Earth’s Interior
12
Earth's Interior begins with a brief look at gravity and layered planets followed by an examination of seismic
waves and the nature of Earth’s interior. The characteristics of the crust, mantle, and core are all examined in
detail along with a discussion of heat and heat flow with Earth. The chapter closes with a look at Earth’s
three-dimensional structure and Earth’s magnetic field.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Discuss the layered nature of Earth and how gravity is responsible for such layering in planets.
Understand and briefly explain the nature of seismic waves.
Explain the importance of seismic waves in determining Earth’s structure.
List and briefly explain the layers of Earth defined by composition.
List and briefly explain the layers of Earth defined by physical properties.
Compare and contrast the mechanical behavior of the lithosphere and asthenosphere.
Briefly discuss Earth’s major boundaries, including the Moho and the crust-mantle boundary.
Discuss the properties and characteristics of Earth’s crust, mantle, and core.
Explain the commonly accepted origin of Earth’s magnetic field.
Briefly discuss internal heat in Earth, including heat flow in the crust and mantle convection.
Chapter Outline___________________________________________________________________
I.
Probing Earth's interior
A. Most of our knowledge of Earth’s
interior comes from the study of
earthquake waves
1. Travel times of P (compressional)
and S (shear) waves through Earth
vary depending on the properties of
the materials
2. Variations in the travel times
correspond to changes in the
materials encountered
B. The nature of seismic waves
1. Velocity depends on the density and
elasticity of the intervening material
2. Within a given layer the speed
generally increases with depth due to
3.
4.
5.
6.
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pressure forming a more compact
elastic material
Compressional waves (P waves) are
able to propagate through liquids as
well as solids
Shear waves (S waves) cannot travel
through liquids
In all materials, P waves travel faster
than do S waves
When seismic waves pass from one
material to another, the path of the
wave is refracted (bent) provided that
the ray is not traveling perpendicular
to the boundary
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II.
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Seismic waves and Earth’s structure
A. The rather abrupt changes in seismicwave velocities that occur at particular
depths helped seismologists conclude
that Earth must be composed of distinct
shells
B. Layers defined by composition
1. Because of density sorting during an
early period of partial melting,
Earth’s interior is not homogeneous
2. Three principal compositional layers
a. Crust – the comparatively thin
outer skin that ranges from 3
kilometers (2miles) at the oceanic
ridges to 70 kilometers (40 miles
in some mountain belts)
b. Mantle – a solid rocky (silicarich) shell that extends to a depth
of about 2900 kilometers (1800
miles)
c. Core – an iron-rich sphere having
a radius of 3486 kilometers (2161
miles)
C. Layers defined by physical properties
1. With increasing depth, Earth’s
interior is characterized by gradual
increases in
a. Temperature – at a depth of 100
kilometers temperature is between
1200°C and 1400°C, whereas the
temperature at Earth’s center may
exceed 6700°C
b. Pressure – with depth, increased
pressure tends to increase rock
strength
c. Density
2. Depending on the temperature and
depth, a particular Earth material may
behave
a. Like a brittle solid
b. Deform in a putty-like manner, or
c. Melt and become liquid
3. Main layers of Earth’s interior based
on physical properties and hence
mechanical strength
a. Lithosphere (sphere of rock)
1. Earth’s outermost layer
b.
2. Consists of the crust and
uppermost mantle
3. Relatively cool, rigid shell
4. Averages about 100 kilometers
in thickness, but may be 250
kilometers or more thick
beneath the older portions of
the continents
Asthenosphere (weak sphere)
1.
Beneath the lithosphere, in
the upper mantle to a depth
of about 600 kilometers
2.
c.
d.
e.
III.
Small amount of melting in
the upper portion
mechanically detaches the
lithosphere from the layer
below allowing the
lithosphere to move
independently of the
asthenosphere
Mesosphere or lower mantle
1. Between the depths of 660
kilometers and 2900
kilometers
2. Rigid layer
3. Rocks are very hot and
capable of very gradual
flow
Outer core
1. Composed mostly of an
iron-nickel alloy
2. Liquid layer
3. 2270 kilometers (1410
miles) thick
4. Convective flow within
generates Earth’s magnetic
field
Inner core
1. Sphere with a radius of
3486 kilometers (2161
miles)
2. Material is stronger than the
outer core
3. Behaves like a solid
Discovering Earth’s major boundaries
A. The Moho (Mohorovicic discontinuity)
Earth’s Interior
1.
Discovered in 1909 by Andriaja
Mohorovicic
2. Separates crustal materials from
underlying mantle
3. Identified by a change in the velocity
of P waves
B. The core-mantle boundary
1. Discovered in 1914 by Beno
Gutenberg
2. Based on the observation that P
waves die out at 105 degrees from the
earthquake and reappear at about 140
degrees, but about 2 minutes later
than expected – this 35 degree wide
belt is named the P wave shadow
zone
3. Characterized by bending (refracting)
of the P waves
4. The fact that S waves do not travel
through the core provides evidence
for the existence of a liquid layer
beneath the rocky mantle
C. Discovery of the inner core
1. Predicted by Inge Lehmann in 1936
2. Region of seismic reflection and
refraction within the core
3. P waves passing through the inner
core show increased velocity
suggesting that the inner core is solid
IV.
