Laboratory Manual for Introductory Geology 4e

plate between two continents, continental collision occurs, forming a collisional
mountain belt like the Himalayas, Alps, or Appalachians. Folding during the collision
thickens the crust to the extent that the thickest continental crust is found in
these mountains.
3. At a transform boundary, two plates slide past each other along a vertical zone of
fracturing called a transform fault. Most transform faults break ocean ridges into
segments; these faults are also called oceanic fracture zones. A few transform faults,
however, such as the San Andreas fault in California, the Alpine fault in New
Zealand, the Enriquillo–Plantain Garden fault in Haiti, and the Great Anatolian
fault in Turkey, cut through continental plates.
■ In the tectonic cycle, new oceanic lithosphere is created at the mid-ocean
ridges, moves away from the ridges during seafloor spreading, and returns to the
mantle in subduction zones. However, oceanic lithosphere is neither created nor
destroyed at transform faults, where movement is almost entirely horizontal.
EXERCISE 2.1
Recognizing Plates and Plate Boundaries
Name:
Section:
Course:
Date:
Using Figure 2.1 as a reference, answer the following questions:
(a) What is the name of the plate on which the contiguous United States and Alaska reside?
(b) Where is the eastern edge of this plate?
(c) Does this plate consist of continental lithosphere, oceanic lithosphere, or both?
(d) On what plate is Hawaii located?
(e) Does this plate consist of continental lithosphere, oceanic lithosphere, or both?
(f) Where and how does the lithosphere of the Atlantic Ocean form?
(g) What kind of plate boundary occurs along the west coast of South America?
(h) Is the west coast of Africa a plate boundary? Explain.
2.3 Early Evidence for Plate Tectonics
The simple problem of scale and the slow rate of plate movement delayed the discovery
of plate tectonics until the late 1960s: lithosphere plates are so big, and move so slowly,
that we didn’t realize they were moving at all. Today there is no question that they
move because global positioning satellites and sensitive instruments can measure their
directions and rates of movement. In the rest of this chapter we will look at some of the
evidence that led geologists to accept the hypothesis that plates move and then see how
we can deduce the nature and rates of processes at the three types of plate boundaries.
2.3.1 Evidence from the Fit of the Continents
As far back as 500 years ago, mapmakers drawing the coastlines of South America and
Africa noted that the two continents looked as if they might have fit together once in
a larger continent. Those foolish enough to say it out loud were ridiculed, but today
this fit is considered one of the most obvious lines of evidence for plate tectonics
(FIG. 2.3). Note that the true edge of a continent is not the shoreline, but rather the
edge of the continental shelf: the shallow water shown in light blue in Figure 2.3a.
28 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS