# Chapter 2 Study Guide: Internal Structure of Earth and Plate Tectonics

Prepare to summarize each numbered section, define the chapter's key terms, compare plate boundaries, explain evidence and mechanisms, and connect plate tectonics to natural hazards.

## Chapter Learning Objectives

The chapter states these objectives. Each section below identifies the objective or objectives it supports.

- **LO:1:** Describe the basic internal structure and processes of Earth.
- **LO:2:** Summarize the various lines of evidence that support the theory of plate tectonics.
- **LO:3:** Compare and contrast the different types of plate boundaries.
- **LO:4:** Explain the mechanisms of plate tectonics.
- **LO:5:** Outline how plate tectonics has changed the appearance of Earth's surface over time.
- **LO:6:** Compare and contrast the two fundamental processes that drive plate tectonics.
- **LO:7:** Link plate tectonic processes to natural hazards.

## Key Terms

- **Crust:** Earth's outer rock layer. Oceanic crust is thinner and denser than continental crust.
- **Mantle:** The thick layer of mostly solid, iron- and magnesium-rich silicate rock between the crust and core. It can flow slowly over geologic time.
- **Core:** Earth's metallic inner region, consisting of a solid inner core and a liquid outer core.
- **Inner core:** The solid, mostly iron center of Earth, held solid by immense pressure despite its high temperature.
- **Outer core:** The liquid, mostly metallic layer surrounding the inner core. Its convection and rotation help generate Earth's magnetic field.
- **Moho:** The Mohorovicic discontinuity, the boundary between the crust and mantle.
- **Lithosphere:** Earth's strong, rigid outer layer, consisting of the crust and uppermost mantle. It is broken into tectonic plates.
- **Asthenosphere:** The hot, weak, slowly flowing layer beneath the lithosphere and entirely within the mantle.
- **Tectonics:** Large-scale geologic processes that deform Earth's lithosphere and produce continents, mountains, and ocean basins.
- **Plate tectonics:** The theory that Earth's lithosphere is divided into moving plates whose interactions create, move, and recycle lithosphere and produce many geologic hazards.
- **Tectonic plate:** A large, rigid piece of lithosphere that moves over the asthenosphere. It may contain both continental and oceanic crust.
- **Convection:** Temperature-driven circulation in which hot, less-dense material rises and cooler, denser material sinks.
- **Continental drift:** The hypothesis that continents move across Earth's surface, proposed by Alfred Wegener in 1915.
- **Seafloor spreading:** The formation of new oceanic lithosphere at mid-ocean ridges and its movement away from the ridge.
- **Divergent boundary or spreading center:** A boundary where plates move apart and new oceanic lithosphere forms.
- **Convergent boundary:** A boundary where plates move toward one another. It may involve subduction or continent-continent collision.
- **Subduction zone:** A convergent boundary where one plate sinks into the mantle beneath another plate.
- **Submarine trench:** A long, narrow, deep depression on the ocean floor commonly formed at a subduction zone.
- **Volcanic arc:** A curved chain of volcanoes formed above a subduction zone; it may be a continental or island arc.
- **Transform boundary:** A boundary where plates slide laterally past one another.
- **Triple junction:** A location where three plate boundaries meet.
- **Paleomagnetism:** The study of the magnetic record preserved in rocks.
- **Magnetic reversal:** A change between normal and reversed polarity in Earth's magnetic field.
- **Magnetic anomaly:** A local difference in the magnetic field caused by magnetized rocks.
- **Curie point:** The critical temperature at which iron-bearing minerals in cooling volcanic rock become aligned with the magnetic field.
- **Thermoremanent magnetization:** Permanent magnetization preserved in rock as it cools through the Curie point.
- **Hot spot:** A volcanic center associated with hot, buoyant material rising from deep in the mantle. If it remains relatively fixed while a plate moves over it, it can form a volcanic chain.
- **Seamount:** A submarine volcanic mountain, often an eroded volcano that has sunk below sea level.
- **Wadati-Benioff zone:** An inclined zone of shallow, intermediate, and deep earthquakes tracing a descending subducting plate.
- **Ridge push:** Gravity-driven movement away from the elevated crest of a mid-ocean ridge.
- **Slab pull:** Gravity-driven force produced when cold, dense oceanic lithosphere sinks at a subduction zone and pulls the rest of the plate toward the trench.
- **Suture zone:** The boundary where colliding continental plates join after compression and crustal thickening.

