CHAPTER 12: EARTH'S INTERIOR
Terms to Know and Define:
Asthenosphere: The partially molten, ductile (capable of flowing), and weak layer of the Earth's
mantle beneath the lithosphere, allowing for the movement of tectonic plates.
Conduction: The transfer of heat through a material without any movement of the material
itself. In the Earth, conduction is a less important mechanism for heat transfer
compared to convection.
Convection: The transfer of heat by the movement of a fluid (in the Earth's case, mantle
material) with different temperatures. This process is a crucial mechanism driving the movement
of tectonic plates.
Core: The innermost layer of the Earth, consisting of the solid inner core and the liquid outer
core. The core is primarily composed of iron and nickel.
Crust: The outermost layer of the Earth, divided into the continental crust (thicker, less dense)
and the oceanic crust (thinner, more dense).
Geothermal Gradient: The rate at which temperature increases with depth beneath the Earth's
surface.
Inner Core: The solid, dense, and extremely hot central region of the Earth's core.
Lithosphere: The rigid outer layer of the Earth, including the crust and a portion of the upper
mantle, divided into tectonic plates.
Lower Mantle: The part of the Earth's mantle that lies below the asthenosphere, extending to the
outer core.
Mantle: The layer of the Earth between the crust and the core, composed of solid rock that can
flow over geological timescales.
Magnetic Field: The region around a magnet or a moving electric charge within which the force
of magnetism acts. Earth's magnetic field is generated in the outer core.
Mesosphere: The third layer of the Earth's interior, located between the asthenosphere and the
outer core.
Moho (Mohorovičić Discontinuity): The boundary between the Earth's crust and mantle,
marked by a change in seismic wave velocity.
Outer Core: The liquid layer of the Earth's core, situated beneath the mantle and above the solid
inner core.
Shadow Zones: Areas on the Earth's surface where seismic waves from a particular earthquake
are not detected due to the bending (refraction) and reflection of these waves by the Earth's core.
Seismic Reflection: The process by which seismic waves encounter a boundary and some of the
energy is reflected back to the surface.
Seismic Refraction: The bending of seismic waves as they pass from one material to another
with different seismic wave velocities.
Geoid: The shape that the surface of the oceans would take under the influence of Earth's gravity
and rotation, disregarding other influences such as winds and tides.
Seismic Rays: Paths along which seismic waves travel.
D" layer: A layer at the core-mantle boundary, composed of partially molten material, and
believed to play a role in generating Earth's magnetic field.
Concepts to know:
1. How do the velocities of P- and S-waves change with depth in passing through the
following layers: crust, asthenosphere, transition zones, lower mantle, outer core, and
inner core (see Figure 12.6 in the text book)?
Crust: seismic wave velocity increase abruptly below the crust due to compositional change
from crustal rocks to
Mantle peridotite.
Asthenosphere: seismic waves velocities decrease within the upper asthenosphere because
peridotite contains at a few percent partial melt but not enough to completely stop S-waves.
Transitional zones- at -400 km depth, olvine changes to the spinel crystal structure of higher
density. At 660 km depth, spinal changes to the higher density perovskite crystal structure
Lower mantle: velocities of S- and P waves increase with depth in the lower mantle
(mesosphere).
Outer core- P-waves bend downward when entering due to decrease in velocity and bend
when they leave again, slows down P waves meaning it's a liquid waves - can't travel
through, stops S waves from traveling through indicating it's liquid.
Inner core- P-waves speed up again
2. What is seismic refraction? Why do seismic rays bend outward with depth in the
mantle?
P-waves generally bend outward as they travel through the mantle due to the increased
density of mantle rocks with depth. The bending of seismic waves is called refraction.
3. What is seismic reflection?
Some seismic waves also reflect when reaching the boundary between two different
materials. Reflect back to the surface.
4. What causes P-wave and S-wave shadow zones? Which is wider and why?
P-waves slow down in the outer core, suggesting that this layer has a significantly different
composition from the mantle and may actually be liquid. This bending in the outer core
creates a P-wave shadow zone where no P-waves are detected. S-waves do not travel
through the outer core, creating an even bigger shadow zone for S-waves. The fact that Swaves do not travel through the outer core suggests that the latter is liquid.
