Brief Review from yesterday
• We claimed that the Earth has 3 main layers
 Crust
 Mantle
 Core
• 2 of these layers (Mantle & Core) are broken into subgroups due to
differences in composition and physical properties
 Mantle consists of (order from least to greatest depth) lithosphere, asthenosphere, and
mesosphere
 The core consists of an “outer core” and an “inner core.” Though both are part of what
we call the core due to their composition, they have very distinct physical properties
• Because we just barely “scratched the surface” yesterday, let’s break these
layers down a bit and get a “deeper” understanding
 2 puns in one sentence without intention….not bad….not good…..but not
bad ;)
The Crust
• Though we generally refer to the crust as any
continental plates, geologists separate the crust into
continental and oceanic crust for 3 reasons:
 Difference in density and therefore composition
 Difference in actual depth
 Difference in formation and therefore age
Characteristics
Continental
Oceanic
Density
Less dense
More dense
Depth
20-70 km
About 7 km
Age
Around 4 billion yrs
Around 200 m.y.
The Crust
 The oceanic crust at the island of Hawaii is about 5 km thick
 Thickness of the continental crust under eastern California ranges from 25 km
under the Great Valley to 60 km under the Sierra Nevada where Mount Whitney is
The Crust
• So why is oceanic crust much thinner AND
younger than continental crust?
• Unlike continental crust, oceanic
crust is actively being created
• At mid-ocean ridges, magma
erupts on ocean floor and
horizontally pushes previously
created ocean crust in a conveyorbelt fashion
• When oceanic crust encounters
continental crust, it becomes
“subducted” because of its greater
density
• Oceanic crust then returns back to
mantle where it is re-melted into
magma
The Mantle
• Constitutes about 84% of Earth’s volume
Lithosphere
 The uppermost mantle plus
overlying crust
 Rigid
Asthenosphere
 Resembles a much more “plasticlike” substance due to the T & P
conditions that allow rock to
become bendable w/o breaking
 Area of low density
 Important to tectonic plate
movements
Mesosphere
 Lower part of mantle
 Though the rock is hot, the
pressure makes the rock solid
The Core
• Huge ball of liquid and solid iron (mostly)
Outer Core
 Radius of 3400 km
 Liquid due to low enough pressure (which is still huge
amounts!) and extreme temperature
 Contains Iron and Nickel
 Creates Earth’s magnetic field which protects life from
solar radiation
Inner Core
 Radius about 1200 km
 Solid ball of iron-nickel alloy
 Temperatures comparable to the sun! (over 10,000
degrees Fahrenheit)
 SLOWLY growing in size due to the cooling of the outer
core where the 2 cores meet
Sounds good, but how do we know it’s
true?
• What about the “hollow Earth theory?” 
• http://www.youtube.com/watch?v=ZlkjtiebZw
k
• Look for the evidence!!
Why waves???
One of the properties
about any type of wave is
that it will change its
speed & direction as it
moves from one
substance to another
If we have a good
understanding of how certain
waves move through certain
substances, we can use this
information to infer what kind
of substance a wave moves
through without having even
seen the actual substance
Represent different layers
in the Earth
Notice how waves are “bent” each time it encounters a
new layer
 Time it takes waves to reach microphone also allows
us to determine depth
Oil companies use this concept
 Help determine where the oil is
 Help determine depth
 Provides information as to what material they will need to drill through
So lets get back to the inside of the
Earth
Here was the hypothesis:
 IF the crust and the mantle were composed of the same material, then the wave
traveling the shortest distance would be the first to register at the seismograph
But that’s not what actually
happened…
So what did happen?
 The seismic waves that traveled through the mantle, which was a greater distance,
actually arrived at the seismograph FIRST!
• Andrija Mohorovicic, a Croatian seismologist discovered this in 1909
 Moho realized that the waves must have been traveling through a different substance,
specifically a substance with greater density, in order to arrive earlier
 Labeled this boundary between the crust and mantle as Mohorovicic Discontinuity or
simply “Moho”
What about the rest of the Earth?
Only high energy seismic waves from nuclear explosions or earthquakes can travel all the
way through the Earth and back to the surface
• If the interior of the Earth was made of the same substance and density, we would
expect to see the seismic waves travel in a path like the diagram below
However…..
Instead, what we see is something like this…
 What this means is that something caused these waves to bend and travel at
different velocities
 The increase in density increases the speed at which the waves travel
Therefore, the Earth
is not made of the
same material and
has different density
throughout!
In addition….
We noticed something else very interesting….
 Some of the waves, called S waves, never reached the opposite side of the Earth
 Scientists knew from previous experiments that S waves CANNOT travel through
liquid, only through solid
What this meant was that
something inside the Earth
stopped these S waves from
traveling through
 Therefore, there must be a
liquid layer inside the Earth
(outer core)
 Size of the shadow zone
helped us understand the
size of the layer that was
liquid
What about the inner core?
We cannot rely on S waves to reveal the inner core because, as we saw, they stopped at
the liquid outer core
 Because of this, we have to rely on another type of seismic wave called a P wave
 P waves can travel through both liquid and solid
The waves bend AGAIN
after passing through
outer core!
 This has to mean that
there must be another
layer within the outer
core
To summarize….
Mostly made of
basalt
Mostly made of
granite
What I expect you to know about
Earth’s interior
• Names of the different layers
• General depths (which is bigger and smaller)
• General physical properties of different layers
(pressure and state of matter)
• Explain how we know of the existence of
different layers
 Use evidence to support such claims
So why do scientists laugh at such a
theory as the hollow Earth?
• Use of seismic waves
Seismic waves
 Vibrations that carry energy from the source
of the shaking outward in all directions
 Concept is similar to the “pebble in pond”
experience we’ve all had
 To no surprise, an earthquake is a much
more complicated process and therefore has
much more varied seismic waves
So what waves are we talking about then?
 P waves (primary)
 S waves (secondary)
P and S waves
P & S waves
When an earthquake occurs, the seismic waves (P
& S) spread out in all directions through Earth’s
interior
Scientists analyze this seismic data by specifically
looking for a few things
 The velocity the waves travel at throughout the
interior of the Earth
 Whether or not they actually travel through
them at all
What are some characteristics of P and S waves while traveling through interior?
• Depend on the material properties such as composition, temp., pressure, and density
• Travel more quickly through denser materials
• Hot areas slow down seismic waves
• Seismic waves move more slowly through a liquid than a solid
• Molten areas slow down P waves and stop S waves (cannot travel through liquid)
It’s all about the Moho
Mohorovicic Discontinuity (Moho)
 Boundary between the crust and the
mantle
 Discovered by Andrija Mohorovicic, a
Croatian seismologist, in 1909
 The word “discontinuity” in geology is
used for a surface at which seismic
waves change velocity
 Moho saw this increase in velocity and
inferred that the acceleration must be
caused by a higher density material (a
different layer)
Has anyone ever seen the Moho?
• No one has ever been deep enough into the Earth and no wells have ever been
drilled deep enough
• Drilling wells to that depth (avg. of 32 km) are insanely expensive and very difficult
• Deepest well ever drilled about 12 km