As we discussed in the previous tutorial, geologists who study the interior of Earth
have determined that there are three main layers: the crust, the mantle, and the core.
Some evidence pertaining to the composition of Earth’s layers exists in the form of
strange rocks, called xenoliths, brought to the surface from the upper mantle.
In some locations on the surface, ancient oceanic crust was thrusted upward and overturned
during mountain building events.
These deposits, called ophiolites, bring pieces of the upper mantle to the surface.
Pieces of the mantle can also be brought to the surface in ultra-deep seated volcanic
eruptions from kimberlite pipes.
That is about as good as it gets in relation to physical samples of Earth’s interior.
In addition, the study of a type of meteorite, called a C1 chondrite, can provide some insight
into the composition of Earth, it being Earth’s parent material.
But is that the only evidence geologists have regarding Earth’s interior?
How can geologists know anything about the physical properties of Earth’s inner layers
without actually visiting them?
Geologists have come up with a clever way to answer this question using seismic waves.
Seismic waves are vibrations that rumble through the Earth.
Seismologists, who are geologists that focus on seismic waves, study events like earthquakes,
which release massive amounts of energy and produce seismic waves that ripple through
Earth’s layers.
In an earlier tutorial on geologic structures, we discussed how rocks can undergo elastic
deformation.
When Earth’s tectonic plates move past each other, they build up stress along their boundaries,
which are massive faults.
Over time, the stress gradually increases until the crust can hold on no longer.
The crust then ruptures and slides along the fault, releasing all the built-up stress in
a burst of energy.
The precise location within the crust that this burst occurs is called the focus.
If you were to draw a straight line from the focus to the closest point on Earth’s surface,
you’d find the epicenter of the earthquake.
By tracking the epicenters of earthquakes through time along a given fault zone, seismologists
can predict the location of future earthquakes and provide early warnings to citizens who
live in earthquake-prone areas like California and Japan.
When an area experiences a large earthquake, most of the stress is released, and the fault
needs time to build up more stress before another one can occur.
Once an earthquake begins and energy is released at the focus, several different types of seismic
waves travel through the Earth’s interior and along its surface.
Surface waves are the seismic waves that travel through the shallow crust, potentially causing
massive amounts of destruction.
Surface waves from the 1960 Valdivia earthquake in Chile, the most powerful earthquake ever
recorded, caused a tsunami that moved a ship almost a mile down the Valdivia River.
There are two main types of surface waves: Love waves and Rayleigh waves.
Love waves wiggle through Earth’s surface side to side, like a snake slithering along
the ground.
Rayleigh waves roll along the surface, as do waves in the ocean.
The seismic waves that travel through the Earth’s interior are called body waves.
Body waves come in two types: S-waves and P-waves.
P-waves are compressional, like sound waves, and wave motion is parallel to propagation.
S-waves are shear waves, with wave motion being perpendicular to propagation.
A slinky is often used to represent P and S waves.
If a stretched-out slinky is waved up and down at one end, that S-wave will move along
the slinky to the other end.
If the same slinky is nudged forward or backward, a P-wave will form.
P-waves are the fastest seismic wave and have velocities of around 8 kilometers per second,
which is why P stands for primary, as these waves arrive first.
S-waves are a bit slower with velocities of around 3.5 kilometers per second, which is
why S stands for secondary, since they arrive second.
By measuring the types of body waves that travel through the planet’s interior during
earthquakes, and their velocities, seismologists have built up a sort of echogram of Earth.
The waves interact with Earth differently depending on the density, temperature, and
state of the rocks they travel through.
When a propagating wave encounters a change in these properties, say for example, at the
boundary between the crust and the mantle, part of the wave will be reflected back toward
the focus, with the remainder refracting into the new layer as the wave’s velocity changes.
Seismologists use instruments sensitive to Earth’s vibrations called seismometers to
measure the S- and P-waves from earthquakes, as well as man-made sources like nuclear explosions
and mining activity.