5. How does continental crust differ from oceanic crust in terms of composition and
thickness?
(a) The oceanic crust is thin (~5 km average) under oceans and composed primarily of
basalt.
(b) The cont. crust is much thicker (~40-65 km) under continents and has an average
composition of granite. Continental crust is therefore lighter (more buoyant) than oceanic
crust.
6. What is the Moho and where does it occur?
Beneath the crust, seismic waves increase abruptly indicating a sharp boundary between the
crust and upper mantle. This is due to the compositional change from granite, or basalt, to
peridotite that comprises the upper mantle. The boundary between the crust and upper
mantle is called the Moho.
7. Where is the asthenosphere? How do S-waves behave when passing through
the asthenosphere? What does the behavior of S-waves tell us about
the asthenosphere?
the velocity of S-waves decreases within a zone just below the lithosphere. This suggests
that the peridotite within this zone contain a few percent partial melt, but not enough to
completely stop the S-waves. This region is therefore referred to as the low velocity zone or
asthenosphere.
8. What is the composition of the mantle? How do P- and S-waves generally behave
when traveling deeper into the mantle?
The velocity of S-waves decreases within a zone just below the lithosphere (asthenosphere.)
From 200-400 km depth, the velocity of S-waves gradually increases again until reaching the
400 km transition zone where the S-wave velocity increases rapidly.
Another abrupt increase in S-wave velocity occurs at the ~670 km transition zone,
Below the 670 km transition zone, S-wave and P-wave velocity increase in a less dramatic
manner until reaching the mantle-core boundary
The mantle is composed mainly of silicate rocks.
P-waves generally increase in velocity with depth, and S-waves show a gradual decrease,
reflecting the increasing rigidity of the mantle.
9. Why do seismic waves speed up within the 400km and 670km transition zones?
The 400 km increase may the associated with a change in the crystal structure of olivine to a
closer atomic packing referred to as the spinel structure.
The 670 increase indicates another change to even closer atomic packing where the spinel
structure changes to that of perovskite.
10. Where is the D" layer? What are its characteristics?
A region in roughly the lowermost 200 kilometers of the mantle where P-waves experience a
sharp decrease in velocity
- comprises the bottom few hundred km of the lower mantle (just above the outer core)
- exhibits large horizontal variations in both temperature and composition
- possible graveyard of subducted oceanic lithosphere, birthplace of some mantle plume.
11. What is the composition of the outer core? How does the velocity of P-waves
change when entering the outer core? What happens to S-waves in the outer core?
The slowing down of P-waves in the outer core, coupled with the failure of S-waves to pass
through it, tells us that the outer core is liquid. Experimental measurements of seismic waves
through various materials, coupled with the fact that the core contains one third of the Earth's
mass, suggests that the outer core is composed of molten iron.
12. What is the composition of the inner core?
P-waves speed up again through the inner core and S-waves also travel through it,
suggesting that the inner core is composed of solid iron and nickel.
13. What produces the earth's magnetic field? Do the geographic poles coincide with
the magnetic poles?
The liquid iron in the outer core is stirred into convective motion by heat generated from
radioactivity in the core. Circulation of the liquid iron in the outer core produces electric
currents that, in turn, generate the earth's magnetic field.
The magnetic poles are near, but not quite in the same places as, the geographic poles. The
needle in a compass points towards a magnetic pole.
14. What is the geothermal gradient? List three processes that have contributed to
Earth's internal heat.
earth's temperature gradually increases with depth at a rate known as the geothermal
gradient:
* varies considerably from place to place * averages between about 20 C to 30 C per km in
the crust * rate of increase of heat with depth significantly less in the mantle and core
Processes contributing to Earth's internal heat include radioactive decay, residual heat from
planetary formation, and heat from ongoing differentiation.
15. Describe the process of conduction. Which of Earth’s layers transfers heat mainly
by conduction?
