Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

4.1 Sketch and describe the mechanism that generates most earthquakes.

4.2 Compare and contrast the types of seismic waves and describe how a seismograph works.

4.3 Locate Earth’s major earthquake belts on a world map.

4.4 Distinguish between intensity scales and magnitude scales.

4.5 List and describe the major destructive forces that earthquake vibrations can trigger.

4.6 Explain how Earth acquired its layered structure and name and describe each of its major layers.

4.7 Compare and contrast brittle and ductile deformation.

4.8 List and describe the major types of folds.

4.9 Sketch and describe the relative motion of rock bodies located on opposite sides of normal, reverse, and strike-slip faults.

4.10 Locate and name Earth’s major mountain belts on a world map.

4.11 Sketch a cross section of an Andean-type mountain belt and describe how its major features are generated.

4.12 Summarize the stages in the development of an Alpine-type, mountain belt such as the Appalachians.

Concept Checks 

4.1

1. What is an earthquake?

2. How are hypocenters and epicenters related?

3. Explain what is meant by elastic rebound.

4.2

1. Briefly describe how a seismograph works.

2. List the major differences between P, S, and surface waves.

3. Which of the three basic types of seismic wave is likely to cause the greatest destruction to buildings?

4.3

1. What zone on Earth has the greatest amount of seismic activity? 

2.Which type of plate boundary is associated with Earth’s most destructive earthquakes?

3. What information does a travel–time graph provide?

4. Briefly describe the triangulation method used to locate the epicenter of an earthquake.

4.4

1. What does the Modified Mercalli Intensity scale tell us about an earthquake?

2. How much more energy does a magnitude 7.0 earthquake release than a magnitude 6.0 earthquake?

3. Why is the moment magnitude scale favored over the Richter-like magnitude scales for large earthquakes?

4.5

1. List three factors that influence the amount of destruction that seismic vibrations cause to human-made structures.In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.

2. What is a tsunami? 

3. How are tsunamis generated?

4.6

1. How do continental crust and oceanic crust differ?

2. Contrast the physical characteristics of the asthenosphere and the lithosphere.

3. How are Earth’s inner and outer cores different? How are they similar?

4.7

1. Define deformation.

2. Describe elastic deformation.

3. How is strain different from stress?

4.8

1. Distinguish between anticlines and synclines, domes and basins, anticlines and domes.

2. The Black Hills of South Dakota is a good example of what type of geologic structure?

3. Where do we find the youngest rocks in an eroded basin: near the center or near the flanks?

4.9

1. Contrast the movements that occur along normal and reverse faults.

2. How are reverse faults different from thrust faults? 

3. In what way are they the same?

4. Describe the relative movement along a strike-slip fault.How are joints different from faults?

4.10

1. Define orogenesis.

2. Which type of plate boundary is most directly associated with Earth’s major mountain belts?

4.11

1. Compare and contrast mountain building at a volcanic island arc with mountain building at an Andean-style continental margin.

2. Describe and give an example of a passive continental margin.

3. In what ways are the Sierra Nevada and the Andes similar?

4. What is an accretionary wedge? 

5. Briefly describe its formation.

6. What is a batholith? 

7. In what tectonic setting are batholiths being generated?

4.12

1. Explain why the continental crust of Asia was deformed more than the Indian subcontinent during formation of the Himalayas.

2. How does the plate tectonics theory help explain the existence of fossil marine life in rocks atop collisional mountains?

3. Differentiate between terrane and terrain.

Chapter 4 Concepts In Review Restless Earth: Earthquakes and Mountain Building

4.1 What Is an Earthquake?

Sketch and describe the mechanism that generates most earthquakes.

Key Terms: earthquake fault, hypocenter (focus), epicenter, seismic wave, elastic rebound megathrust, fault, fault creep

*The sudden movements of large blocks of rock on opposite sides of faults cause most earthquakes. The location where the rock begins to slip is called the hypocenter, or focus. During an earthquake, seismic waves radiate outward from the hypocenter in all directions. The point on Earth’s surface directly above the hypocenter is the epicenter.

*Earthquakes are caused by differential stress that gradually bends Earth’s crust over tens to hundreds of years. Up to a point, frictional resistance along the fault keeps the rock from rupturing and slipping. Once that point is reached, the fault slips, allowing the bent rock to “spring back” to its original shape, generating an earthquake. The springing back is called elastic rebound.

*Convergent plate boundaries and associated subduction zones are marked by megathrust faults, which are responsible for most of the largest earthquakes in recorded history.

*The San Andreas Fault in California is an example of a large strike-slip fault that forms a transform plate boundary capable of generating destructive earthquakes.

QUESTION: Label the blanks on the diagram to show the relationship between earthquakes and faults using the following terms: epicenter, seismic waves, fault, fault trace, and hypocenter. 

4.2 Seismology: The Study of Earthquake Waves

Compare and contrast the types of seismic waves and describe how a seismograph works.

Key Terms: seismology, seismograph (seismometer), inertia, seismogram body, waves, surface waves, primary (P) waves, secondary (S) waves

*Seismology is the study of seismic waves. A seismograph measures these waves, using the principle of inertia. While the body of the instrument moves with the waves, the inertia of a suspended weight keeps a sensor stationary to record the displacement between the two.

*A seismogram, a record of seismic waves, reveals two main categories of earthquake waves: body waves (primary [P]) waves and secondary [S] waves), which are capable of moving through Earth’s interior, and surface waves, which travel only along the upper layers of the crust. P waves are the fastest, S waves are intermediate in speed, and surface waves are the slowest. However, surface waves tend to have the greatest amplitude and produce the strongest shaking, so surface waves usually account for most damage during earthquakes.

*P waves momentarily push (compress) and pull (stretch) rocks as they travel through a rock body, thereby changing the volume of the rock. S waves impart a shaking motion as they pass through rock, changing the rock’s shape but not its volume. Because fluids do not resist forces that change their shape, S waves cannot travel through fluids, whereas P waves can.

4.3 Where Do Most Destructive Earthquakes Occur?

Locate Earth’s major earthquake belts on a world map.

Key Term: circum-Pacific belt

*Most earthquake energy is released in the circum-Pacific belt, the ring of megathrust faults rimming the Pacific Ocean. Another earthquake belt is the Alpine–Himalayan belt, which runs along the zone where the Eurasia plate collides with the Indian subcontinent and African plate.

*Earth’s oceanic ridge system produces another belt of frequent but small-magnitude quakes. Transform faults in the continental crust, including the San Andreas Fault, can produce large earthquakes.

*Some earthquakes occur at considerable distances from plate boundaries. Examples include the 1811–1812 New Madrid, Missouri, earthquakes and the 1886 Charleston, South Carolina, earthquake.

*The distance separating a recording station from an earthquake’s epicenter can be determined by using the difference in arrival times between P and S waves. When the distances are known from three or more seismic stations, the epicenter can be located using a method called triangulation.

QUESTION: Outline the circum-Pacific earthquake belt on the accompanying map that has plate boundaries drawn in red. Do the same for the Alpine–Himalayan belt.

4.4 Determining the Size of an Earthquake

Distinguish between intensity scales and magnitude scales.

Key Terms: intensity, magnitude, Modified Mercalli Intensity scale, moment magnitude

*Intensity is a measurement of the amount of ground shaking at a location due to an earthquake, and magnitude is an estimate of the amount of energy released during an earthquake.

*The Modified Mercalli Intensity scale is a 12-point scale that uses structural damage to quantify earthquake intensity.

*Richter-like scales take into account both the maximum amplitude of the seismic waves measured at a given seismograph and that seismograph’s distance from the earthquake. These scales are logarithmic, meaning that a number on the scale represents seismic amplitudes that are 10 times greater than those represented by the next lower number. Furthermore, each larger number on a Richter-like scale represents the release of about 32 times more energy than the number below it.

*Because the Richter scale does not effectively differentiate between very large earthquakes, the moment magnitude scale was devised. This scale measures the total energy released from an earthquake by considering the strength of the faulted rock, the amount of slippage, and the area of the fault surface that slipped.

4.5 Earthquake Destruction

List and describe the major destructive forces that earthquake vibrations can trigger.

Key Terms: liquefaction, tsunami

*Factors influencing how much destruction an earthquake might inflict on human-made structures include (1) intensity and length of time the shaking persists, (2) building construction, and (3) the nature of the ground that underlies the structure. Buildings constructed of unreinforced bricks and blocks are more likely than other types of structures to be severely damaged in a quake.

*In general, unconsolidated sediments amplify seismic shaking, while bedrock-supported buildings fare best in an earthquake.Liquefaction may occur when waterlogged sediment or soil is severely shaken during an earthquake. 

*Liquefaction can reduce the strength of the ground to the point at which it may not support buildings.

*Earthquakes may also trigger landslides or ground subsidence, and they can break gas lines, which may initiate devastating fires.

*Tsunamis are large ocean waves that form when water is displaced, usually by a megathrust fault rupturing on the seafloor. Traveling at the speed of a jet aircraft, a tsunami is hardly noticeable in deep water. However, upon arrival in shallower coastal waters, the tsunami slows down and piles up, producing a wall of water sometimes more than 30 meters (100 feet) in height. Tsunami warning systems have been established in most of the large ocean basins.

4.6 Earth’s Interior

Explain how Earth acquired its layered structure and name and describe each of its major layers.

Key Terms: crust, mantle, lithosphere, asthenosphere, core, outer core, inner core

*The layered internal structure of Earth developed due to gravitational sorting of Earth materials early in the history of the planet. The densest material settled to form Earth’s core, while the least dense material rose to form Earth’s crust, oceans, and atmosphere.

*Seismic waves generated from large earthquakes allow geoscientists to “look” into Earth’s interior. Like a sonogram used to image human bodies, seismic waves reveal details about Earth’s layered structure.

*Earth has two distinct kinds of crust: oceanic and continental. Oceanic crust is thinner, denser, and younger than continental crust. Oceanic crust also readily subducts, whereas the less dense continental crust does not.

*The uppermost mantle and crust make up Earth’s rigid outer shell, called the lithosphere, which overlies the asthenosphere—a solid but relatively weak layer. The lower mantle is a strong solid layer but capable of very gradual flow.

