Geology Lecture Outline

The Earth's Interior (Ch 12)


I. Lecture Content

Introduction Earth Anatomy 101

Seismic Waves - Tool for "X-Raying" the Earth

The Dense Metallic Core - How Do We Know It is There?

The Thick Stony Mantle - The "Meat" of our Planet

The Thin Rocky Crust - The "Skin" of our Planet

Earth's Interior Heat - What Makes the Earth Dance

Gravity - Its Nature and How to Measure It

The Principle of Isostacy - The "Floating" Concept

Earth's Magnetic Field - A Spin-induced Bar Magnet


I. Introduction

A. Indirect Means of Calculating Earth's Density

1. Comparison of gravitational bodies


2. Examining orbital relationships of Sun and Planets


3. Average density for the Earth is 5.5 grams/cubic cm


B. Earth's Layering is Recognized from Seismic Data

1. Seismologists use seismic waves that travel

through the Earth to image Earth's interior.


        Much like a doctor or dentist who use X-rays to

image a patient's bones and internal organs


        Totally indirect means of recognizing Earth's

internal makeup and structure


C. The Earth is a Concentrically Layered Body

(See Figure 10.2 and Table 10.1)

Right Brace: 34Ur8843 Mantle - solid (plastic) 1. Inner core - solid - 1% of volume 32% mass

2. Outer core - liquid - 16% volume

3. Mantle - solid - 83% of volume - 68% mass


4. Crust - solid - 0.6% of volume - 0.4% mass


II. Seismic Waves - A MEans of "X-raying" the Earth

A. The Behavior of P&S Waves Varies Systematically as

They Travel Through the Earth


1. The velocity or speed (i.e. travel time) of seismic waves

traveling through the Earth varies as a function of both

rock density and elasticity. (See Fig. 10.6)


        Speed decreases with increasing density


        Speed increases with increasing elasticity


        Wave speed generally increases with depth because the

elasticity of the rock increases much faster with depth

than does the density.


        Changes in rock density and/or elasticity are due to

changes in three things:

1) rock composition

2) lithostatic pressure (depth)

3) temperature


        S-waves do not travel through liquids (outer core).

(See Fig. 10.9)

2. The direction of seismic waves traveling through the

Earth also varies as a function of both rock density and

elasticity, and is intimately related to changes in the

speed of the waves. (See Figs. 10.5, 10.7 and 10.8)


        Seismic waves refract (bend) when they travel

through rocks that are changing in density and

elasticity character, due to changes in composition

and physical conditions.


        Seismic waves may also reflect when they meet

a sharp boundary between rocks of greatly different

character; this sort of boundary between two distinct

rock layers is called a discontinuity.


    Crust-mantle boundary (the "Moho")

    Core-mantle boundary

    Inner-Outer core boundary


B. The Systematic Changes in the Speed and Direction

of Seismic Waves Traveling Through the Earth are

Recorded by Seismographs from around the World


1. Seismologists use the data to calculate the depth, density

physical character, and composition of the Earth's

rock layers, and develop a map of the Earth's interior.


2. Seismic waves reveal several major discontinuities:

        Interface between inner and outer core

        Interface between upper and lower mantle

        Interface between athenosphere and the lithosphere

        Interface between crust and the mantle

       Interface between oceanic and continental crust



III. Earth's Metal Core - Imaged by Seismic Shadows

A. Discovery of a Seismic "Slow Zone"

1. Seen at stations 130 degrees or more from focus


2. Indicates that Earth has a core material different

from the overlying mantle.


3. P-wave slowdown marked at depth of 2900 km images

the core-mantle discontinuity.

B. Discovery of Two Seismic Wave "Shadow Zones"

1. Faint P-wave shadow zone between 103 and 143 degrees

(Fig. 10.7 and 10.8)


2. Total S-wave shadow zone at locations greater than 103

degrees (see Fig. 10.9)


3. P-waves abruptly speed up at depth of 5200 km.

4. The two seismic shadows and the P-wave speed jump at

5200 km indicate that the Earth's Core is divided into

a liquid outer core and solid inner core.


C. Nature of the Earth's Core - An Iron-rich Metallic Ball

1. Density of core varies from 10 to 13 g/cm3


2. Pressure at Earth's center is 3.5 million times that of

normal atmospheric pressure.