## 2.1 Internal Structure of Earth (LO:1)

Earth is a dynamic planet with layers that can be described by composition and density or by physical properties such as strength and whether material is solid or liquid. Internal processes produce continents, ocean basins, mountains, plateaus, volcanoes, and earthquakes. Topography created by tectonics also affects ocean circulation, atmospheric circulation, weather, climate, and the distribution of life.

### Compositional layers

- **Inner core:** Solid, metallic, and mostly iron. It is more than 1,200 km thick and remains solid under immense pressure despite its very high temperature.
- **Outer core:** Liquid, metallic, and just over 2,200 km thick. Convection and Earth's rotation generate electric currents that sustain Earth's magnetic field.
- **Mantle:** Nearly 2,900 km thick and composed mostly of solid, iron- and magnesium-rich silicate rock. It can flow very slowly.
- **Crust:** Earth's outer rock layer. The Moho separates the less-dense crust from the denser mantle.

### Oceanic and continental crust

- **Oceanic crust:** About 6 to 7 km thick, denser, and generally less than 200 million years old.
- **Continental crust:** About 35 km thick on average, up to about 70 km beneath mountains, less dense, and potentially several billion years old.
- Continental crust is relatively buoyant and difficult to subduct. Oceanic crust is denser and is recycled into the mantle.

### Physical layers

The **lithosphere** includes the crust and part of the upper mantle. It is cool, strong, rigid, and broken into tectonic plates. The **asthenosphere** lies beneath it, is entirely within the mantle, and is hot, weak, and capable of slow flow. The lithosphere is not the same thing as the crust: the lithosphere includes both crust and upper mantle.

### Internal heat and convection

Earth's internal heat comes from its original formation, heat released as the core crystallizes, and radioactive decay of elements such as uranium. Heating lowers the density of material and causes it to rise; cooling increases density and causes material to sink. Mantle convection is a proposed explanation for plate movement, but the exact mechanism remains complex.

### Check Your Understanding

1. **What are the major differences between the inner and outer cores of Earth?**

   **Answer:** Both cores are primarily metallic and mostly iron, but the inner core is solid while the outer core is liquid. The inner core is held solid by immense pressure and is more than 1,200 km thick. The outer core is just over 2,200 km thick; convection and rotation in its electrically conductive liquid generate the currents that sustain Earth's magnetic field.

2. **In what ways are the major properties of the lithosphere different from those of the asthenosphere?**

   **Answer:** The lithosphere is the cool, strong, rigid layer made of the crust and uppermost mantle. It is broken into brittle tectonic plates. The asthenosphere lies beneath it, is entirely within the mantle, and is hotter, weaker, and able to flow slowly. Plates move over the weak asthenosphere.

## 2.2 Plate Tectonics (LO:3 and LO:4)

Plate tectonics is the theory that the lithosphere is divided into moving plates. The plates include continental and oceanic portions and move a few centimeters per year. Their boundaries are broad, active zones rather than perfectly narrow cracks. New oceanic lithosphere forms where plates diverge, and old oceanic lithosphere is destroyed where it subducts, keeping Earth's overall size approximately constant.

### Movement of tectonic plates

The weak, slowly flowing asthenosphere permits the rigid lithospheric plates to move. Continents do not move independently through oceanic crust; they are embedded in and carried by moving plates. The continental-drift hypothesis was proposed by Alfred Wegener in 1915. It was reasonable because of evidence such as matching continental shapes and fossils, but it was initially rejected because Wegener could not explain a mechanism capable of moving continents. Seafloor spreading later supplied that mechanism.