Crust
A. Thinnest of Earth’s divisions
1. Varies in thickness
a. Exceeds 70 kilometers in some
mountainous regions
b. Oceanic crust ranges from 3 to 15
kilometers thick
2. Two parts
a. Continental crust
1. Average rock density about 2.7
g/cm3
2. Rocks exceed 4 billion years in
age
3. Composition comparable to the
felsic igneous rock
granodiorite
b.
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Oceanic crust
1. Density about 3.0 g/cm3
2. Rocks are 180 million years or
less in age
3. Composed mainly of the
igneous rock basalt
V.
Mantle
A. Contains 82 percent of Earth’s volume
B. Solid, rocky layer
C. Upper potion has the composition of the
ultramafic rock peridotite
D. Two parts
1. Mesosphere (lower mantle)
a. From core-mantle boundary to a
depth of 660 kilometers
b. The mineral perovskite (perhaps
Earth’s most abundant mineral) is
thought to be pervasive in the
lower mantle
c. D layer
1. Lowermost 200 kilometers of
the mantle
2. Sharp decrease in P-wave
velocities
3. Partially molten is some places
2. Asthenosphere or upper mantle
VI.
Core
A. Larger than the planet Mars
B. Earth’s dense central sphere
C. Two parts
1. Outer core – liquid outer layer about
2270 kilometers thick
2. Inner core – solid inner sphere with a
radius of 1216 kilometers
D. Density and composition
1. Average density is nearly 11 g/cm3
and at Earth’s center approaches 14
times the average density of water
2. Mostly iron, with 5 to 10 percent
nickel and lesser amounts of lighter
elements
E. Origin
1. Most accepted explanation is that the
core formed early in Earth’s history
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2.
Early Earth went through a molten
period because of heat released by
the collisions of in-falling matter
a. Heavy iron-rich materials
collected and sank to the center
3. As Earth began to cool, iron in the
core began to crystallize and the
inner core began to form
F. Earth’s magnetic field
1. The requirements for the core to
produce Earth’s magnetic field are
met in that
a. It is made of material that
conducts electricity and
b. It is mobile
2. Affects the rotation of the solid inner
core
a. Inner core rotates in a west-to-east
direction about 1 degree a year
faster than the surface – one extra
rotation about every 400 years
VII. Earth’s internal heat engine
A. Earth’s temperature gradually increases
with an increase in depth at a rate known
as the geothermal gradient
1. Varies considerably from place to
place
2. Averages between about 20C and
30C per kilometer in the crust (rate
of increase is much less in the mantle
and core)
B. Major processes that have contributed to
Earth’s internal heat
1. Heat emitted by radioactive decay of
isotopes of uranium (U), thorium
(Th), and potassium (K)
2. Heat released as iron crystallized to
form the solid inner core
3. Heat released by colliding particles
during the formation of Earth (no
longer operating)
C. Heat flow in the crust
1. Process called conduction
a.
The transfer of heat through
matter by molecular activity
b. Operates at a relatively slow rate
in crustal rocks, accounting for the
fact that the crust has a steep
temperature gradient – cool on top
and hot on the bottom
2. Rates of heat flow in the crust varies
a. Relatively high rates along the
axes of mid-ocean ridges
b. Relatively low heat flow in
ancient shields (such as the
Canadian and Baltic Shields)
D. Mantle convection
1. There is not a large change in
temperature with depth in the mantle
2. Mantle must have an effective
method of transmitting heat from the
core outward
a. Mass transport of rock must exist
within the mantle
b. Convection is the transfer of heat
by mass movement or circulation
in a substance
c. The rock in the mantle must flow
3. An important process in the Earth
a. The force that propels the rigid
lithospheric plates across the
globe, ultimately generating
1. Mountain belts
2. Earthquakes, and
3. Volcanic activity
b. Associated with convection are
1. Rising plumes of hot rock that
form near the core-mantle
boundary, and
2. The subduction of cool, dense
slabs of lithosphere where
downwelling occurs
4. Because the mantle transmits S
waves and at the same time flows, it
is referred to as exhibiting plastic
behavior – solid under certain
conditions and fluid under other
conditions (similar to manufactured
substances such as Silly Putty)
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103
Answers to the Review Questions
1. Gravity is the force responsible for the layering of planets because it causes the more dense materials to
sink inwards as a planet is forming. Each successive layer outward towards a planets surface is composed
of less dense materials, resulting in the layered structure observed in planets.
2. One reason for the increase in density within the mantle is due to the compression of existing minerals by
the intense pressures. Also, at such intense pressures certain minerals are no longer stable and they
undergo a mineral phase change that produces a new, more stable mineral.
3. Seismology is responsible for lower gasoline prices in that it provides the means by which geologists can
“see” into our planet and find the more favorable structures where petroleum is located.
