Great question! We have several sources of
evidence that suggest the earth is layered.
source of information is meteorites that fall to
Earth. How do these meteorites from outer
tell us about our own planet? Well, geologist and
planetary scientists think that small meteorites
represent the same stuff (elements) that our
planet is made out of. However, the rocks at the
surface of the earth are more enriched in some
elements (for example, silicon, oxygen, aluminum,
potassium...) and more depleted in others (nickel,
iron…) than these meteorites. That tells us that
there must be complimentary reservoirs in
different layer of the earth. For example, there
must be layers with more iron and nickel than in
the crust to account for the depletion of these
elements in the crust relative to meteorites. In
fact, we think that much of the iron and nickel
are concentrated in the core of the earth.
Field geology can sometimes tell us
layer of the earth. There are some places where
cross-sections of rock show different layers of
the crust and mantle. A good example of these
field exposures are ophiolites, which are
of oceanic crust that have been thrust up on land.
In many ophiolites, we can see the upper layers of
the earth from the upper mantle through the crust
(check out the wikipedia article on ophiolites):
Another line of evidence for different layers
comes from the study of rocks known as
(these literally means foreign rock) that are
brought from very deep in the earth to the surface
in volcanoes. These rocks have very different
compositions than the rocks on the surface of the
earth, so we know that they must have come from a
different layer. Some of these rocks contain the
mineral diamond, which forms at pressures of ~ 50
kilobars (about 50,000 times the pressure of our
atmosphere at sea level; Kennedy & Kennedy, 1976).
This pressure corresponds to a depth of ~150 km
below the surface of the earth. This is how we
know they came from very deep.
Another major (maybe the most important) line
of evidence comes from the study of seismic waves
released by earthquakes. Seismic wave travel
through different materials at different speeds,
and some waves don’t even travel through some
types of material. In general, seismic waves
travel faster through material that is denser.
Geophysicists can locate the source of earthquakes
and measure how fast it takes a wave to arrive at
a seismograph station far away. If the wave
arrives faster than expected, that usually means
that it sped up by passing through something
dense. When waves are collected at a lot of
stations, we can determine where the dense layer
is. This type of technique allows geophysicists to
determine the location of the boundary between the
crust and the mantle of the earth, where seismic
p-waves increase in velocity from ~6.7 kilometers
per second to ~8.1 kilometers per second.
seismic waves also led to the discovery that the
outer core of the earth is molten (liquid) because
it does not transmit s-waves, which only travel
through solid materials.
Kennedy, C. S., & Kennedy, G. C. (1976). The
equilibrium boundary between graphite and diamond.
Journal of Geophysical Research, 81(14),
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