Conduction: the transfer of heat through matter by molecular activity
- inner core, outer core and D" layer are involved in conduction
- heat flow in the crust, not very efficient at transferring heat
The lithosphere transfers heat mainly by conduction.
16. Describe mantle convection and how this process transports heat through the
mantle.
outer core, mantle and lithosphere involved in convection
Convection: the transfer of heat by the mass movement or circulation of a substance
provides the force that moved rigid lithospheric plates
Mantle convection involves the upward movement of warm mantle material and the
downward movement of cooler material, transferring heat through the mantle.
17. Is gravity on earth everywhere uniform? What are the various factors that affect
Earth’s gravity? What is the geoid?
gravita�onal accelera�on is less at the equator (9.78 m/s^2) than at the poles (9.83 m/s^2) for two reasons:
1. centrifugal force greater at the equator 2. earth surface further from center of gravity at the equator than at
the poles
1. eleva�on higher eleva�on puts earths surface further from the center of gravity so that gravity decreases 2.
density differences of rocks below earths surface higher density rocks below the surface increase gravita�onal
atrac�on locally
the hypothe�cal shape of the earth, coinciding with mean sea level and its imagined extension under (or over)
land areas. differences in the height of the geoid, rela�ve to the perfect spheroid, are caused by density
differences below earths surface
Ch. 12 Study Bank Ques�ons
List the three composi�onally dis�nct layers of Earth's interior.
Core, mantle, crust
Arrange the following Earth layers in order from least dense to most dense: crust, atmosphere, core, mantle,
oceans.
atmosphere, oceans, crust, mantle, core
The two transi�on zones in Earth's mantle, one at 400 km and the other at 670 km depths, are characterized by...
Abrupt increases in P- and S- wave veloci�es due to closer atomic packing of crystal structures in mantle minerals
_____________ is probably closest in chemical composi�on to the upper mantle.
Perido�te
Differences in the height of Earth's geoid is measured rela�ve to a perfect ellipsoid.
True
As P waves pass from the mantle into the outer core, veloci�es increase abruptly.
False, When P-waves strike the outer core, however, they bend downward when traveling through the outer core
and bend again when they leave. This indicates that P-waves slow down in the outer core, sugges�ng that this
layer has a significantly different composi�on from the mantle and may actually be liquid.
Which of the following statements about the 'D' layer is false?
Occurs directly below con�nental and oceanic crust
Comprises the botom few hundred kilometers of the lower mantle
Exhibits large horizontal varia�ons in both temperature and composi�on
Birthplace of some mantle plumes
________ zone of incipient par�al mel�ng in the upper mantle
the low-velocity zone
Which of the following did not contribute to Earth's internal heat?
Heat released by colliding par�cles during the forma�on of Earth 4.5 billion years ago
Solar radia�on from the early sun
Heat released as iron crystallized to form the solid inner core of early Earth
Heat emited by radioac�ve decay of isotopes of uranium, thorium, and potassium
Which one of the following best characterizes the asthenosphere?
A layer of so�, solid, magnesium silicates and molten iron droplets at the base of the mantle
A zone of hardened, silicate rock at the base of the oceanic crust
A zone of so�ened perido�te in the upper mantle
The zone in the mantle where the velocity of P- and S-waves drama�cally increase
The P-wave shadow zone is largely the result of...
Reflec�on of P waves from the inner core-outer core boundary
Lower P-wave veloci�es in the mantle than in the crust
Refrac�on of P waves crossing the mantle-core boundary
P waves not being able to pass through the outer core
Con�nental crust is generally thicker than oceanic crust.
True
The __________ of the Earth does not transmit S waves.
Outer mantle
Inner crust
Outer core
Deep mantle
As P waves pass from the mantle into the outer core, veloci�es increase abruptly.
True
The Earth's magne�c field originates by...
Weak electrical currents associated with hot, rising mantle plumes
Weak electrical currents associated with fluid mo�ons in the outer core
Magne�c minerals in the inner core
Magne�za�on of oxygen and nitrogen atoms in the atmospheric ozone layer by solar radia�on
_____________ is probably closest in chemical composi�on to the upper mantle.
Granite
Shale
Andesite
Perido�te
The ___________ is the seismic discon�nuity that forms the boundary between the crust and mantle.
Asthenosphere
'D' layer
400 km transi�on zone
Moho
Heat from the core is transferred through the mantle to the base of the crust mainly by __________
Convec�on
Earth's radius is slightly longer at the pole, but shorter at the equator.
False
The example of seismic tomography illustrated below shows hoter and colder regions of Earth's mantle based
on measured varia�ons in __________
Gravity
Veloci�es of S waves
Heat flow
Earth's geoid
Which of the following is good evidence for a solidified, inner core?
P waves are faster in the inner core than in the outer core
S waves do not pass directly through the core
S waves are slower in the inner core than in the outer core
S waves are focused at the center of the P-wave shadow zone
The geothermal gradient is a curve showing the increase of temperature with depth within Earth's interior.
True
The two transi�on zones in Earth's mantle, one at 400 km and the other at 670 km depths, are characterized by...
Changes in mantle temperature
Changes from a completely solid to partly molten mantle perido�te
Changes in gravity in the mantle due to changes in mantle chemistry
Abrupt increases in P- and S-wave veloci�es due to closer atomic packing of crystal structures in mantle
minerals
A mixture of ________ represents the average chemical composi�on of the con�nental crust.
quartz and calcite
________ are closest to the average chemical composi�on of the oceanic crust
Basalt and gabbro
Convec�on in the mantle causes ________
warm rock to rise as narrow pipe-like plumes
________ zone of incipient par�al mel�ng in the upper mantle
the low-velocity zone
________ most of the lower mantle; more rigid than the asthenosphere
the mesosphere
________ caused by the transi�on from the spinel to the perovskite mineral structure
the 660-km-depth discon�nuity
________ implicated in the upward flow of warmed mantle from near the core-mantle boundary
the "D" layer
The ________ of the Earth has the smallest volume
Crust
1. What is the approximate distance from the surface to the center of Earth?
A) 700 km
B) 2900 km
C) 6400 km
D) 24,000 km
C) 6400 km
2. How fast do P waves travel through granite?
A) 4 kilometers per second
B) 6 kilometers per second
C) 8 kilometers per second
D) 12 kilometers per second
B) 6 kilometers per second
3. The S-wave shadow zone extends from ____ to 180° angular distance from the earthquake focus.
A) 45°
B) 75°
C) 105°
D) 145°
C) 105°
4. Which of the following statements is true?
A) Liquids do not transmit P waves.
B) P-wave veloci�es are greater in the core than in the mantle.
C) Seismic waves follow straight paths through the interior of Earth.
D) The S-wave shadow zone is larger than the P-wave shadow zone.
D) The S-wave shadow zone is larger than the P-wave shadow zone.
5. Which of the following seismic waves will arrive at a seismograph first?
A) P
B) S
C) PP
D) SS
A) P
6. Which layer in Earth does not transmit S waves?
A) the crust
B) the inner core
C) the mantle
D) the outer core
D) the outer core
We have an expert-writen solu�on to this problem!
7. Where do P waves travel fastest?
A) in the upper mantle
B) in the outer core
C) in the lower mantle
D) in the inner core
C) in the lower mantle
8. Con�nental crust beneath mountains can be up to ___ kilometers thick.
A) 10
B) 40
C) 70
D) 100
C) 70
9. How thick is the oceanic crust?
A) approximately 7 km
B) approximately 70 km
C) approximately 35 km
D) approximately 240 km
A) approximately 7 km
10. The oceanic crust consists mostly of
A) felsic rocks.
B) mafic rocks.
C) sedimentary rocks.
D) ultramafic rocks.
B) mafic rocks.
12. Which of the following statements about the Mohorovičić discon�nuity (Moho) is false?
A) The Moho marks the top of a par�ally molten layer.
B) The Moho separates denser rocks below from less dense rocks above.
C) The Moho separates the crust from the mantle.
D) The speed of seismic waves increases as they pass down through the Moho.
A) The Moho marks the top of a par�ally molten layer.
13. Which of the following statements is false?
A) P waves travel slower in the crust than in the mantle.
B) The crust is denser than the mantle.
C) The crust-mantle boundary is called the Mohorovičić discon�nuity.
D) The oceanic crust consists of basalt and gabbro.
B) The crust is denser than the mantle.
14. Which of the following marks the boundary between the crust and the mantle?
A) a decrease in P-wave velocity and a change in rock type
B) an increase in P-wave velocity and a change in rock type
C) a decrease in P-wave velocity, but no change in rock type
D) an increase in P-wave velocity, but no change in rock type
B) an increase in P-wave velocity and a change in rock type
15. What principle is based on the con�nents being less dense than the underlying mantle?
A) epeirogeny
B) isostasy
C) superposi�on
D) tomography
B) isostasy
16. Why is the average eleva�on of the con�nents higher than the average eleva�on of the seafloor?
A) because mel�ng of large ice caps has caused isosta�c rebound of the con�nents
B) because oceanic crust is composed of denser rocks than the con�nental crust
C) because the con�nents are supported by upwelling mantle currents
D) because subduc�on pulls down the seafloor
B) because oceanic crust is composed of denser rocks than the con�nental crust
17. A�er mel�ng of a con�nental ice cap, the surface of the con�nent will tend to
A) rise.
B) sink.
C) rise or sink depending on the thickness of the ice cap.
D) remain the same.
A) rise.
18. Which of the following regions in Earth consists primarily of olivine and pyroxene?
A) the crust
B) the upper mantle
C) the lower mantle
D) the inner core
B) the upper mantle
19. How thick is average lithosphere?
A) 25 km
B) 100 km
C) 250 km
D) 1000 km
B) 100 km
20. Which of the following statements about the asthenosphere is false?
A) The asthenosphere lies beneath the lithosphere.
B) The asthenosphere is stronger than the lithosphere.
C) The asthenosphere rises close to the surface beneath mid-ocean ridges.
D) The asthenosphere is par�ally molten.
B) The asthenosphere is stronger than the lithosphere.
21. The boundary between the mantle and the core lies at a depth of approximately
A) 300 kilometers.
B) 1000 kilometers.
C) 3000 kilometers.
D) 10,000 kilometers.
C) 3000 kilometers.
22. Which of the following regions is the source of most basal�c magma?
A) the asthenosphere
B) the lithosphere
C) the lower mantle
D) the outer core
A) the asthenosphere
23. What causes the sharp increases in the velocity of S waves at a depth of 400 and 660 km in the mantle?
A) changes in the composi�on of the mantle
B) changes in the mineral structures
C) changes in the pressure of the mantle
D) changes in the temperature of the mantle
B) changes in the mineral structures
24. At what depth does the transi�on from the upper mantle to the lower mantle occur?
A) approximately 100 kilometers
B) approximately 3000 kilometers
C) approximately 700 kilometers
D) approximately 5000 kilometers
A) approximately 100 kilometers
25. What element makes up most of Earth's core?
A) iron
B) oxygen
C) magnesium
D) silicon
A) iron
26. According to a recent hypothesis, mantle plumes that feed hot spots originate from
A) the asthenosphere-lithosphere boundary.
B) the core-mantle boundary.
C) the crust-mantle boundary.
D) the inner core-outer core boundary.
B) the core-mantle boundary.
27. Which of the following statements about Earth's core is true?
A) The inner core and the outer core are both liquid.
B) The inner core and the outer core are both solid.
C) The inner core is liquid and the outer core is solid.
D) The inner core is solid and the outer core is liquid.
D) The inner core is solid and the outer core is liquid.
28. The mechanical transfer of heat by vibra�on of atoms and molecules is called
A) conduc�on.
B) convec�on.
C) magne�sm.
D) radia�on.
A) conduc�on
29. What drives plate tectonics?
A) erosion
B) solar energy
C) thermal conduc�on
D) thermal convec�on
D) thermal convec�on
30. Over geologic �me, most of the heat lost from Earth's interior has been transported by
A) conduc�on.
