Geoscience Reference
In-Depth Information
chains and why earthquakes and volcanoes occur along plate
boundaries. The first part of the chapter closely examines this the-
ory and how it explains the formation and geography of mountain
ranges. The second part of the chapter focuses on the specific pro-
cesses and landforms associated with volcanism and earthquakes.
Plate Tectonics
If you frequently travel in the United States, or live in the east-
ern or western parts of the country, you are probably somewhat
familiar with the Appalachian and Rocky Mountains. These re-
gions contain prominent mountain chains that have formed over
the course of geologic time. Although many of these mountains
are partially composed of igneous rocks that represent some
kind of volcanic origin, many contain marine sedimentary
rocks that are now thousands of meters above the modern sea-
floor. How is that possible? You are also probably aware that
earthquakes tend to occur in distinct places such as California,
and that volcanoes are common in the Pacific Northwest of the
United States, as well as other regions on Earth. Why is this so?
This chapter focuses on these questions by investigating the
concept of
plate tectonics
, which is the now accepted theory that
Earth's crust consists of individual pieces (plates) that move indi-
vidually and collectively. Understanding plate tectonics is impor-
tant because it explains the geographic distribution of mountain
The Lithospheric Plates
A good place to begin a discussion of plate tectonics is the early
20th century, when a German geophysicist named Alfred Wegener
noticed that many of the continents look as if they could have fit
together at one time as a single landmass (Figure 13.1). Wegener no-
ticed that the east coast of South America, for example, mirrors the
southwestern edge of Africa. Wegener looked more deeply into the
problem and discovered that portions of some continents share the
same kind of rocks and fossils with other continents (Figure 13.1),
which is precisely what one would expect if the continents were in-
deed connected at some point in the past. As a result of these stud-
ies, Wegener proposed in 1915 that a
supercontinent
existed about
300 million years ago that he named
Pangaea
(meaning
whole
Fossils of the Triassic reptile
Lystrosaurus
are found in Africa,
Madagascar, India, and Antarctica
AFRICA
INDIA
SOUTH AMERICA
AUSTRALIA
ANTARCTICA
Fossils of the fern
Glossopteris
are found in parts of all southern
continents and India, implying
they were all once joined together
Fossils of the land reptile
Cynognathus
are found in
middle Africa and middle
South America
Fossils of the freshwater
reptile
Mesosaurus
are
found in southern Africa
and South America
Figure 13.1 Fossils and continental drift.
A strong line of evidence supporting the theory of continental drift is the occurrence of
specific fossil plant and animal species on continents that are now far apart. (
Source
: U.S. Geological Survey.)
Plate tectonics
The theory that the Earth's crust is divided
into a number of plates that move because they float on the
asthenosphere.
Pangaea
The hypothetical supercontinent, composed of
all the present continents, that existed between 300 and 200
million years ago.