Live Earthquake Map

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:

3.1 Summarize the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

3.2 List and explain the evidence Wegener presented to support his continental drift hypothesis.

3.3 List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

3.4 Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

3.5 Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found.

3.6 Describe the relative motion along a transform fault boundary and locate several examples of transform faults on a plate boundary map.

3.7 Explain why plates such as the African and Antarctic plates are increasing in size while the Pacific plate is decreasing in size.

3.8 List and explain the evidence used to support the plate tectonics theory.

3.9 Describe plate–mantle convection and explain two of the primary driving forces of plate motion.

Concept Checks 

3.1 

1. Briefly describe the view held by most geologists prior to the 1960s regarding the ocean basins and continents.

2. Name the early twentieth-century hypothesis that was at first rejected by geologists and the more comprehensive theory that later replaced it.

3.2

1. What was the first line of evidence that led early investigators to suspect that the continents were once connected?

2. Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis.

3. Early in the twentieth century, what was the prevailing view of how land animals apparently migrated across vast expanses of open ocean?

4. Describe two aspects of Wegener’s continental drift hypothesis that were objectionable to most Earth scientists.

3.3

1. What new findings about the ocean floor did oceanographers discover after World War II?

2. Compare and contrast Earth’s lithosphere and asthenosphere.List the three types of plate boundaries and describe the relative motion along each.

3.4

1. Sketch or describe how two plates move in relation to each other along divergent plate boundaries.

2. What is the average rate of seafloor spreading in modern oceans?List four features that characterize the oceanic ridge system.

3.5

1. Why does oceanic lithosphere subduct while continental lithosphere does not? 

2. What characteristic of a slab of oceanic lithosphere explains the formation of a deep oceanic trench as opposed to one that is less deep?  

3. What distinguishes a continental volcanic arc from a volcanic island arc?

3.6

1. Sketch or describe how two plates move in relation to each other along a transform plate boundary.

2. List two characteristics that differentiate transform faults from the two other types of plate boundaries.

3.7

1. Name two plates that are growing in size. 

2. Name a plate that is shrinking in size.

3. What new ocean basin was created by the breakup of Pangaea?

3.8

1. What is the age of the oldest sediments recovered using deep-ocean drilling? 

2. How do the ages of these sediments compare to the ages of the oldest continental rocks?

3. What did the study of preserved magnetism in ancient lava flows tell researchers about the geographic locations of North America and Europe about 180 million years ago?

4. Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the Hawaiian Islands were forming?

5. Describe how magnetic reversals provide evidence of the seafloor-spreading hypothesis.

3.9

1. Which of these forces, slab pull or ridge push, contributes more to plate motion?

2. Briefly describe the whole mantle convection (plume) model.

Concepts In Review

3.1 From Continental Drift to Plate Tectonics

Summarize the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

Key Term: plate tectonics

*Geologists once thought that ocean basins were very old and continents were fixed in place. The theory of plate tectonics changed that. Supported by multiple kinds of evidence, plate tectonics is the foundation of modern Earth science.

3.2 Continental Drift: An Idea Before Its Time

List and explain the evidence Wegener presented to support his continental drift hypothesis.

Key Terms: continental drift, supercontinent, Pangaea

*German meteorologist Alfred Wegener formulated the continental drift hypothesis in 1915, which suggested Earth’s continents are not fixed but move slowly over geologic time.

*Wegener proposed that Pangaea, a supercontinent, existed about 200 million years ago during the late Paleozoic and early Mesozoic eras.

*Evidence that Pangaea existed and later broke into pieces that drifted apart included (1) the shape of the continents, (2) continental fossil organisms that matched across oceans, (3) matching rock types and modern mountain belts on separate continents, and (4) sedimentary rocks that recorded ancient climates, including glaciers on the southern portion of Pangaea.

*Continental drift hypothesis suffered from two flaws: It proposed tidal forces as the mechanism for motion of continents, and it implied that continents plowed through weaker oceanic crust, like boats cutting through a thin layer of sea ice. 

*Most geologists rejected continental drift when Wegener proposed it.

QUSETION: Why did Wegener choose organisms such as Glossopteris and Mesosaurus as evidence for continental drift as opposed to other fossil organisms such as sharks or jellyfish?

3.3 The Theory of Plate Tectonics

List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

Key Terms: theory of plate tectonics, lithosphere, asthenosphere, lithospheric plate (plate),

*Research conducted after World War II led to new insights that revived interest in continental drift hypothesis. 

*Seafloor exploration uncovered an extremely long mid-ocean ridge system. 

*Sampling of the oceanic crust revealed that it was quite young relative to the continents.

*The lithosphere, Earth’s outermost rocky layer, is relatively stiff and deforms by bending or breaking. 

*The lithosphere consists of crust (either oceanic or continental) and the upper most mantle. *Beneath the lithosphere is the asthenosphere, a relatively weak layer of solid rock that deforms by flowing.

*The lithosphere consists of numerous segments of irregular size and shape called lithospheric plates. 

*There are seven large plates, another seven intermediate-size plates, and many relatively small microplates. 

*Plates meet along boundaries that may be divergent (moving apart from each other), convergent (moving toward each other), or transform (moving laterally past each other).

3.4 Divergent Plate Boundaries and Seafloor Spreading

Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

Key Terms: divergent plate boundary (spreading center), oceanic ridge system, rift valley, seafloor spreading, continental rift

*Seafloor spreading leads to formation of new oceanic lithosphere at mid-ocean ridge systems. As two plates move apart from one another, tensional forces open cracks in the plates, allowing magma to well up and generate new slivers of seafloor. This process generates new oceanic lithosphere at a rate of 2 to 15 centimeters (1 to 6 inches) each year.

*As it ages, oceanic lithosphere cools and becomes denser. It therefore subsides as it is transported away from the mid-ocean ridge. At the same time, the underlying asthenosphere cools, adding new material to the underside of the plate, which consequently thickens.

*Divergent plate boundaries (spreading centers) are not limited to the seafloor. Continents can break apart, too, starting with a continental rift (as in modern-day east Africa) that produces a rift valley, potentially producing a new ocean basin between the two sides of the rift.

3.5 Convergent Plate Boundaries and Subduction

Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found.

Key Terms: convergent plate boundary (subduction zone), deep-ocean trench, partial melting, continental volcanic arc, volcanic island arc (island arc)

*Plates move toward one another at convergent plate boundaries (subduction zones). Here oceanic lithosphere subducts into the mantle, where it is recycled. Subduction manifests as a deep ocean trench. The subducting slab of oceanic lithosphere can descend at a variety of angles, from nearly horizontal to nearly vertical.

*Aided by the presence of water, subducted oceanic lithosphere triggers partial melting in the mantle, which produces magma. The magma is less dense than the surrounding rock and will rise. It may cool at depth, thickening the crust, or it may make it all the way to Earth’s surface, where it erupts as a volcano.

*A line of volcanoes emerging through continental crust is a continental volcanic arc. A line of volcanoes arising in an overriding plate of oceanic lithosphere is a volcanic island arc.

*Continental crust resists subduction due to its relatively low density, and so when an intervening ocean basin is completely destroyed through subduction, the continents on either side collide, generating a new mountain range

3.6 Transform Plate Boundaries

Describe the relative motion along a transform fault boundary and locate several examples of transform faults on a plate boundary map.

Key Terms: transform plate boundary (transform fault), fracture zone

*At a transform plate boundary (transform fault), lithospheric plates slide horizontally past one another. No new lithosphere is generated, and no old lithosphere is consumed. Shallow earthquakes signal the movement of these slabs of rock as they grind past their neighbors.

*The San Andreas Fault in California is an example of a transform boundary in continental crust, while the fracture zones between segments of the Mid-Atlantic Ridge are transform faults in oceanic crust.

QUESTION: On the accompanying tectonic map of the Caribbean, find the Enriquillo Fault. (The location of the 2010 Haiti earthquake is shown as a yellow star.) What kind of plate boundary is shown here? Are there any other faults in the area that show the same type of motion?

3.7 How Do Plates and Plate Boundaries Change?

Explain why plates such as the African and Antarctic plates are increasing in size while the Pacific plate is decreasing in size.

*Although the total surface area of Earth does not change, the shape and size of individual plates constantly change as a result of subduction and seafloor spreading. Plate boundaries can also be created or destroyed in response to changes in the forces acting on the lithosphere.

*The breakup of Pangaea and the collision of India with Eurasia are two examples of how plates change through geologic time.

3.8 Testing the Plate Tectonics Model

List and explain the evidence used to support the plate tectonics theory.

Key Terms: mantle plume, hot spot, hot-spot track, Curie point, paleomagnetism (preserved magnetism), magnetic reversal, normal polarity, reverse polarity, polarity, magnetic time scale, magnetometer

*Multiple lines of evidence verify the plate tectonics model. For instance, deep-sea drilling found that seafloor age increases with distance from a mid-ocean ridge. Thickness of sediment atop this seafloor is also proportional to distance from the ridge: Older lithosphere has had more time to accumulate sediment.

*A hot spot is an area of volcanic activity where a mantle plume reaches Earth’s surface. Volcanic rocks generated by hot-spot tracks provide evidence of both the direction and rate of plate movement over time.

*Magnetic minerals such as magnetite align themselves with Earth’s magnetic field as rock forms. This paleomagnetism (preserved magnetism) is a record of the ancient orientation of Earth’s magnetic field. This allows a given stack of rock layers to be interpreted in terms of their orientation relative to the magnetic poles through time. Magnetic reversals in the orientation of the Earth’s magnetic field are preserved as “stripes” of normal polarity and reversed polarity in the oceanic crust. Magnetometers reveal this signature of seafloor spreading as a symmetrical pattern of magnetic stripes parallel to the axis of the mid-ocean ridge.

3.9 What Drives Plate Motions?

Describe plate–mantle convection and explain two of the primary driving forces of plate motion.

Key Terms: convection, slab pull, ridge push

*In general, convection (upward movement of less dense material and downward movement of more dense material) appears to drive the motion of plates.

*Slabs of oceanic lithosphere sink at subduction zones because the subducted slab is denser than the underlying asthenosphere. In this process, called slab pull, Earth’s gravity tugs at the slab, drawing the rest of the plate toward the subduction zone. As oceanic lithosphere slides down the mid-ocean ridge, it exerts an additional but smaller force, called ridge push.

*Plate tectonics and convective flow in the mantle are part of the same system. Subducting oceanic plates drive the cold downward-moving portion of convective flow, while shallow upwelling of hot rock along the oceanic ridge and buoyant mantle plumes are the upward-flowing arms of the convective mechanism.

QUESTION: Compare and contrast mantle convection with the operation of a lava lamp.