At mid-ocean ridges, plates diverge. Hot material rises to fill the opening, cools, and adds new oceanic lithosphere to the plate edges. At subduction zones, cool, dense oceanic lithosphere sinks into the asthenosphere. This creates a continuous cycle of plate creation, movement, and destruction.

### Types of plate boundaries

| Boundary | Plate motion and products | Examples and hazards |
|---|---|---|
| Divergent | Plates move apart; mid-ocean ridges or continental rifts form and new oceanic lithosphere is produced | Mid-Atlantic Ridge and East African Rift; shallow earthquakes and generally nonexplosive volcanism |
| Convergent, ocean-continent subduction | Dense oceanic plate sinks beneath continental plate; trench, continental volcanic arc, folding, faulting, and mountains form | Andes; great earthquakes, explosive volcanoes, tsunamis, flooding, mass wasting, and subsidence |
| Convergent, ocean-ocean subduction | Older, denser oceanic plate sinks beneath younger oceanic plate; trench and island arc form | Aleutian and Caribbean island arcs; great earthquakes, explosive volcanoes, tsunamis, flooding, mass wasting, and subsidence |
| Convergent, continent-continent collision | Buoyant continents collide; compression, folding, faulting, crustal thickening, and a suture zone form | Himalayas, Alps, and Appalachians; major earthquakes, flooding, mass wasting, and high-topography effects |
| Transform | Plates slide horizontally past one another; faults may offset ridge segments | San Andreas fault; strong to major earthquakes, especially where faults cross continents |

### Subduction and collision

At an ocean-continent or ocean-ocean convergent boundary, a dense oceanic plate can descend into the mantle. At a continent-continent collision, neither buoyant continental plate easily subducts, so compression shortens and thickens the crust and builds a high mountain chain. Earthquakes occur in both settings, but subduction zones also commonly produce explosive volcanism and tsunamis.

### Check Your Understanding

1. **Why was the continental drift hypothesis not taken seriously?**

   **Answer:** Wegener had useful evidence, including the fit of continents and matching fossils, but he could not explain what force or mechanism moved continents across Earth's surface. The missing mechanism was later supplied by seafloor spreading and the theory of plate tectonics.

2. **What enables the tectonic plates to move?**

   **Answer:** The lithosphere is broken into rigid plates that move over the hot, weak, slowly flowing asthenosphere. Plate movement is driven by forces such as ridge push and slab pull; mantle convection is also a proposed part of the mechanism, although the exact process is complex.

3. **What are the three major types of plate boundaries?**

   **Answer:** The three major types are divergent boundaries, where plates move apart; convergent boundaries, where plates move together and may involve subduction or collision; and transform boundaries, where plates slide laterally past one another.

4. **How is a subduction zone different from a collision zone?**

   **Answer:** A subduction zone forms when a dense oceanic plate sinks beneath another plate, producing a trench, earthquakes, and commonly a volcanic arc. A collision zone forms when two buoyant continental plates converge; neither easily sinks, so compression folds and thickens the crust and builds high mountains.

## 2.3 A Detailed Look at Seafloor Spreading (LO:2 and LO:4)

Seafloor spreading explains how new oceanic lithosphere forms at mid-ocean ridges and moves laterally away from them. The chapter identifies three major sources of evidence: mapping the global system of oceanic ridges, dating volcanic rocks on the ocean floor, and studying the paleomagnetic history of ocean basins.

### Earth's magnetic field

Convection in the hot, liquid, iron-rich outer core, together with Earth's rotation, produces electric currents that sustain the magnetic field. The field has existed for at least 3 billion years. Because the core is too hot to remain permanently magnetized, it must be continuously generated.

### Magnetic reversals and stripes

Earth's magnetic field periodically reverses between normal and reversed polarity. Basalt formed at a mid-ocean ridge contains iron-bearing minerals. As the basalt cools through the Curie point, the minerals align with the current magnetic field and preserve that orientation as thermoremanent magnetization.

