Chapter 4: The Earth’s Interior
Chapter 4: The Earth’s Interior
What percent of the
Earth’s total volume is
made of crust?
 How can we study the
interior of the Earth?
1%


Why can’t we just drill
down to the mantle?
1.
2.
3.
4.
5.
6.
Drilling
Seismic waves
Earth’s magnetism
Measurement of gravity
Meteorites
Heat flow
1. Crust is too thick
2. Too expensive
3. Takes too long
What can we learn from the study
of seismic waves?
1. One important way for learning about the
Earth’s interior is the study of seismic
reflection. With seismic reflection,
seismic waves bounce (or reflect) from a
rock boundary deep within the Earth, and
return to a seismograph station on the
surface. This is just like light bouncing off
a mirror. Scientists can use this process
to calculate the depth of the rock layer.
Seismic Reflection
2. Another method is seismic refraction.
With seismic refraction, seismic waves
bend (or change paths) as they pass from
one material to another. Seismic waves
will bend toward the rock layer that is
made of lower-velocity (or slower
material). Refer to Fig. 4.2 on pg. 110.
Seismic Refraction
What is inside the Earth?
Seismic reflection and seismic refraction
have enabled scientists to plot the three
main zones of the Earth’s interior:
1. Crust - outer layer of rock; thin skin on
the surface
2. Mantle - thick shell of rock that
separates the crust above from the core
below
3. Core - central zone of the earth,
probably metallic and probably the
source of the Earth’s magnetic field
Interior of the Earth
Crust
Apple Analogy
Moho
boundary
The Crust
Studies of the crust have shown the
following:
1. The crust is thinner beneath the oceans
than beneath the continents
2. Seismic waves travel faster in oceanic
crust than continental crust (so, it’s
assumed that each is made of a different
type of rock)
Characteristics of Oceanic &
Continental Crust
Characteristic
Oceanic Crust
Continental
Crust
Avg. thickness
7 km
30-50 km (thickest
under mountains)
Density
3.0 g/cm3
2.7 g/cm3
Composition
Various types of Granite rock
covered with
rock
sedimentary rock
layer
The Crust (cont’d)
Mohorovičić discontinuity (Moho boundary):
This is the boundary that separates the
crust from the mantle
Note: The mantle lies closer to the Earth’s
surface beneath the ocean than it does
beneath the continents
(The Mohorovičić discontinuity [MOE-HOE-ROE-vee-cheech], usually
referred to as the Moho, is the boundary between the Earth's crust and the
mantle. Named after the pioneering Croatian seismologist Andrija
Mohorovičić)
The Mantle
Scientists believe that the mantle is made
mostly of solid rock. However, a few
isolated chambers of melted rock
(magma) do exist. Also, the rock of the
mantle is quite different than the rock of
the crust.
 The crust and uppermost mantle together
form the lithosphere which is relatively
strong and brittle.

The Mantle (cont’d)
Beneath the lithosphere is a 200 km thick zone
called the asthenosphere. Here, the seismic
waves travel more slowly, which suggests that
the rocks are closer to their melting point.
These rocks may be partially melted forming a
“crystal-and-liquid slush”.
 This is an important fact for two reasons:
1. Magma is probably produced here
2. Rocks have less strength & they probably flow
 So, the asthenosphere acts as a “lubricating
layer” which allows the plates to move.

The Core

Seismic wave data tells us a great deal
about the core. P-waves bounce off the
core or refract through the core. But
there is a “P-wave shadow” that has
allowed scientists to calculate the size and
shape of the core.
P-wave Shadow
Here, P-waves reflect (or
bounce) off the core
Here, the size and shape of the Pwave shadow can be used to
determine the size and shape of
the entire core.
Here, the P-waves refract (or
bend) as they pass though the
core
More on the P-wave Shadow
Videos
P-wave & S-wave Shadows
The Core (cont’d)
S-waves do not travel through the core at
all, which indicates that the core is liquid
or that it acts like a liquid.
 The way P-waves behave in the core
suggest that the core has two parts:
1. a liquid outer core
2. a solid inner core

What is the composition of the
core?
The core is made of metal (probably iron),
with small amounts of oxygen, silicon,
sulphur or nickel).
 The core is extremely heavy, and has a
density of between 10 and 13 g/cm3

How does the elevation of
continents change?



Isostasy is a balance between blocks of the crust that
are floating on the upper mantle. Remember, the crust
is not as dense as the mantle, so it floats.
The blocks of crust will rise or sink depending on their
thickness. Thicker blocks (such as mountains) will
extend into the mantle more deeply than other blocks.
In other words, the crust rises or sinks gradually until a
balance is achieved.
This balanced is called isostatic adjustment, and occurs
when “high spots” erode or when the crust bounces
back after a glacier has melted (please refer to pages
120 & 121 in the soft-covered books for diagrams and
more information).
Isostasy
Crust that is less dense will float higher than crust this is more dense.
Isostasy of Plates
Isostatic Adjustment
What can gravity tell us about the
Earth’s crust?




The force of gravity is greater between bigger objects.
For example, the force of gravity between the moon
and the Earth is greater than the force between two
bowling balls.
Scientists use a gravity meter (a weight on a spring) to
sense the amount of gravity.
More gravitational attraction is present when a heavy,
dense mass of rock is in the crust underneath the
gravity meter. Less attraction is present when a cave
or light rock is underneath.
Such gravity measurements can be used to learn more
about the structure of the Earth and to locate valuable
metals, minerals, and oil.
Earth’s Magnetic Field
What is the magnetic field?
A region of magnetism surrounds the Earth.
These invisible lines of force surrounding
the Earth deflect magnetized objects, such
as compass needles. The magnetic lines
connect at both the North and South Poles
How is the magnetic field
generated?
One widely accepted idea is that the mag. Field is created
by currents within the liquid outer core. The outer core
is hot and actually flows several kilometres per year.
What are magnetic reversals?
This happens when the magnetic lines of force run in the
opposite direction. So, the South Pole becomes the
North Pole and vice versa. In other words, the polarity
reverses. Evidence exists for this in rocks that contain
metal. One can see the lines in the rock change
direction.
What are magnetic anomalies?
Variations (or anomalies) in the magnetic
field can indicate different types of rocks.
Scientists use instruments called
magnetometers to measure the strength
of the magnetic field. For example, rocks
with more iron or metal will give off a
stronger magnetic field.
Geothermal Gradient
Geothermal Gradient:

This is the rate of temperature increases
with depth. The average temperature
increase is 25°C for every kilometre of
depth for the first few km’s. Some areas
have a much higher gradient, and some
have potential for geothermal energy
(such as Iceland). This temperature
gradient makes mines hot (near the
boiling point of 100°C in South Africa) and
makes drilling deep oil wells difficult.
Geothermal Gradient (cont’d)
The temperature gradient of 25°C/km
actually decreases substantially a short
distance into the Earth, down to about
0.3°C/km within the mantle.
 The core-mantle boundary has a
temperature of about 3800°C, 6300°C at
the inner-core/outer-core boundary, and
6400°C at the Earth’s centre. The
temperature at the centre of the core is
hotter than the surface of the sun!!!

Heat Flow
A small amount of measureable heat from the
Earth’s interior is gradually being lost through
the surface. This gradual loss of heat is called
heat flow. This heat could be “original heat” or
new heat that is created from radioactive decay.
This probably happens within rock that is rich in
uranium. Also, the average heat loss is about
the same for continental crust and oceanic crust.
END OF NOTES BEFORE MID-TERM EXAM!!
Next: Ch. 5 and Mineral Term Project (5%)