B) convec�on.
C) radioac�ve decay.
D) solar radia�on.
B) convec�on.
31. Which of the following regions is hotest?
A) the crust
B) the inner core
C) the mantle
D) the outer core
B) the inner core
32. The temperature at the center of Earth is approximately
A) 200°C.
B) 500°C.
C) 2000°C.
D) 5000°C.
D) 5000°C.
33. Seismic wave speeds increase with
A) decreasing density and temperature.
B) decreasing density and increasing temperature.
C) increasing density and decreasing temperature.
D) increasing density and temperature.
C) increasing density and decreasing temperature.
34. Seismic tomography has revealed that seismic waves in the uppermost mantle are rela�vely
A) fast beneath both mid-ocean ridges and subduc�on zones.
B) fast beneath mid-ocean ridges and slow beneath subduc�on zones.
C) slow beneath mid-ocean ridges and fast beneath subduc�on zones.
D) slow beneath both mid-ocean ridges and subduc�on zones.
C) slow beneath mid-ocean ridges and fast beneath subduc�on zones.
35. Based on seismic tomographic images, how deep do subduc�ng slabs extend?
A) to the crust-mantle boundary
B) to the upper mantle-lower mantle boundary
C) to the core-mantle boundary
D) to the inner core-outer core boundary
C) to the core-mantle boundary
36. What cause large-scale varia�ons in Earth's geoid?
A) varia�ons in gravity
B) varia�ons in radioac�vity
C) varia�ons in magne�sm
D) varia�ons in solar energy
A) varia�ons in gravity
37. Who first proposed that Earth acted as a large magnet whose field forces the needle of a magne�c compass
to align north-south?
A) Charles Darwin
B) William Gilbert
C) James Huton
D) Charles Lyell
38. Where is Earth's magne�c field generated?
A) in the crust
B) in the mantle
C) in the outer core
D) in the inner core
C) in the outer core
39. Above what temperature do materials lose their permanent magne�sm?
A) 100°C
B) 500°C
C) 1000°C
D) 5000°C
B) 500°C
40. Approximately how o�en does Earth's magne�c field reverse itself?
A) every 50 years
B) every 5000 years
C) every 500,000 years
D) every 50,000,000 years
C) every 500,000 years
41. How much of Earth's magne�c field can be described by a simple dipole?
A) 10%
B) 25%
C) 75%
D) 90%
D) 90%
42. Which of the following Earth systems powers Earth's magne�c field?
A) the climate system
B) the plate tectonic system
C) the geodynamo system
D) all of the above
C) the geodynamo system
43. Which of the following processes is responsible for the recording of the magne�c reversals on the seafloor?
A) deposi�onal remanent magne�sm
B) tectonosilicate magne�sm
C) metamorphic magne�sm
D) thermoremanent magne�sm
D) thermoremanent magne�sm
44. When was there a 35-million-year period during which Earth's magne�c field did not undergo reversals?
A) the Cretaceous
B) the Eocene
C) the Miocene
D) the Pliocene-Pleistocene
A) the Cretaceous
45. Seismic waves that bounce off a boundary between two different materials are said to be ________, whereas
seismic waves that cross the boundary and are bent are said to be ________.
A) refracted; reflected
B) reflected; transmited
C) reflected; refracted
D) refracted; transmited
C) reflected; refracted
46. Which of the following statements is false?
A) S waves slow down as they travel through the mantle.
B) P waves slow down in the outer core.
C) S waves cannot transmit through the outer core.
D) P waves travel through the inner core.
A) S waves slow down as they travel through the mantle.
47. The area on the surface of Earth where P waves and S waves are not recorded by seimograms following an
earthquake event are referred to as
A) reflec�on zones.
B) aseismic areas.
C) geoid highs.
D) shadow zones.
D) shadow zones.
48. A P wave that travels from the earthquake focus through the mantle, outer core, back through the mantle,
and to Earth's surface is labeled a=
A) PcP wave.
B) PKP wave.
C) PKS wave.
D) PcS wave.
B) PKP wave.