As new basalt forms and the seafloor spreads, it records each magnetic reversal. The resulting bands of normal and reversed polarity are parallel and symmetrical on opposite sides of the ridge. The youngest oceanic rocks are at the ridge, and rocks become progressively older away from it. Present ocean floors are no more than about 200 million years old, whereas continental rocks may be about 4 billion years old, because oceanic lithosphere is recycled at subduction zones and buoyant continental crust generally remains at the surface.

### Hot spots

Hot spots are volcanic centers produced by hot, buoyant material rising from deep in the mantle. If a hot spot remains relatively fixed while a plate moves over it, a chain of volcanoes and seamounts forms. The Hawaiian-Emperor chain becomes older toward the northwest, recording Pacific plate movement. Its sharp bend indicates a past change in plate-motion direction. Yellowstone is an example of a continental hot spot.

### Wadati-Benioff zones

An inclined zone of shallow, intermediate, and deep earthquake foci traces a descending slab at a subduction zone. The existence of a Wadati-Benioff zone is strong evidence that rigid, breakable lithosphere is descending into the mantle.

### Check Your Understanding

1. **Describe the major processes that are thought to produce Earth's magnetic field.**

   **Answer:** Convection and compositional changes in the liquid, iron-rich outer core make buoyant material rise and denser material sink. Earth's rotation also affects the moving conductive liquid. Together these motions generate electric currents that produce and sustain Earth's magnetic field.

2. **How have magnetic reversals and the study of paleomagnetism been important in understanding plate tectonics?**

   **Answer:** Cooling basalt at a mid-ocean ridge records whether Earth's magnetic field is normal or reversed at the time the rock forms. Because seafloor spreading carries new basalt away from the ridge, it creates matching, mirror-image magnetic stripes on both sides. The stripes, combined with rock ages and their position at ridges, provide strong evidence that seafloor spreading and plate movement occur.

3. **What are hot spots?**

   **Answer:** Hot spots are volcanic centers associated with hot, buoyant mantle material rising from deep within Earth. If a hot spot is relatively fixed while a tectonic plate moves over it, a linear chain of volcanoes forms. Increasing ages away from the active hot spot can reveal the direction and history of plate movement.

## 2.4 Pangaea and Present Continents (LO:5)

Pangaea was a supercontinent containing nearly all modern continents. It was present about 200 million years ago and began breaking apart about 180 million years ago as seafloor spreading opened ocean basins. Pangaea included Laurasia in the north and Gondwana in the south.

Plate movement changed the positions of continents and the shapes of ocean basins. The Tethys Sea closed as Africa and Europe-Asia converged, contributing to formation of the Alps; a remnant remains as the Mediterranean Sea. About 50 million years ago, India collided with Asia, and the continuing collision produced the Himalayas and Tibetan Plateau. Plate movement also changes mountain ranges, ocean circulation, climate, habitats, weather patterns, and natural-resource distribution. High mountains can increase precipitation and create steep slopes prone to landslides and flooding.

### Check Your Understanding

1. **What was Pangaea and when did it exist?**

   **Answer:** Pangaea was an enormous supercontinent containing nearly all modern continents. The chapter places it at about 200 million years ago, with the most recent breakup and continental drift beginning about 180 million years ago. It consisted of northern Laurasia and southern Gondwana.

2. **What geologic problems were solved when the existence of Pangaea was confirmed?**

   **Answer:** Reassembling the continents into Pangaea explains why identical fossil plants and animals occur on continents now separated by oceans. It also explains the distribution and directions of ancient glacial deposits: patterns that seem unlikely on the present continents make sense when the southern continents are restored to their former positions near the South Pole.

## 2.5 How Plate Tectonics Works: Putting It Together (LO:6)

The chapter identifies two likely gravity-driven forces that move plates from spreading centers toward subduction zones.

### Ridge push

Mid-ocean ridges stand above the surrounding seafloor because hot, newly formed lithosphere is buoyant. As the lithosphere cools and becomes denser, gravity causes it to slide downslope away from the ridge. This force is called ridge push.

### Slab pull

Oceanic lithosphere cools and becomes denser as it moves away from a ridge. At a subduction zone, the cold slab sinks through the less-dense asthenosphere. The weight of the descending slab pulls the rest of the plate toward the trench. This force is called slab pull.

### Relative importance

Both processes are gravity-driven. Calculations indicate that ridge push is relatively weak compared with slab pull, and plates with large attached subducting slabs tend to move faster than plates driven mainly by ridge push. The chapter therefore considers slab pull probably more influential, while recognizing that the complete mechanism remains an active area of study.

### Check Your Understanding

1. **What is the difference between ridge push and slab pull?**

   **Answer:** Ridge push is the gravity-driven sliding of cooling, denser lithosphere away from an elevated mid-ocean ridge. Slab pull is the force produced when cold, dense oceanic lithosphere sinks at a subduction zone and pulls the rest of the plate toward the trench.

2. **What evidence suggests that slab pull may be the dominant driver of plate tectonics?**

   **Answer:** Calculations of gravitational effects show that ridge push is relatively weak. In addition, plates with large subducting slabs attached to them, especially around the Pacific Basin, tend to move faster than plates driven primarily by ridge push. These observations suggest that slab pull may be the more influential force.

## 2.6 Plate Tectonics and Hazards (LO:7)

Plate tectonics determines the distribution of many earthquakes and volcanoes and indirectly affects hazards through the topography and environments it creates. The key connections are:

- **Divergent boundaries:** Usually produce shallow earthquakes and volcanic eruptions. Much of the activity is beneath the ocean, so risk is often lower, with important on-land exceptions such as Iceland and East Africa. Subglacial volcanic activity in Iceland can also produce floods.
- **Transform boundaries:** Produce appreciable earthquake risk where faults pass through populated areas, including parts of California, Haiti, and New Zealand. Associated highlands may also contribute to landslides and flooding.
- **Convergent subduction boundaries:** Produce the greatest combination of hazards, including the largest earthquakes, explosive volcanoes, tsunamis, flooding, mass wasting, and subsidence.
- **Continent-continent collisions:** Build high mountains and plateaus. Earthquakes, steep slopes, increased precipitation, landslides, and flooding are common, and high topography can influence monsoons and global climate.
- **Tectonic topography:** Mountains and plateaus alter atmospheric circulation, severe weather, erosion, climate, ecosystems, and the distribution of resources.

Plate tectonics cannot be stopped, but losses can be reduced through hazard mapping, site investigation, building codes, retrofitting, monitoring, land-use planning, and preparedness.

### Check Your Understanding

1. **Explain why convergent plate boundaries represent a greater hazard than other plate boundaries.**

   **Answer:** Convergent boundaries combine several hazardous processes. Subduction zones can generate great earthquakes, explosive volcanic eruptions, tsunamis, subsidence, flooding, and mass wasting. Continent-continent collisions produce the highest mountains and plateaus, which are associated with earthquakes, steep slopes, increased precipitation, landslides, flooding, severe weather, and climate effects. Divergent boundaries are often beneath oceans, and transform boundaries are dominated mainly by earthquakes, so their combination of hazards is generally less extensive.

## Important Case Studies and Examples (supports LO:1 through LO:7)

- **San Andreas fault:** A continental transform boundary between the Pacific and North American plates. Its movement produces earthquakes and has built parts of coastal California's mountainous topography.
- **1906 San Francisco earthquake:** Movement on the San Andreas fault destroyed much of San Francisco and helped establish the connection between fault movement and earthquakes. Ruptured gas mains caused fires that destroyed additional buildings.
- **California's future earthquake risk:** Los Angeles and San Francisco are moving toward one another along the plate boundary and may be side by side in about 20 million years. Population growth means a future large earthquake could produce a catastrophe, although building codes and retrofitting reduce vulnerability.
- **Andes:** Ocean-continent subduction formed a trench, continental volcanic arc, mountains, and a combination of earthquake and volcanic hazards.
- **Himalayas and Tibetan Plateau:** Continuing collision between India and Asia produced the world's highest mountains and high plateau, with related earthquake, landslide, flooding, monsoon, and climate effects.
- **Hawaii:** A hot-spot volcanic chain whose islands become older toward the northwest, recording Pacific plate movement.
- **Iceland:** An above-sea-level part of the Mid-Atlantic Ridge where divergent-boundary volcanism and earthquakes occur on land.

## Likely Comparison Questions

Be ready to answer these directly:

- How are Earth's compositional layers different from its physical-property layers?
- What are the differences between the inner and outer cores?
- How are the lithosphere and asthenosphere different?
- How are oceanic and continental crust different in thickness, density, age, and buoyancy?
- Why was Wegener's continental-drift hypothesis not initially accepted?
- How does seafloor spreading work?
- What evidence confirms seafloor spreading and plate tectonics?
- How do divergent, convergent, and transform boundaries differ?
- How does subduction differ from continent-continent collision?
- How do hot spots and Wadati-Benioff zones provide evidence for plate movement or subduction?
- How did Pangaea's breakup and the India-Asia collision change Earth's surface?
- How do ridge push and slab pull differ, and which is thought to be more important?
- Why is the seafloor younger than most continental crust?
- Why do convergent boundaries generally represent the greatest natural-hazard risk?

## Practice Questions

1. Describe Earth's compositional layers from the surface to the center.
2. Compare the lithosphere and asthenosphere in terms of location, strength, and behavior.
3. Explain why the inner core is solid while the outer core is liquid.
4. Explain how convection and Earth's rotation contribute to the magnetic field.
5. Define plate tectonics and explain why it is considered a unifying theory.
6. Why was continental drift a reasonable hypothesis even before its mechanism was known?
7. List at least four lines of evidence supporting plate tectonics.
8. Explain how magnetic reversals create symmetrical stripes on the seafloor.
9. Why does oceanic crust become older with increasing distance from a mid-ocean ridge?
10. Explain why oceanic crust is recycled but continental crust generally remains at the surface.
11. Identify the boundary type and likely hazards at the San Andreas fault.
12. Compare ocean-continent subduction, ocean-ocean subduction, and continent-continent collision.
13. What is a Wadati-Benioff zone, and what does it reveal?
14. How can a hot spot produce a chain of volcanoes?
15. What was Pangaea, and what evidence shows that it existed?
16. How did the collision of India and Asia change Earth's surface and climate systems?
17. Explain ridge push and slab pull. Why is slab pull thought to be dominant?
18. Trace a chain of hazards that begins with a convergent plate boundary.
19. Why can slow plate motion still produce sudden, damaging earthquakes?
20. How can society reduce the consequences of hazards related to plate tectonics?

## Quick Review Checklist

Before the quiz or test, make sure you can:

- summarize sections 2.1 through 2.6;
- label and describe the core, mantle, crust, Moho, lithosphere, and asthenosphere;
- compare oceanic and continental crust;
- explain convection and the sources of Earth's internal heat;
- define divergent, convergent, and transform boundaries and the three convergent settings;
- match each boundary type with its landforms, hazards, and an example;
- explain why Wegener's idea was rejected and how seafloor spreading supplied the missing mechanism;
- describe evidence from fossils, glaciation, magnetic reversals, seafloor ages, hot spots, and Wadati-Benioff zones;
- explain why the seafloor is younger than continental crust;
- describe Pangaea's breakup and the India-Asia collision;
- distinguish ridge push from slab pull; and
- connect plate tectonics to earthquakes, volcanoes, tsunamis, landslides, flooding, severe weather, and climate.