Earth’s Internal Structure and Dynamics Practice Quiz (19 questions)

1. What limits direct investigation of Earth’s interior?

A. Geothermal measurements and fossils reveal about 3% of Earth’s depth

B. Seismic waves and gravity measurements reach about 3% of Earth’s depth

C. Meteorites and surface rocks represent about 3% of Earth’s depth

D. Volcanic eruptions and drilling reach about 3% of Earth’s depth

Answer: (D)

Explanation

Direct investigation relies on volcanic eruptions and drilling, but these methods reach only about 3% of Earth’s depth. Seismic and gravimetric techniques are indirect methods used to study the remaining interior.

2. How does the gravimetric method identify a gravity anomaly?

A. It compares seismic-wave speed with gravity values predicted from Earth’s temperature profile

B. It calculates actual gravity from rock density and compares it with a value measured by seismometers

C. It measures theoretical gravity with gravimeters and compares it with a value calculated from rock composition

D. It measures actual gravity with gravimeters and compares it with a value calculated from altitude and latitude

Answer: (D)

Explanation

The gravimetric method measures actual gravity using gravimeters and compares it with the theoretical value expected from altitude and latitude. Theoretical gravity is calculated rather than directly measured by the gravimeter.

3. What does a positive gravimetric anomaly indicate?

A. Actual gravity matches theoretical gravity, suggesting uniform rock density

B. Theoretical gravity exceeds actual gravity, suggesting a low-density salt dome

C. Actual gravity exceeds theoretical gravity, suggesting unusually dense rocks

D. Theoretical gravity matches actual gravity, suggesting unusually dense rocks

Answer: (C)

Explanation

A positive anomaly occurs when measured gravity is greater than the theoretical value, which indicates denser-than-expected rocks. A salt dome is associated with a negative anomaly because its low density reduces actual gravity.

4. Which comparison correctly distinguishes P waves from S waves?

A. P waves are longitudinal and travel through solids, liquids, and gases, whereas S waves are transverse and travel through solids

B. P waves are transverse and travel through all media, whereas S waves are longitudinal and travel through solids

C. P waves are transverse and travel through solids, whereas S waves are longitudinal and travel through all media

D. P waves are longitudinal and travel through solids, whereas S waves are transverse and travel through liquids and gases

Answer: (A)

Explanation

P waves are longitudinal and can propagate through solids, liquids, and gases, while S waves are transverse and require a solid medium. The inability of S waves to travel through liquids is an important distinction between the two wave types.

5. What is a seismic discontinuity?

A. A zone of uniform composition where seismic waves maintain constant speed and direction

B. A surface location above an earthquake focus, identified by the strongest ground motion

C. A boundary between layers with different composition or physical state, marked by an abrupt change in wave speed and direction

D. A region where earthquakes begin, identified by a sudden increase in surface displacement

Answer: (C)

Explanation

A seismic discontinuity separates layers that differ in composition and/or physical state and produces an abrupt change in seismic-wave speed and direction. An earthquake’s origin is the focus, while the surface location above it is the epicenter.

6. How does seismic tomography construct an image of Earth’s interior?

A. It measures gravity at a few locations and converts the results into a two-dimensional map of rock density

B. It analyzes volcanic materials at the surface to calculate the age and thickness of underground layers

C. It follows one seismic wave from its focus to determine the composition of every interior layer

D. It combines thousands of seismic waves recorded from different positions to map seismic velocities in three dimensions

Answer: (D)

Explanation

Seismic tomography uses many seismic waves observed from different positions to create a three-dimensional map of seismic velocities. It therefore provides a spatial image based on wave behavior rather than relying on a small set of gravity measurements.

7. What does a zone of low seismic velocity generally suggest about temperature?

A. It suggests that the temperature has decreased

B. It suggests that the density has become uniform

C. It suggests that the layer has changed from solid to gaseous

D. It suggests that the temperature has increased

Answer: (D)

Explanation

Low seismic velocity indicates an increase in temperature, whereas increased velocity indicates lower temperatures. Seismic velocity is therefore useful for inferring thermal variations inside Earth.

8. How does the geochemical model divide Earth’s interior?

A. It divides it by age into ancient, intermediate, and young regions, separated by volcanic deposits

B. It divides it by physical behavior into rigid, weak, and liquid zones, separated by temperature boundaries

C. It divides it by chemical composition into crust, mantle, and core, separated by seismic discontinuities

D. It divides it by seismic-wave type into P-wave, S-wave, and silent zones, separated by density changes

Answer: (C)

Explanation

The geochemical model classifies Earth’s interior according to chemical composition into the crust, mantle, and core. These major compositional layers are separated by seismic discontinuities; physical behavior belongs to a different type of model.

9. Which description best characterizes oceanic crust?

A. It averages 50 km thick, has a density near 2.7 g/cm³, and contains extensive ancient sedimentary rocks

B. It averages 9 km thick, has a density near 3.0 g/cm³, and is no more than 180 million years old

C. It averages 50 km thick, has a density near 3.0 g/cm³, and consists mainly of metamorphic rocks

D. It averages 9 km thick, has a density near 2.7 g/cm³, and is generally older than continental crust

Answer: (B)

Explanation

Oceanic crust is about 9 km thick, has an average density of 3.0 g/cm³, and is no more than 180 million years old. The 50 km thickness and 2.7 g/cm³ density describe average continental crust instead.

10. Which statement correctly describes the mantle?

A. It is the shallowest layer, composed mainly of sediments and separated from the atmosphere by the Moho

B. It is the deepest liquid layer, composed mainly of an iron-nickel alloy below the Gutenberg discontinuity

C. It is the intermediate and most voluminous layer, composed mainly of iron- and magnesium-rich peridotites

D. It is the outermost rigid layer, composed mainly of basalt and separated from the crust by the Gutenberg discontinuity

Answer: (C)

Explanation

The mantle is Earth’s intermediate and most voluminous layer and consists mainly of iron- and magnesium-rich peridotites. The Gutenberg discontinuity separates it from the core at about 2,900 km depth, whereas the external core is the liquid layer below that boundary.

11. Why is the internal core solid while the external core is liquid?

A. The internal core is cooler because it is shallower, while the external core is warmer because it is deeper

B. Lower pressure freezes the internal core, although the external core contains a different iron-rich material

C. The internal core contains silicate minerals, while the external core contains an iron-nickel alloy

D. High pressure keeps the internal core’s atoms together, although both regions consist of an iron-nickel alloy

Answer: (D)

Explanation

Both parts of the core consist of an iron-nickel alloy, but the much higher pressure in the internal core keeps its atoms together and makes it solid. The external core remains liquid despite having the same general alloy composition.

12. What principle distinguishes the dynamic model of Earth’s interior from the geochemical model?

A. It divides the interior by depth intervals rather than internal structure.

B. It divides the interior by physical behavior rather than chemical composition.

C. It divides the interior by seismic age rather than material behavior.

D. It divides the interior by chemical composition rather than physical behavior.

Answer: (B)

Explanation

The dynamic model identifies layers according to how materials behave physically, while the geochemical model classifies them by chemical composition. A chemical classification therefore describes the other model, not the dynamic one.

13. Which description correctly characterizes the oceanic and continental lithosphere?

A. Oceanic lithosphere is thicker, while continental lithosphere is thinner.

B. Both types are fluid, with continental lithosphere flowing more slowly.

C. Both types have roughly the same thickness but different compositions.

D. Oceanic lithosphere is thinner, while continental lithosphere is thicker.

Answer: (D)

Explanation

The lithosphere is solid and rigid, and its oceanic thickness is about 60–100 km compared with roughly 150–250 km beneath continents. The opposite thickness relationship reverses the observed distinction.

14. Why can the solid mesosphere flow under Earth’s deep interior conditions?

A. Partial melting turns the entire mesosphere into a liquid layer.

B. High pressure and temperature make it behave like a very slow fluid.

C. Seismic activity fractures it into separate rigid plates.

D. Low pressure and temperature make it behave like a rapidly moving fluid.

Answer: (B)

Explanation

Although the mesosphere remains solid, high pressure and temperature allow it to deform and flow slowly, more slowly than the asthenosphere. Complete melting is associated with inferred conditions in level D, not the mesosphere as a whole.

15. How is the currently most accepted definition of the asthenosphere described?

A. It is a weak layer limited to depths between 100 and 200 km.

B. It is the entire sublithospheric upper mantle extending to 660 km.

C. It is a narrow seismic low-velocity zone beneath the lithosphere.

D. It is the entire mantle because all mantle material flows.

Answer: (B)

Explanation

Kearey and colleagues and Braun described the asthenosphere as the whole upper mantle beneath the lithosphere, extending to the lower-mantle boundary at 660 km. A narrow seismic zone reflects Gutenberg’s earlier definition rather than the broader current one.

16. What feature defined Gutenberg’s 1926 interpretation of the asthenosphere?

A. A seismic low-velocity zone beneath the lithosphere.

B. A deep mantle region where thermal plumes originate.

C. The complete sublithospheric upper mantle down to 660 km.

D. A mechanically weak layer required for isostatic compensation.

Answer: (A)

Explanation

Gutenberg defined the asthenosphere as a seismic low-velocity zone beneath the lithosphere. The mechanically weak interpretation belongs to Barrell, while the broad 660-km definition is the current interpretation.

17. What does level D produce as part of deep Earth circulation?

A. Thermal plumes consisting of hot material rising toward the surface.

B. Rigid plates moving laterally through the lower mantle.

C. Cold currents consisting of subducted material descending toward level D.

D. Seismic waves traveling through the external core.

Answer: (A)

Explanation

Level D generates thermal plumes, which are columns of hot material that rise toward the surface. Descending cold currents are associated with deep subduction rather than with the definition of thermal plumes.

18. What happens to deeply subducted material after it accumulates in level D?

A. It cools the upper endosphere and later rises when heated.

B. It moves upward first and then sinks after losing heat.

C. It immediately melts in the asthenosphere and remains near the surface.

D. It stays permanently at the mesosphere–endosphere boundary.

Answer: (A)

Explanation

Deep subduction carries cold material to the mesosphere–endosphere boundary, where it accumulates in level D, cools the upper endosphere, and eventually rises after heating. Permanent storage would omit the later upward part of the convection cycle.

19. Which comparison correctly distinguishes continental from oceanic volcanism?

A. Both forms result from melting subducting oceanic lithosphere at the same mantle boundary.

B. Continental volcanism involves melting subducting oceanic lithosphere, whereas oceanic volcanism involves rising hot material.

C. Continental volcanism involves rising hot material, whereas oceanic volcanism involves melting subducting oceanic lithosphere.

D. Both forms result from hot material rising, but continental volcanism occurs at greater depth.

Answer: (C)

Explanation

Continental volcanism is linked to the ascent of hot material, whereas oceanic volcanism involves melting of subducting oceanic lithosphere that may halt at the asthenosphere–mesosphere boundary. Treating the two settings as having reversed or identical mechanisms confuses their causes.