49. An S wave that is reflected once off the surface of Earth is labeled an ______ wave.
A) SR
B) SP
C) SS
D) SK
C) SS
50. How many mineralogical phase changes do scien�sts interpret to occur in the mantle based on the jumps in
S-wave speeds?
A) one
B) two
C) three
D) four
B) two
51. Using a composi�onal model of pure iron-nickel alloy for the inner core renders the inner core ________ to
match the data for the outer core.
A) too dense
B) not dense enough
C) too magne�c
D) not magne�c enough
A) too dense
52. The graphical curve that describes how temperature changes with depth in Earth is called the
A) tempotherm.
B) hea�ng gradient.
C) geoid gradient.
D) geotherm.
D) geotherm.
53. Exploring Earth's interior using seismic tomography is analogous to exploring the human body using a(n)
A) CAT scanner.
B) MRI machine.
C) stethoscope.
D) fMRI machine.
A) CAT scanner.
54. Seismic tomography has revealed a superplume rising from the core-mantle boundary below
A) Iceland.
B) South Africa.
C) the Himalayas.
D) Mount St. Helens.
B) South Africa.
55. Earth's magne�c field shields the surface of Earth from
A) asteroids.
B) solar winds.
C) intergalac�c radio waves.
D) geotherms.
B) solar winds.
56. Varia�ons in the pull of gravity at the surface of Earth are linked to
A) varia�ons in la�tude.
B) varia�ons in the liquid outer core.
C) varia�ons in lithospheric densi�es.
D) varia�ons in the mantle temperature.
D) varia�ons in the mantle temperature.
57. What do sound, light, and seismic waves have in common?
A) They can travel trough all types of materials.
B) They travel at the same speeds.
C) Their speed depends on the materials they travel through.
D) Their speeds are independent of the material they travel through.
C) Their speed depends on the materials they travel through.
58. Evidence for the internal structure of Earth is mostly based on
A) heat flow measurements.
B) waves generated by earthquakes.
C) existence of a magne�c field.
D) deep mines and drilling.
B) waves generated by earthquakes.
59. The P wave shadow extends from 105° to roughly __________distance from an earthquake's focus.
A) 120°
B) 130°
C) 140°
D) 160°
C) 140°
60. The P wave shadow extends from roughly __________ to 142° distance from an earthquake's focus.
A) 25°
B) 50°
C) 100°
D) 125°
C) 100°
61. Which of the following layers will not transmit S waves?
A) crust
B) mantle
C) outer core
D) inner core
C) outer core
62. As seismic waves travel through Earth, they generally
A) curve upward.
B) travel in straight lines.
C) curve downward.
D) travel along irregular paths.
A) curve upward.
63. Earth's inner core was discovered by seismologist
A) Beno Gutenberg.
B) Andrija Mohorovičić.
C) Hugo Benioff.
D) Inge Lehmann.
D) Inge Lehmann.
64. Which of the following foods would best serve a proxy for Earth's layering?
A) apple
B) orange
C) peach
D) onion
C) peach
65. Calcula�ons suggest that the inner core
A) spins at a faster rate than the rest of Earth.
B) spins in the opposite direc�on of the rest of Earth.
C) spins at the same rate as Earth.
D) does not spin at all.
A) spins at a faster rate than the rest of Earth
66. Over �me, the size of the inner core will __________ and the size of the outer core will __________.
A) grow; grow
B) shrink; shrink
C) shrink; grow
D) grow; shrink
D) grow; shrink
67. Geologists have formulated a simple mathema�cal equa�on that can predict the depth of oceans. According
to their equa�on, on what does the water depth primarily depend?
A) distance from shore
B) age of the seafloor
C) distance from the nearest mid-ocean ridge
D) none of the above
B) age of the seafloor
68. Recently scien�sts have suggested that __________, thus ac�ng like a "planet within a planet."
A) mantle spins faster than the core
B) outer core spins faster than the mantle
C) inner core spins faster than the mantle
D) lower mantle spins faster than the outer core
C) inner core spins faster than the mantle
69. Given that the P wave veloci�es are related to rock density, choose the sequence that shows the rocks in
terms of increasing P wave speeds. Rock densi�es are: rock I = 2.9 gm/cc, rock II = 2.6 gm/cc, and rock III = 3.3
gm/cc.
