Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Become familiar with the
geographic, geologic, and
geophysical evidence that led
geologists to develop plate
tectonics theory
■■
Explore plate-tectonic
processes by observing
modern features of the Earth
■■
Use 21st-century technology
to measure the direction and
rate at which plates are moving
today
MATERIALS
NEEDED
■■
Tracing paper
■■
Colored pencils
■■
Ruler with divisions in tenths
of an inch or millimeters
(included in the Geo Tools
section at the back of this
manual)
■■
Protractor (included in the
Geo Tools section at the back
of this manual)
■■
Calculator or pencil and paper
for simple arithmetic
2.1 Introduction
Earthquakes and volcanic eruptions show that the Earth is a dynamic planet with
enough energy beneath its surface to cause disasters for those who live atop it.
Humans wondered about the causes of these events until the 1960s and 1970s, when
geologists developed the theory of plate tectonics: a unifying theory that answered
geologic questions that had puzzled us for thousands of years. According to this
theory, the outer layer of the Earth is made up of separate plates that move with
respect to one another and change the Earth’s surface as they move. At first it was
difficult to accept the concept that the Earth’s oceans, continents, and mountains
are only temporary features that move and change over time because the changes
are so slow that they could not be detected. Yet, according to plate tectonics theory,
planet-wide processes break continents apart, open and close oceans, and build and
shrink great mountain chains. Local earthquakes and volcanoes are simply results
of the energy released as these processes occur.
No one ridicules plate tectonics now because geologists have drawn on conclusive
evidence to prove that these processes are happening today and to show that
they have been operating for billions of years. In this chapter we will explore the
evidence and geologic reasoning that led to plate tectonics theory.
2.2 The Theory of Plate Tectonics
Plate tectonics theory is based on many kinds of information about the Earth that
you will examine during this course, including the origin and distribution of different
rock types, the topography of the continents and ocean basins, and the geographic
distribution of earthquakes and volcanic eruptions. The basic concepts of
the theory include the following:
■ The Earth’s crust and the uppermost part of the layer below it, called the mantle,
form a relatively rigid outermost layer, called the lithosphere, which extends to a
depth of 100 to 150 km.
■ The lithosphere is not a single shell, but consists of several large pieces called
lithosphere plates, or simply plates (FIG. 2.1). There are about 12 major plates that
are thousands of kilometers wide and several minor plates that are hundreds of
kilometers wide.
FIGURE 2.1 The Earth’s major lithosphere plates.
Trench or
collision zone
Ridge
Transform
boundary
Arabian
Plate
African
Plate
Iran
Plate
Eurasian Plate
Indian-Australian
plate
Philippine
Plate
Bismarck
Plate
Juan
de Fuca
Plate
Pacific
Plate
Cocos
Plate
North
American
Plate
Nazca
Plate
South
American
Plate
Caribbean
Plate
Scotia Plate
Antarctic
Plate
Antarctic
Plate
26 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS