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.

          Examples: 

ü    Crust-mantle boundary (the "Moho")

ü    Core-mantle boundary

ü    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 = 25°C/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)

D2

 

       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

 

Athenosphere

Continental crust

Core

Discontinuity

Geothermal gradient

Gravity anomaly

Heat flow

Isostatic rebound

Lithosphere

Low-velocity zone

Magnetic declination

Magnetic reversal

Mantle

Moho (discontinuity)

Normal polarity

Oceanic crust

Principle of Isostacy

P-wave shadow zone

Reflection

Refraction

Reversed polarity

S-shadow zone