A) I, II, III
B) II, III, I
C) I, III, II
D) II, I, III
D) II, I, III
70. Assume you are astronaut geophysicist working on Titan (Moon of Saturn) and measure a P-wave velocity of
8 km for the moon's crust. Assuming what you know from Earth, what kind of rock is this surface made of?
A) granite
B) basalt
C) gabbro
D) perido�te
D) perido�te
71. According to the principle of isostasy
A) oceanic crust should stand higher than con�nental crust.
B) con�nental crust should be thinner than oceanic crust.
C) crustal thickness is directly related to surface eleva�on.
D) eleva�on is independent of crust type.
B) con�nental crust should be thinner than oceanic crust.
72. The P wave shadow extends from roughly ___ to roughly ____ distance from an earthquake's focus.
A) 60°; 100°
B) 100°; 140°
C) 140°; 180°
D) 180°; 220°
B) 100°; 140°
73. Let's assume that the inner core was liquid and the outer core was solid. What would one expect to see in
terms of P and S wave shadows in terms of distance from an earthquake focus?
A) P-wave and S-wave shadows would be the same size.
B) S-wave shadow would start where the P-wave shadow ends.
C) P-wave shadow would start where the S-wave shadow ends.
D) There would only be a P-wave shadow.
B) S-wave shadow would start where the P-wave shadow ends.
74. Between 410 and 670 km depths, S-waves show several jumps atributed to phase changes in the upper
mantle. What do geophysicists call this region?
A) the seismic jump region
B) the sudden change zone
C) the low-velocity zone
D) the transi�on zone
B) the sudden change zone
75. The base of transi�on zone layer marks the beginning of the
A) outer core.
B) inner core.
C) lower mantle.
D) upper mantle.
B) inner core.
76. Which por�on of the mantle is rela�vely free of seismic discon�nui�es?
A) 30 to 100 km
B) 100 to 400 km
C) 400 to 1000 km
D) 1000 to 2500 km
D) 1000 to 2500 km
77. Where, inside Earth, is the largest change in material density?
A) base of the crust
B) base of the lithosphere
C) core-mantle boundary
D) outer core-inner core boundary
C) core-mantle boundary
78. In a deep mine, temperatures increase at the rate of approximately ______ per kilometer.
A) 2.5°C
B) 25°C
C) 250°C
D) 2500°C
B) 25°C
79. What sort of geothermal gradient is consistent with having convec�on in the mantle?
A) 0.5°C/km
B) 2.5°C/km
C) 25°C/km
D) 250°C/km
A) 0.5°C/km
80. Geologists have discovered a simple mathema�cal equa�on that relates the depth of the seafloor to crustal
age. This rela�onship implies that the oceanic lithosphere
A) floats on the asthenosphere.
B) cools by conduc�on.
C) cools by convec�on.
D) is rigid.
B) cools by conduc�on.
81. William Gilbert, the physician of Queen Elizabeth I, was the first to suggest that
A) Earth had a dipole magne�c field.
B) gravity varied over the surface of Earth.
C) isostacy worked for the crust.
D) oceanic depth was related to seafloor age.
A) Earth had a dipole magne�c field.
82. What process generates Earth's magne�c field?
A) Earth's rota�on
B) the magne�c stripes on the seafloor when it moves
C) flow in the outer core
D) movement of the con�nents that drag the magne�c poles with it
C) flow in the outer core
83. If Earth's magne�c field disappeared, then
A) seafloor spreading would cease.
B) con�nental dri� would stop.
C) the atmosphere would be blown away by the solar wind.
D) volcanic ac�vity would increase.
B) con�nental dri� would stop.
84. Earth's magne�c field would disappear if
A) Earth stopped rota�ng.
B) the outer core grew larger.
C) plate tectonics stopped.
D) the outer core solidified.
D) the outer core solidified.