4. Oceanic crust and continental crust are different in several ways. Oceanic crust is formed at oceanic
ridges, it is fairly similar in composition and thickness everywhere, and nowhere is it older than 200
million years. Continental crust is highly variable with many different compositions, it is formed in a
variety of ways, and it can be as old as 4 billion years. The thinnest crust is found underneath the oceanic
ridges while the thickest crust occurs underneath major mountain belts on the continents.
5. The cross-over distance can be used to determine the depth of the Moho, which also gives the thickness of
the crust at a particular location.
6. Since S waves travel only through solids and not liquids, the fact that they are readily transmitted through
the mantle indicates it is solid.
7. If significant amounts of water are contained in the mantle, they are most likely within the Transition
Zone.
8. In the upper part of the transition zone, olivine converts into a phase called B-spinel. In the lower portion
of the transition zone B-spinel converts into a true spinel structure called Ringwoodite.
9. At over 82 percent of the total Earth volume, the mantle is by far the most voluminous layer. Within the
mantle, the lower mantle contains 56 percent of the total Earth volume.
10. The “D” layer is similar to the lithosphere in that both of them exhibit large variations in composition and
temperature.
11. False. P and S waves do arrive in the shadow zone, but they are typically weaker than would be expected
if Earth did not have a core. Also, S waves generally do not arrive after entering the outer core since S
waves are blocked by liquid.
12. Earth’s core is 1/3 of the mass (despite being only 1/6 of the total volume) of the planet because it is
composed mainly of iron, which is a heavy element.
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13. Earth’s inner core continues to grow as the planet cools and iron crystallizes, thus increasing the size of
the inner core.
14. Heat flow is not evenly distributed from Earth’s surface because it is highest where magma is rising
towards the surface (at mid-ocean ridges) or in regions where high levels of radioactive isotopes exist.
15. Earth apparently increased in heat during its early formation because of several factors. Early collisions of
planetesimals converted kinetic energy into heat energy, many short-lived radioactive isotopes released
heat energy through decay, compression due to gravitational forces caused an increase in temperatures,
the collapse of the iron core released additional heat, and finally a collision with a Mars-sized object
created the Moon and heated the planet further.
16. Long-lived radioactive isotopes throughout Earth’s history have provided the heat energy for convection
that drives plate tectonics and keeps our planet from being a cold, motionless sphere of rock and metal.
17. Conduction is a heat transfer mechanism that operates through matter by molecular activity. Heat is
transferred by molecules vibrating into one another and therefore conduction is a slow process for many
materials. Convection refers to the transfer of heat by moving material in a fluid-like manner, operating in
both liquids and solids.
18. Convection is less efficient in high viscosity materials since the heat transfer occurs in a fluid-like manner
(remember that higher viscosity materials resist movement).
19. Because the lithosphere is a relatively stiff, rigid medium, conduction is more important for heat transfer
than is convection.
20. As the geotherm approaches the melting point, rock begins to weaken and get soft.
21. When the geotherm and melting curves cross one another, rock begins to melt.
22. Tectonic plates would have trouble moving without the asthenosphere since the weak, soft nature of the
asthenosphere allows the more rigid lithosphere to slide across it, thus creating plate movement.
23. The lithosphere is stiffer than the asthenosphere primarily because it is much cooler and therefore more
brittle and rigid.
24. Earth’s rotation has caused its shape to be slightly flattened, with the equator further from the center than
the poles. If Earth rotated faster in the past than it does now, it would have been much flatter with even
less of a spherical shape than it exhibits now.
25. A large layer of iron ore would cause a positive gravity anomaly compared to the rocks surrounding it
because the increased mass of the iron would cause a greater gravitational force.
26. The mid-Atlantic ridge appears as a slow seismic velocity anomaly because the increased heat flow along
the ridge causes seismic velocities to slow down.
Earth’s Interior
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27. The three sources of convection in the outer core are 1) thermally-driven convection by the outermost
core fluid cooling and sinking; 2) chemically-driven convection caused by the crystallization of solid iron
at the bottom of the outer core; and 3) additional thermal convection from radioactive decay.
28. During a magnetic reversal, Earth’s magnetic field reverses polarity so that the north needle on a compass
would point to the south. Also, during the reversal the strength of the magnetic field decreases to about 10
percent of normal and the locations of the poles begin to wander.
29. Along with the atmosphere, Earth’s magnetic field protects the surface from ionized particles emitted by
the sun. If the strength of the magnetic field decreases greatly during a reversal, the increases amounts of
solar wind reach Earth’s surface could cause health hazards for humans.
30. Magnetic reversals would probably occur less frequently if large volumes of subducted oceanic
lithosphere sank to the bottom of the mantle. This would most likely displace hot rock at the base of the
mantle, causing some of it to rise to the surface and erupt as flood basalts. The sudden displacement of
cold oceanic lithosphere next to the core would chill the uppermost core, causing more vigorous core
convection that would prevent the magnetic field from weakening and reversing.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).
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