth were built of a uniform substance, that would be the
only variation measured.
Scientists, however, noticed variations
that could not be accounted for based simply on the distance traveled
from the epicenter. For
instance, they noticed places in the Earth through which S waves
didn’t travel. Geologists inferred that these sections of
the Earth were liquid, through which S waves (which, remember,
are shear waves) cannot travel. You may not know it, but you are
probably already familiar with this phenomenon. In a bathtub, if
you submerge your arm underwater and push your hand straight out
from your body, you can see a wave arrive as it hits the edge of
the tub. Consider this an example of a P wave. If you then move
your hand side to side in the water, you should notice that the
wave does not hit the edge of the tub in front of you. Consider
this to represent an S wave. What happened to it?
Solids and liquids
both transmit P waves because their particles transfer energy in
the direction of the wave as they compress and
elastically spring back along its length. As with the child at
the end of the Slinky, the seismograph at the end of a P wave detects
a “push” — the energy of this action. Although
S waves don’t compress, they still travel through solids
because the particles in solids elastically spring back even when
moved only from side to side. This is not a property of liquids.
In liquids, the energy of an S wave simply dissipates. So technically,
the second Slinky described above represented an S wave travelling
through a solid.
What were scientists able to learn from P and
S waves?
Simulation of an Earth
cross section.
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The absence of S waves in certain places along with an
understanding of S wave behavior in solids and liquids led scientists
to conclude
that the outer core is liquid and effectively absorbs S waves.
As the number of seismic readings increased along with their precision,
a worldwide community of scientists uncovered patterns that indicated
a much more complicated picture of the Earth’s interior than
was previously believed. Scientists have been able to distinguish
the layers of the Earth that are made of different materials that
transmit waves at different speeds. Based on these seismic observations,
the Earth’s interior has been divided into the following
layers:
Crust: A very thin, solid outer layer. The oceanic crust
is about 5 km (3 miles) thick. The continental crust is from 30–40
km (18–24 miles) thick.
Moho: The boundary between the crust
and the mantle.
Mantle: The layer beneath the crust. The mantle
is about 2885 km (1790 miles) thick.
Upper mantle: Includes
a solid layer fused to the crust. This layer combined with
the crust is called the lithosphere. Beneath
this
is the asthenosphere, which is a partly molten layer. The
asthenosphere is thought to be the layer upon which tectonic
plates ride.
The upper mantle is about 700 km (420 miles) thick.
Lower
mantle: Is composed of solid rock under conditions of extremely
high temperature and pressure. This layer is about
2,185 km (1,370
miles) thick.
Outer Core: A layer about 2,270 km (1,400
miles) thick, having the properties of a metallic liquid.
Inner
Core: A solid, metallic, spherical layer about 1,216 km (755
miles) thick.
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