3. Temperature in the core is over 5000 degrees C.


4. Inner Core composed of a crystalline alloy of iron and nickel

5. Outer Core composed of liquid blend of mainly iron and

some sulphur, plus a little silicate material

       Addition of sulphur lowers melting temperature

which helps keep outer core molten


IV. Earth's Stony Mantle - The Silicate 'Meat' of Our Planet

A. Discovery of Two Distinct Sets of P- and S-Waves at

Shallow Depths Marks the "Moho" (see Fig. 10.10)


1. A deeper faster set of P-and S-waves arrives sooner

than a shallower slower set of P- and S-waves.


2. The two sets of waves delineate the boundary between

the crust and the top of the upper mantle, called the

Mohorovicic discontinuity or "Moho" for short.


3. The Moho is present everywhere except under the mid-

ocean ridge spreading centers.


4. The Moho varies in depth: very shallow beneath ocean

basins, and deeper beneath continents.

       5 to 10 km beneath ocean crust

       20 to 90 km beneath continents (35 km average)


B. Variation in P- and S-wave patterns Indicate that the

Mantle is Vertically Layered

1. Lithosphere mantle (upper Upper Mantle)


2. Athenosphere (lower Upper Mantle)


3. Lower Mantle


C . Nature of the Earth's Mantle - It's All Stony Stuff


1. Density of mantle varies from 3.3 to 5.7 gm/cm3


2. Mantle is fairly homogeneous in composition


       Consists mainly of Peridotite - Fe/Mg-rich silicate

mineralogy = Equivalent to Olivine + Pyroxene


3. Very thick layer - Almost 3000 km-thick


4. Multiple seismic profiles made across the entire Earth,

(similar to doing a CAT scan of a person's body) has

revealed numerous anomalous "hot" and "cold" regions

throughout the mantle.


       This method is called seismic tomography.


       These hot and cold regions are a good indicator

of active mantle convection (cells).


       These convection cells are believed to be the

driving force for plate tectonics.



V. Earth's Thin & Complicated Crust

A. Seismic Imaging of the Earth's Crust Reveals the

Greatest Variations of All of Earth's Solid Layers


1. Crust has considerable vertical and lateral variation


       Chemically and Physically


       Relatively very thin


       Least dense of Earth's solid layers


2. Crust is classified into two distinctive types, based on

seismic velocities and densities.


       Continental crust - lighter and thicker


       Oceanic crust - denser and thinner


B. The Nature of the Earth's Crust

1. Continental Crust

        Density = 2.0 to 3.0 g/cm3 (avg. = 2.7 g/cm3)


        Composition = Variable (Average = Granodiorite)


       Thickness = 20 to 90 km (avg. = 35 km)


2. Oceanic Crust

        Density = 2.9 to 3.2 g/cm3 (avg. = 3.0 g/cm3)


        Composition = More consistent (Average = Gabbro)


       Thickness = 5 to 10 km (avg. = 35 km)


VI. Earth's Interior Heat - Driving Force of Change

A. Temperature Increases with Depth


1. Crustal Geothermal Gradient = 25C/km


2. Roughly 1000 C at base of crust


3. Roughly 4000 C at base of mantle


4. Maxing out at around 6500 C at the Earth's center

B. Sources of Heat Coming from the Interior of the Earth


1. Radioactive decay of elements in the mantle

       The major heat contributor


2. Residual gravitational heat of planet accretion

       Mostly from cooling of the core


C. The Earth is a Massive Heat Engine


1. Heat drives convection in the core and mantle


2. Heat off the core fuels mantle plumes


3. Mantle convection drives plate tectonics


VII. Gravity and The Principle of Isostacy

A. Gravity is a Very Powerful Universal Force

1. Law of Universal Gravity


       Attractive Force


       Proposed by Sir Isaac Newton

       F = G(m1 x m2)



B. Variations in the Force of Gravity on Earth

1. Measured with a gravimeter


2. Value depends on geographic location

       Equator versus the poles


       Sea level versus mountaintop


       Normal values are those expected for a given location

with a simple, hypothetical cross section


3. Anomalous values are associated with unique geology

that have either a mass excess (positive anomaly) or

mass deficiency (negative anomaly) (see Figs. 10.15/16)

        Ore deposits (+)

        Rootless mountain ranges (+)

        Extended crust (+)

        Ocean trenches (-)

        Salt Domes (-)

        Deep Sediment Basins (-)

        Magma chambers (+ or -)

       Uncompensated crust (+ or -)


C. The Principle of Isostacy - Layer Floating on Layer

1. Gravitational studies of massive mountain ranges

reveals a "floating" equilibrium between the lighter

crust and the denser mantle. (see Fig. 10.16)


       Isostatic "compensation" effect

       Similar to floating icebergs and ships (Fig. 10.17)


2. The Principle of Isostacy explains why the lighter and

thicker continents stand high and the more dense and

thinner ocean bottoms sit low in relation to the even

denser underlying mantle.


