Now that we’ve discussed the stresses that affect Earth’s rocks, let’s go into detail about the
different layers of Earth. On the surface, Earth may seem like a relatively stable place, with the
occasional earthquake or volcanic eruption, but our experiences are limited to the uppermost part
of Earth, the crust. Earth’s crust comes in two varieties: continental and oceanic. Continental
crust is felsic, or rich in silica, and is around 20 to 70 kilometers thick. Oceanic crust is mafic,
or rich in iron and magnesium, and is around 5 to 10 kilometers thick. However, the most important
difference between oceanic and continental crust is their density. Oceanic crust is more dense
than continental crust, therefore when a piece of oceanic crust collides with continental crust,
the oceanic crust slides beneath the continental crust in a process that we’ve mentioned,
called subduction. The subducted oceanic crust will then sink deeper and deeper into the mantle
until eventually reaching the bottom of the mantle. Oceanic crust is constantly
being subducted and therefore destroyed. There are large areas of ancient, 4-billion-year-old
continental crust called cratons, but the oldest oceanic crust is only 340 million years old.
Below the crust is the mantle, a solid, but malleable layer with temperatures reaching
3500 degrees Celsius and pressures up to a million times greater than atmospheric pressure. Although
the mantle is solid, over millions of years, it convects like the world’s largest lava lamp,
as earth’s primordial and radiogenic heat is transferred to the surface and radiated to space.
Mid-ocean ridges and rift zones form beneath large areas of hot,
upwelling mantle, while subduction zones form beneath areas of cold, sinking mantle.
When subducting oceanic crust sinks into the mantle, it creates a suction-like force
in its wake, which pulls the subducting slab further into the mantle. This slab-pull force
is driven by buoyancy, due to oceanic crust becoming more dense with age. Eventually it
becomes more dense than the mantle beneath it, which is when subduction can begin. According
to the current scientific consensus, slab pull is the main force that causes plate tectonics.
The mantle extends from 70 to 2900 kilometers below the surface and is mainly composed of an
ultramafic rock called peridotite, which is mainly composed of the mineral olivine, or MgFeSiO4.
Geologists have found xenoliths, or strange rocks, of peridotite in massive volcanic eruptions.
By analyzing the minerals in peridotite, scientists can determine what depth the peridotite
came from. Diamonds, for example, are formed at depths of about 150 kilometers or deeper,
and are primarily found in another ultramafic xenolith called kimberlite.
At the bottom of the mantle lies a very mysterious, but geologically important
layer called D’’. This layer is marked by a large thermal gradient and an increased heterogeneity.
The exact nature of this layer is unknown, but there are some hypotheses. Some believe that
the D’’ layer is the “slab graveyard”, or the place where subducted slabs eventually settle.
Some believe that mantle plumes, or cylindrical areas of hot, rising mantle, initiate from within
D’’ in areas where there is increased heat flux from the core. Mantle plumes have caused some
of the largest volcanoes in Earth’s History, with modern examples being located in Hawaii,
Iceland, and Yellowstone. The accumulation of subducted slabs at D’’ also has implications
for triggering magnetic field reversals; but before we get there we must discuss the core.
At 2900 kilometers, the D’’ layer gives way to a liquid outer core composed mainly of iron and
nickel. Here, temperatures reach 5500 degrees Celsius, and pressures exceed 300 million times
the atmospheric pressure on the surface. Since this layer is a liquid, it can easily convect,
carrying heat from the inner core into the mantle through the D’’ layer. The rotation
of Earth on its axis further modifies the flow and creates large, cylindrical helices.
Since iron is an electrically conductive material, when it swirls through Earth’s magnetic field it
creates an electric field, which in turn creates an additional magnetic field, strengthening the
original field. This is called the geodynamo, and it is how earth maintains its magnetic field.
The geodynamo does require a “seed” magnetic field to get going, which was likely provided
during the T-Tauri stage of the early sun. At 5100 kilometers, the pressure becomes so great
that iron is forced into the solid phase. This boundary is called the Bullen discontinuity,
and it marks the division between the liquid outer core and the dense, solid inner core. The
inner core is about the size of the moon and has a temperature equivalent to the surface of the sun.
Though the inner core does not generate a magnetic field, it does help lock-in the field lines and
stabilize the magnetic field generated in the outer core. The inner core also rotates
faster than Earth as a whole. This phenomenon is called super-rotation. It is thought to be caused
by rotating magnetic fields in the outer core exerting a magnetic torque on the inner core,
similar to how an induction motor works. Magnetic fields induce electric fields in conductors,
and the created electric field manifests an additional magnetic field that interacts with
the original magnetic field. In an electric motor, the stator windings produce a rotating
magnetic field, and the rotating magnetic field induces an electric current in the rotor,
which induces a secondary magnetic field that interacts with the original rotating field causing
the rotor to spin, as it is “dragged” along by the rotating magnetic field of the stator.
Earth’s magnetic field undergoes periodic reversals where the field strength drops to
near zero, and the poles switch. Failed reversals also occur, where the field strength drops to
near zero and the original field eventually reestablishes. The last reversal was around
780,000 years ago and we may be due for one soon. It is not known what triggers magnetic reversals.
One hypothesis is the sudden collapse of large amounts of cold, subducted slabs
into the D’’ layer, which can reorganize outer core convection, altering the magnetic field.
Now that we know all about the Earth’s layers, lets discuss how they formed. When Earth first
accreted, it was an extremely hot, homogeneous planet, since the material present was primarily
chondrite meteorites. At some point early in its history, the combined heat of radioactive decay
and kinetic energy of the accretion process caused Earth to melt. The process of differentiation was
then able to begin, with the heaviest materials like iron and nickel, sinking toward the center
of the Earth, and the lightest materials, like silicon and oxygen, rising toward the surface.
Eventually, earth cooled enough to where the crust and mantle crystallized, with the core
remaining entirely molten for a long period of time. The start of crystallization of the inner
core is debated, with estimates ranging from about 2 billion to 570 million years ago. Earth has been
cooling, radiating its heat to space, since its formation. At the moment, the inner core
is growing at a rate of a millimeter per year as the liquid iron in the outer core solidifies,
forming new crystals of solid iron. If Earth were to exist in 91 billion years,
the Earth would have an entirely solid iron core, and Earth’s magnetic field would be gone. However,
it’s likely that the sun will engulf the Earth in a red giant phase long before then. A poetic
end to our hero’s journey, and an amazing sight for any sentient beings that were able to escape
the same fate, by venturing into the cosmos. So, with this overview of Earth’s structure
complete, let’s go into more detail about the physical properties of each of Earth’s layers,
and how geologists can detect those properties with seismic waves.