P-waves are able to travel from the focus in the crust through the solid mantle, onwards
through the liquid and solid core, and then back through the mantle to the opposite side
of Earth.
S-waves, however, can only travel through solid rock.
Since S-waves are dissipated at the outer core, a large area of earth, opposite the
focus, lacks transmitted S-waves.
This is called the S-wave shadow zone.
There is also a P-wave shadow zone caused by the refraction of P-waves that occurs at
the core-mantle boundary due to the dramatic increase in density that occurs there.
This is how we know that the outer core is a in a dense, liquid state.
With an increasing network of seismometers all over the planet, seismologists have been
able use seismic waves to resolve Earth’s layers with increasing accuracy, especially
the many sub-layers of the mantle.
Speaking of the mantle, both P and S-wave velocities suddenly increase at around 30-40
kilometers below continental crust and 5-10 kilometers beneath oceanic crust.
This layer, the Mohorovičić discontinuity or MOHO, marks a sudden increase in density
and is the boundary between the crust and denser mantle.
Then at around 100 kilometers of depth, there is a large decrease in body wave velocity,
which is called the low-velocity zone.
This marks the transition from strong, brittle rocks to weak, ductile rocks.
The brittle layer above the low-velocity zone is called the lithosphere, and it contains
the crust and uppermost mantle.
The ductile layer below is called the asthenosphere and this is where the mantle is able to flow,
marking the boundary between brittle and plastic deformation.
The low-velocity zone is unusual because seismic velocities usually increase with depth.
Its existence is attributed to 1-5 percent partial melting of mantle rock.
The asthenosphere is the most ductile area of the mantle and extends down to 350 kilometers
below the surface.
Continent-sized slabs of the lithosphere, which again are called tectonic plates, move
along the top of the asthenosphere.
Earthquakes and volcanic activity typically occur at the boundaries of tectonic plates
where they collide or slide past each other.
The lower asthenosphere contains two additional abrupt increases in seismic velocities, one
at 410 and another at 660 kilometers.
The area in between these two discontinuities is called the transition zone.
Here, olivine, the dominant mineral in the mantle, changes crystalline structure, becoming
more dense to accommodate extreme pressures.
Below the 660 kilometer boundary is the lower mantle, or mesosphere, which is marked by
a very steady increase in seismic velocities.
At the bottom of the mantle lies the D’’ layer, which is identified by an area of anomalously
low and variable seismic velocities.
The D’’ layer is about 200 kilometers thick and is probably heterogenous in composition.
At the very base of D’’, right at the core-mantle boundary, exists an ultra low
velocity zone.
The name of this thin, laterally discontinuous layer speaks for itself, as it is characterized
by extremely low seismic velocities compared to surrounding mantle.
The exact nature of this layer is a topic of much debate.
It may represent an area of partial melting due to increased heat flux from the core.
Ultra low velocity zones only seem to exist in areas where there hasn’t been recent
subduction.
One exists beneath the center of the Pacific Plate, the other beneath western Africa.
If subducted slabs indeed end up sinking to D’’, they will have the effect of lowering
the temperature in that area, potentially influencing convection patterns in the outer
core, as well as core-mantle heat flux.
Therefore, ultra low velocity zones may represent areas beneath the hot, upward convecting outer
core, which causes partial melting of the D’’ layer above, potentially initiating
large mantle plumes, called superplumes.
At a depth of 2889 kilometers is the core-mantle boundary, which is defined by the disappearance
of S-waves and a slowing of P-waves, due to the outer core being in the liquid state.
P-wave velocity steadily increases with depth in the outer core until a jump in velocity
occurs at around 5100 kilometers, marking the boundary between the liquid outer core
and solid inner core.
Incredibly, the P-waves then make the journey through the other side of Earth’s inner
core and back through every layer on their way to the surface.
With Earth’s layers even better understood, let’s now explore them in motion through
one of the largest scale processes on the planet: plate tectonics.