3. Loading or unloading of the crust produces a sinking

(subsidence) or rising (uplifting) equilibrium response,

respectively. (see Figures 10.18 and 10.19)


4. Crust in the process of uplift, after having undergone

unloading, due to either melting of an ice cap or major

crustal erosion, is said to be experiencing isostatic

rebound. (see Figures 10.18 and 10.19)


5. The underlying mantle acts like a viscous liquid (plastic)

during subsidence or uplift events of the crust.


VIII. Earth's Magnetic Field - electromagnetic Dynamo

A. The Earth Has a Self-Generated Dipolar Magnetic Field


1. Acts like a familiar bar magnet (see Figs. 10.21 & 10.22)


2. Convection currents in the iron-rich liquids of the outer

core, coupled with the Earth's rotation, generates a

complex electrical current within the outer core.


3. The electrical currents, in turn, induce a magnetic field.


4. The induced magnetic field, in turn, induces a

secondary electrical current, which in turn generates

another magnetic field which combines and sustains

the already established field.


        This effect is called a "Self-exciting Dynamo"


        The Earth is a self-perpetuating magnetic dynamo.


5. Earth's dipolar magnetic field does not coincide with

the geographic polar axis (off by 11.5 from true North).


B. Inclination of Earth's Magnetic Field


1. Magnetic lines of force envelope the Earth and the space

around it - forming a 3-dimensional "donut-shaped"

magnetic field.


2. Magnetic lines of force near the equator parallel the

Earth's surface (horizontal). (see Figs. 10.22 & 10.23)


       Earth's magnetism is weakest at the equator


       Horizontal lines of force are defined as having a

magnetic inclination angle of 0 (= no inlc.)

3. Magnetic lines of force are vertical near Earth's polar

rotational axes. (see Figs. 10.22 & 10.23)


       Earth's magnetism is strongest at the poles


       Vertical lines of force are defined as having a

magnetic inclination angle of 90


4. Magnetic lines of force are oriented at varying angles to

the Earth's surface at locations between the equator

and the poles.


       Low magnetic inclination angles near equator with

increasing angle size towards the poles


C. Declination of Earth's Magnetic Field

1. Earth's magnetic north and south poles don't coincide

with the planet's geographic poles (axis of rotation).


       Magnetic north is off by 11.5 from True North


       Position of magnetic poles remain close to the planet's

geographic poles for vast majority of recorded time.


2. Locations on Earth where magnetic north and geographic

do not correspond have a magnetic declination angle.

(see Figs. 10.23 and 10.24)


D. Magnetic Polarity Reversals

1. Earth's dipolar magnetic field has reversed its polarity

many times throughout geologic times


       Positive (+) magnetic pole switches to negative (-)

magnetic pole, and vice-versa


2. Normal Polarity is defined as when Earth's north

magnetic pole is positive (+).


3. Reverse Polarity is defined as when Earth's north

magnetic field is negative(-).


4. Currently, the Earth's north magnetic pole is (+) and the

south magnetic pole is (-).


       Earth is now in a normal polarity cycle


5. Each polarity reversal is marked by a weakening of

Earth's magnetic field, typically accompanied by a

migration of the magnetic poles (magnetic excursions)


E. Earth Magnetism Frozen in Rocks (Time)

1. Iron-bearing minerals can align themselves magnetically

with Earth's magnetic field.


  Crystallizing iron-bearing minerals in a magma that

become volcanic and plutonic rocks.


  Deposited iron-bearing minerals sediment that become

sedimentary rocks


2. Magnetization in iron-bearing minerals can only occur

below a certain temperature, called the Currie point.


3. Iron-bearing minerals that are heated beyond the Currie

point lose their magnetization.


4. Rocks like frozen lava flows record ancient magnetic fields

        Records the direction and strength of field

        Paleomagnetism - study of ancient magnetic fields


IX. Vocabulary - Chapter 10



Continental crust



Geothermal gradient

Gravity anomaly

Heat flow

Isostatic rebound


Low-velocity zone

Magnetic declination

Magnetic reversal


Moho (discontinuity)

Normal polarity

Oceanic crust

Principle of Isostacy

P-wave shadow zone



Reversed polarity

S-shadow zone