Learning Objectives
After reading and studying this chapter, students should
▪ Understand the basic internal structure and processes of the Earth
▪ Learn the basic ideas behind and evidence for the theory of plate tectonics
▪ Understand the mechanisms of plate tectonics
▪ Understand the relationship of plate tectonics to environmental geology
Chapter Summary
Internal Structure of Earth and Plate Tectonics describes the internal structure of the Earth and the
movement of lithospheric plates on the Earth’s surface, as well as the evidence for both. The chapter
opens with a discussion of the major layers of Earth structure and explains how seismic waves have
been used to discern those layers. The subsequent sections focus on the broad-scale patterns of plate
movement (including Wegener’s early idea of continental drift), the evidence for plate motion
Chapter Outline
I. Internal structure of the Earth
A. Earth is complex and dynamic
1. Internal processes
a. Affect continents, oceans, climate, weather, and all life on Earth
B. The Earth is layered and dynamic
1. Inner core
2. Outer core
3. Mantle
a. mostly solid
4. Crust
a. Variable thickness
C. Continents and ocean basins have significantly different properties and history
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Internal Structure of Earth
and Plate Tectonics
1. Lithosphere
a. cool, strong, outermost layer
b. average thickness is 100 km (few km near mid-ocean ridges up to 400 km thick beneath
continents)
2. Asthenosphere
3. Continental and oceanic crust have contrasting geologic histories
II. How we know about the internal structure of Earth
A. What we have learned about Earth from earthquakes
B. Wave behavior
1. wave refraction and reflection reveal discontinuities between layers of the Earth’s interior
a. wave reflection: waves may reflect off discontinuities such as the core-mantle boundary
2. more sophisticated seismological techniques have revealed finer details of Earth structure
III. Plate tectonics
A. Movement of the lithospheric plates
1.What is plate tectonics?
2. Locations of earthquakes and volcanoes define plate boundaries
3. Seafloor spreading is the mechanism for plate tectonics
a. Continental drift
2) fit of continents and distribution of fossils on southern continents
3) not taken seriously—no known mechanism
4. Sinking plates generate earthquakes
5. Plate tectonics is a unifying theory
B. Types of plate boundaries
1. Divergent boundaries
a. new lithosphere is produced
2. Convergent boundaries
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a. plates converge
b. higher density plate subducts
c. compression shortens crust, causing folding and thrust faulting
3. Transform boundaries
a. two plates slide past one another
C. Rates of plate motion
1. plate motion is a fast geologic process
2. few cm/yr (about as fast as fingernail or hair growth)
IV. A detailed look at seafloor spreading
A. Introduction
1. Wegener’s idea of continental drift lacked a solid mechanism for continental movement
a. seafloor spreading provided a mechanism for moving continents
B. Paleomagnetism
1. Definition
2. Magnetic field
a. Dipole magnetic field
3. Cooling volcanic rocks record prevailing magnetic polarity
5. What produces magnetic stripes?
6. Why is the seafloor no older than 200 million years?
a. age of ocean floors can be determined by magnetic patterns: youngest near ridges, older
C. Hot spots
1. volcanic centers from hot materials produced deep in mantle
2. as plate moves over hot spot, a chain of volcanoes is formed
a. Yellowstone volcanic region lies over a continental hotspot
V. Pangaea and present continents
A. Plate tectonics shapes continents and dictates the location of mountain ranges
1. The supercontinent of Pangaea
Internal Structure of Earth and Plate Tectonics
2. Understanding plate tectonics solves long-standing geologic problems
a. occurrence of similar fossil plants and animals on different continents
VI. How plate tectonics works: Putting it together
A. Driving mechanisms that move plates
1. Ridge push
2. Slab pull
a. Driven by weight of descending slab
VII. Plate tectonics and environmental geology
A. Plate tectonics affects us all
2. Tectonic processes determine types and properties of rocks, which determine characteristics of
land, resources, and soil
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. The major difference between inner core and outer core is that the outer core is liquid, while the
Critical Thinking Questions
1. An answer to this question should focus on the importance of plate tectonics to the distribution of
continents and landscapes we see today. The ocean basins, which play a large part in determining
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