Chapter 2: Earth’s Interior
Earlier in the course we learned about the overall properties of Earth’s interior (see notes for
Chapter 1). In this chapter we’ll focus on how we know what we know about Earth’s interior.
Your text mentions meteorites and volcanic inclusions as two important sources of such
Earth’s average density of 5.5 grams per cubic centimeter (g/cm 3) is similar to the average
density of meteorites, which supports the idea that planet Earth is itself composed of meteoritic
Furthermore, some meteorites are composed mostly of iron and a small percentage of nickel.
These so-called iron-nickel meteorites are thought to represent the cores of fragmented
asteroids. Such iron-rich material is also thought to comprise Earth’s core.
Here’s a photo of my very own iron meteorite:
My wife got me this specimen one year for Christmas. She was up in Laguna Beach and saw it
for sale at a ridiculously low price in a rock shop.
Normally, an iron meteorite this large would sell for several hundred dollars; she picked it up for
less than $100…The funniest thing to me was she actually recognized a great deal on a
Most of planet Earth is composed of silicate minerals much less dense than such iron-rich
material, so without an iron-rich core, Earth’s average density would be much less than the 5.5
g/cm3 determined from Newton’s equation of universal gravitation.
If Earth’s core is very dense and composed of an iron-nickel mixture, this would yield a
reasonably good approximation of the measured value of Earth’s average density, so it’s likely
that Earth’s core is similar in composition to iron-nickel meteorites.
Deep-sourced magmas sometimes tear off fragments of Earth’s interior on their way up to the
surface, creating mantle-derived inclusions in volcanic rock. Here’s a photo of some mantle
Mantle inclusions in vesicular basalt.
(photo courtesy of Dr. Richard Busch; Image source: Earth Science World Image Bank)
The mantle inclusions are the green blobs, which are composed mostly of the mineral, olivine.
Mantle inclusions yield information about Earth’s interior, although as your text points out, such
inclusions are themselves probably from relatively shallow regions within the mantle.
So…Earth’s deep interior is completely hidden from direct observation.
How do we know anything at all about Earth’s deep interior if we’ve never sampled it directly?
Other than meteorite and laboratory studies, most of what we know about Earth’s deep interior
comes to us through the study of seismic waves—the same S and P waves we learned about in
Chapter 7 (Earthquakes).
You might recall, for example, that S waves can only pass through solid material as they
propagate outward from an earthquake focus, whereas P waves can travel through solids,
liquids, and gases. This bit of information is very important, as we’ll see below.
You should read the section entitled, “Evidence from Seismic Waves” very carefully.
Notice that in Figures 2.1, 2.3, and 2.4, seismic (S and P) waves travel outward from an
earthquake focus along three possible pathways, including direct waves (the straight line in
Figure 2.3), reflected waves (Figure 2.1) and bent, or refracted waves (Figures 2.3, 2.4, and
Here’s a sketch for clarification:
ground surface distant location
Earthquake (low velocity)
focus reflected wave
refracted (bent) (high velocity)
Possible seismic wave pathways.
Whenever a seismic wave crosses a boundary in Earth’s interior where the rock properties (e.g.,
density, compressibility) across the boundary differ, some of the wave energy bounces, or
reflects back upward, toward the surface (Figure 2.1 and above figure).
If we know the velocity of the seismic waves (which can be measured), then we can estimate
the depth to the reflecting layer. Similarly, seismic wave energy refracts (bends) when it crosses
an internal boundary across which rock properties change (Figures 2.3 and 2.4).
Check out this animation of seismic refraction--Figure 2.2.
As you read the above-referenced section and study the figures mentioned, you’ll probably
recognize why the reflection of seismic waves yields information about Earth’s interior.
Basically, strong reflections from multiple depths in Earth’s interior are the best evidence of all
that Earth’s interior is layered.
But seismic waves also strongly refract as they cross internal layer boundaries.
To understand why, let’s discuss the refraction process in a bit more detail. The following figure
will help clarify things:
Wave fronts (low velocity)
Seismic wave refraction.
As seismic waves propagate downward from the earthquake focus (star), the front of each wave
pulse can be represented by a dashed line perpendicular to the travel direction of the waves.
Let’s call these dashed lines wave fronts.
Eventually, a wave front will reach the boundary between the low- and high-velocity layers (A),
and the wave front will straddle this boundary. As it does so, only the lower (left-hand) part of
the wave is in the high-velocity layer, so only this part of the wave front speeds up, while the rest
of the wave (right-hand, upper part) continues to travel at the lower velocity. This causes the
wave front to bend (refract) as it crosses the layer boundary. This process is can be compared
to a line of marching soldiers. If one part of the line starts walking faster, the entire line changes
its marching direction as the other soldiers compensate to maintain the line.
As the refracted wave front moves through the high-velocity layer it continues to refract,
because the deeper part of the wave travels at a slightly higher velocity than the shallower part
of the wave. Eventually, the wave front arrives at B, where it straddles the boundary between
the two layers once again. This time, the upper, left-hand part of the wave front slows down as
it moves back into the low-velocity layer, while the lower (right-hand) part of the wave continues
to move faster because it’s still in the high-velocity layer. This again causes the wave front to
refract, sending it back up to the surface.