*Earth’s core is very dense and composed of a mixture of iron and nickel, with minor amounts of lighter elements. The outer core is liquid, whereas the inner core is solid.

QUESTION: Label the layers of Earth’s interior shown on the accompanying diagram using the following terms: oceanic crust, continental crust, upper mantle, asthenosphere, and lithosphere. 

4.7 Crustal Deformation

Compare and contrast brittle and ductile deformation.

Key Terms: deformation, tectonic structures (geologic structures), stress, confining pressure differential stress, compressional stress, tensional stress, shear, strain, elastic deformation, brittle deformation, ductile deformation

*Elastic deformation is a temporary bending of rock that does not go past the breaking point. When the stress is released, the rock snaps back to its original shape.

*When stress is greater than the strength of a rock, the rock will deform in either a brittle or ductile fashion. Brittle deformation is the breaking of rocks into smaller pieces, whereas ductile deformation is flow in the manner of modeling clay or warm wax.

QUESTION: Which type of deformation (elastic, brittle, or ductile) is best displayed in the accompanying photograph of a flattened quarter?

4.8 Folds: Structures Formed by Ductile Deformation

List and describe the major types of folds.

Key Terms: fold, anticline, syncline, dome, basin, monocline

*Folds are wavelike undulations in layered rocks that develop through ductile deformation in rocks undergoing compressional stress.

*Folds that have an arch-like structure are called anticlines, while folds that have a trough-like structure are called synclines. Monoclines are large step-like folds in otherwise horizontal strata; they result from subsurface faulting.

*Domes and basins are large folds that produce “bull’s-eye”-shaped outcrop patterns. The overall shape of a dome or basin is like a saucer or a bowl, either right side up (basin) or inverted (dome).

QUESTION: What name is given to the rock structure shown in the accompanying photo? 

4.9 Faults and Joints: Structures Formed by Brittle Deformation

Sketch and describe the relative motion of rock bodies located on opposite sides of normal, reverse, and strike-slip faults.

Key Terms: fault, dip-slip fault, hanging wall, block footwall block, fault scarp, normal fault, fault-block mountain, horst, graben, reverse fault, thrust fault, strike-slip fault, transform fault, joint

*Faults are fractures in rock along which there has been displacement (movement) of the rocks on both sides of the fracture. Joints are fractures along which no appreciable displacement has occurred.

*If the movement is in the direction of the fault’s dip (or inclination), then the rock above the fault plane is called the hanging wall block, and the rock below the fault is the footwall block. If the hanging wall moves down relative to the footwall, the fault is a normal fault. If the hanging wall moves up relative to the footwall, the fault is a reverse fault. Reverse faults with low dip angles are thrust faults.

*Areas of tectonic extension, such as the Basin and Range Province, produce fault-block mountains: horsts separated by neighboring grabens or half-grabens.

*Strike-slip faults have most of their movement in the direction of the trend of the fault trace. Transform faults are large strike-slip faults that serve as tectonic boundaries between lithospheric plates.

4.10 Mountain Building

Locate and name Earth’s major mountain belts on a world map.

Key Terms:orogenesis, orogeny, collisional mountain

*Orogenesis is the making of mountains. An episode of orogenesis is an orogeny. Most orogenesis occurs along convergent plate boundaries, where compressional forces cause folding and faulting, thickening the crust vertically and shortening it horizontally.

4.11 Subduction and Mountain Building

Sketch a cross section of an Andean-type mountain belt and describe how its major features are generated.

Key Terms: accretionary wedge, forearc basin

*The type of convergent margin determines the type of mountains that form. Where one oceanic plate overrides another, a volcanic island arc forms. Where an oceanic plate subducts under a continent, Andean-type mountain building occurs.

*Release of water from a subducted slab triggers melting in the overlying mangle wedge, generating basaltic magmas that rise to the base of the continental crust where they often pond. The hot basaltic magma may heat the overlying crustal rocks sufficiently to generate a silica-rich magma of intermediate or felsic (granitic) composition.

*Sediment scraped off the subducting plate builds an accretionary wedge. Between the accretionary wedge and the volcanic arc is a relatively calm site of sedimentary deposition, the forearc basin.

*The geography of central California preserves an accretionary wedge (Coast Ranges), a forearc basin (Great Valley), and the roots of an Andean-style mountain belt (Sierra Nevada).

4.12 Collisional Mountain Belts

Summarize the stages in the development of an Alpine-type mountain belt such as the Appalachians.

Key Terms: suture, terrain, microcontinent

*The Himalayas and Appalachians were formed by collisions between continents when the intervening ocean basin subducted completely. The Appalachians were caused by the collision of ancestral North America with ancestral Africa more than 250 million years ago. The Himalayas were formed by the collision of India and Eurasia starting around 50 million years ago, and they are still rising.

*A terrane is a relatively small crustal fragment (microcontinent, volcanic island arc, or oceanic plateau) that has been carried by an oceanic plate to a continental subduction zone and then accreted onto the continental margin. The North American Cordillera formed by the accretion of many successive terranes.