Chapter 17 Earth's Interior
I. OVERVIEW: Information on the earth's interior is primarily obtained through:
A. Earthquake observation Fig. 17.1
B. Nuclear testing which provides the precise location and time of the seismic activity.
C. High pressure-temperature laboratory experiments
D. meteorites and space exploration
II. CHARACTERISTICS OF SEISMIC WAVES
A Velocity of seismic waves is determined by the elasticity and density of the material it travels through
1. seismic waves travel fastest through rigid materials, where the rock behaves elastically. Crystalline materials transmit waves faster than unconsolidated materials.
2. the denser the material the faster the waves travel. Basalts have a density of 3.0g/cm3 and transmit faster than granites with a density of 2.8 g/cm3. The peridotites of the mantle have a density of 3.5 g/cm3.
3. The density increases with depth because pressure increases, squeezing the rock into a more compact elastic material, therefore the waves speed up.
4. P waves travel through solids and liquids because these materials resist changes in volume and spring back to their original shape.(Fig. 17.2A)
5. S waves do not transmit through liquids or air because these materials do not offer resistance when its shape is changed. (Fig. 17.2B)
6. P waves travel faster than S waves through any solid
7. The waves speed up with depth because increased pressure enhances the elastic properties of rock. The waves are continuously bent and appear to travel along curved paths. (Fig 17.4)
8. When the waves pass the boundary between two dissimilar materials the wave is either reflected or refracted (bent). This surface is called a discontinuity. (Fig. 17.5)
B. The Layering of the Earth
1. World-wide discontinuities revealed that there were distinct layers of materials of varying composition or structure within the Earth.
a. compositional layering: during the Earth's formation, heavier materials sank and the lighter ones floated.
b. structural layering: results when there is phase change of material of the same composition; solid to liquid, solid to plastic; or recrystallized to a denser crystalline structure.
2. Four major layers of the Earth (Fig. 17.6), with a 6371km. radius:
a. the crust- thin outer layer, 5-50 km thick
b. mantle- a rocky layer, 2885 km thick
c. outer core- mobile liquid, 2270 km thick
d. inner core - solid metallic sphere 1216 in radius
III. NATURE OF THE EARTH'S INTERIOR
A. Mohorovičić Discontinuity or "Moho": the boundary between the crust and the mantle. Discovered by Yugoslavian seismologist Andrija Mohorovičić in 1909.
P waves arriving at close recording stations have average velocities of 6 km/sec. At more distant stations the waves arrived faster; having speeds of 8 km/sec. (Fig. 17.7)
B. P wave shadow zone: discovered by German seismologist Beno Gutenberg.
1. P waves die out about 105o from an Earthquake, then reappear about 140o away. This shadow zone is produced by the bending of the P waves which enter the core. (Fig. 17.8)
2. the Gutenberg discontinuity is the boundary between the mantle and the core
3. P wave velocities suddenly decrease about 40% as they enter the core, indicating a reduction in elasticity.
C. S wave shadow zone: S waves do not propagate through the core. Any location more than 105o from the earthquake epicenter will not receive direct S waves. This information indicates that at least a part of the core is liquid.
Fig. 17.9 summarizes the occurrence and paths of P and S waves.
D. Inner core: in 1936 seismologists discovered reflections from a boundary within the core. These reflections were precisely located and measured during nuclear testing in the 1960's and indicated the presence of a solid inner core.
IV. THE CRUST
A. Thickness: Less than 20 km thick on average.
1. ocean crust: 5 km on average.
2. continental crust: 35 km on average.
a. prominent mountains reach 60 km thick
b. stable interiors are 35 km thick on average
B. Composition
1. continental crust: P wave seismic velocities of 6 km/sec.
a. Direct observation and laboratory experiments indicate a density of 2.8 g/cm3 and a composition of andesite.
2. Ocean crust: P wave seismic velocities of 7 km/sec.
a. Direct observation and laboratory experiments indicate of density of 3.3 g/cm3 and a composition of basalt.
V. THE MANTLE
A. Over 80% of the Earth's volume. Information about the mantle's composition comes from experimental data and from rocks known as peridotites found in kimberlite pipes. Composed of olivine, pyroxene and garnet.
B. S waves are propagated; therefore the mantle behaves like an elastic solid.
C. Temperature does not increase greatly with depth like the crust, therefore a different method of heat distribution (other than conduction because rock conduct heat very slowly) transmits the heat. Convection is believed to be the method, and mantle material must be capable of flow.
D. Plastic behavior
1. under stress of short duration, such as seismic waves, the material behaves as an elastic solid
2. under stress of longer duration, the material flows. Under these conditions the material is unable to store strain and cannot generate earthquakes.
E. Lithosphere: the crust plus the brittle portion of the upper mantle. Extends from the surface to 100 km, in depth. (Fig 17.11)
F. Asthenosphere: A global zone of weak rock below the 100 km level extending to a depth of 400 km sometimes to 700 km in some areas.
1. The low velocity zone is found within the upper zone of the asthenosphere. The low velocity zone occurs between the depths of 100-250 km. P and S waves slow down. A zone of partial melting exists: part melt, part crystals. (Fig. 17.11)
a. found below the oceans and some portions of the crust
b. absent under the older, stable shield areas
c. rocks here are nearer to its melting temperature than the rock above or below (fig 17.13)
G. 400-700 km seismic velocities speed up due to phase changes of the silicate minerals. Olivine in peridotite compacts to the mineral spinel having a denser structure.
H. Between 670-2885 km. seismic velocities speed up again, the spinel is believed to transform to perovskite
VI. THE CORE
A. Larger than Mars. The core is 1/6 the volume and 1/3 the mass of the Earth. Temperatures of 4000o-5000o
1. Liquid outer core is 2270 km thick, solid inner core is 1216 km. thick.
2. density of 10-13 g/cm3; cannot be accounted for by the compression of silicates.
B. Evidence from meteorites
1. assumed to be representative of the material from which the Earth originally was formed.
2. stony and iron-nickel meteorites
3. Iron is abundant in solar system and nearly the right density.
4. Other elements such as sulfur, oxygen, silicon and carbon are believed to be present because:
a. the temperature of the core is believed to be below the melting point of iron at these e pressures
b. the density of pure iron is too high
C. The separation of liquid outer and solid inner core is believed to result from sinking of heavier components from lighter ones.
D. Magnetic field is believed to be generated by a core that conducts electricity, and is mobile. (Box 17.2)
1. magnetic field not due to permanently magnetized materials because:
a. the interior of Earth is too hot for magnetic materials to retain their magnetism
b. permanently magnetized materials do not vary their intensity as the Earth’s magnetic field
2. The core behaves like a dynamo, a device that converts mechanical energy into magnetic energy. The driving forces are the Earth’s rotation and the unequal distribution of heat in the interior. The conductive iron churns, interacting with the magnetic field generating an electric current which enhances and sustains the magnetic field.
3. At any point of the surface of the Earth the magnetic field is defined by its strength and direction.