The situation just described only applies when a low-velocity layer is situated on top of a high-
velocity layer. As shown in Figure 2.2, seismic waves will be refracted downward rather than
upward if a high-velocity layer occurs on top of a low-velocity layer.
The overall point to appreciate is that seismic waves strongly refract and reflect at various
internal Earth boundaries, including the boundary between the crust-mantle boundary (called
the “Moho”), the mantle-outer core boundary, and the boundary between the inner and outer
cores. This is the best evidence of all that Earth is internally layered!
Notice in Figure 2.9 that S waves travel through the mantle. This if proof positive that the
mantle is solid, because S waves can only travel through solid material.
Also notice in Figure 2.9 that the liquid, outer core shields, or shadows the backside of the
planet from receiving any S-wave energy from an earthquake at the focus shown.
This so-called S-wave shadow zone is proof positive that the outer core is liquid rather than
To see an animated version of the S-wave shadow zone, check out this animation: S-wave
shadow zone--Figures 2.8 and 2.9.
P-wave shadow zones occur on the fringes of the S-wave shadow zone, which supports the
hypothesis that the mantle and core have very different physical properties, because if they
didn’t, P-waves wouldn’t refract as strongly as they move from the mantle into the outer core
Here’s a sketch of the S- and P-wave shadow zones:
S and P arrivals S and P arrivals
P Wave Shadow P Wave Shadow
P waves received here S Wave
S-Wave Shadow Zone
(no non-reflected S waves)
S and P Wave Shadow Zones.
In the above sketch, non-reflected S and P waves do not occur between 103º and 142º on both
sides of the planet, because these regions lie within both the S-wave and P-wave shadow
What we know and how we know it…
Way back at the beginning of the course, we discussed how Earth is an internally layered
planet, both with respect to composition and mechanical behavior.
Let’s summarize the evidence for the existence of Earth’s various internal layers:
The core-mantle boundary was discovered in 1906 by Richard
Oldham, a British seismologist.
Oldham noted that P waves generated during an earthquake
arrived later than expected at distant locations on the opposite
side of the Earth.
He correctly recognized this as evidence of a relatively dense
planetary core, through which P waves move more slowly than
in the mantle.
(photo courtesy of Wikipedia; http://en.wikipedia.org/wiki/Richard_Dixon_Oldham)
In 1909, a Croatian seismologist, Andrija Mohorovičić,
discovered the crust-mantle boundary based on the sharp
increase in seismic wave velocities across this boundary.
Though formally named the Mohorovičić discontinuity, this
boundary is commonly nicknamed the “Moho” in honor of its
(image courtesy of Wikipedia; http://en.wikipedia.org/wiki/Andrija_Mohorovi%C4%8Di%C4%87)
(courtesy of USGS; http://earthquake.usgs.gov/learning/topics/people/caltech_1929.php)
In 1913, German-American seismologist Beno Gutenberg argued that the Earth’s core must
be liquid, based on the presence of the S-wave shadow zone. Gutenberg was also
instrumental in the development of the Richter earthquake magnitude scale.
In the 1930s, Danish seismologist, Inge Lehmann discovered the
inner core, literally a core within a core, by explaining weak P
wave arrivals within the P-wave shadow zone as reflections off the
In 1929, after a large earthquake near New Zealand, Lehmann was
puzzled by P wave arrivals that should have been deflected by the
core. In a 1936 paper, she hypothesized that these P wave
arrivals had travelled some distance into the core and were
reflected off some boundary.
(photo courtesy of Wikipedia; http://en.wikipedia.org/wiki/Inge_Lehmann)
Generally, seismic wave velocities increase with depth, due to the effects of increasing
pressure. However, beginning at a depth of approximately 100 km, S and P wave velocities
decrease. This so-called “low velocity zone” extends to a depth of about 200 km and represents
a region within Earth’s mantle thought to be partially molten based on laboratory studies. Since
the development of plate tectonic theory, the low velocity zone is understood to be the seismic
expression of the upper part of the asthenosphere, where mantle rock exhibits highly plastic
The 410 and 670 km Discontinuities
Two other mantle discontinuities have been widely recognized at about 410 km and 670 km
(Figure 2.7). Seismic wave velocities increase sharply across both boundaries. The 410 km
discontinuity is thought to represent a change in crystal structure, where intense pressure
collapses the internal structure of olivine into a denser mineral, spinel. The origin of the 670 km
discontinuity is less certain; however, it may represent both a compositional change as well as a
change to a denser crystalline structure (spinel to perovskite).
Feel free to skim the section in Chapter 2 entitled, “Isostasy.” We’ll cover the concept of
isostasy in Chapter 5.
The Geomagnetic Field
After reading the section in your textbook entitled, “Earth’s Magnetic Field” you should
appreciate two things: (1) Earth’s magnetic field probably originates from fluid motions in the
outer core, and (2) Earth’s magnetic field reverses itself every so often at seemingly random
intervals. Check out this interesting computer simulation of a magnetic field reversal.
It’s difficult to explain the existence of Earth’s magnetic field except in terms of a liquid iron outer
core, because Earth’s interior is too hot for permanent magnetism to exist in the way it does for
cooler, surface rocks. So, the presence of a geomagnetic field is itself evidence that Earth
possesses a liquid outer core.
Feel free to skim the sections on gravity measurement and heat. Both topics are better treated
in upper-division courses and so I won’t hold you responsible for these topics on the exam.