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Guide
to Reading 
This chapter deals with a relative newcomer to geologic
thought, the theory of plate tectonics. It supplies the fundamental
explanation for so many geologic processes that it has become a unifying
principle in the modern study of geology.
The author begins by drawing a picture of Alfred
Wegener who, in the early 1900s, proposed the idea of continental
drift, the idea that the continents have moved around in relation
to one another. His arguments in support of continental drift included
- the fit of the continents
- paleoclimatic studies that showed evidence of past
glaciation, coal deposits, ancient reef deposits, ancient sand dunes,
and salt beds that make no sense in today’s world climate
belts
- the occurrence of the same fossils on lands now
separated by oceans
- the matching of geologic units (distinct assemblages
of rocks) on lands now separated by oceans
For several reasons, Wegener’s ideas were not
accepted for decades. First, although he was a scientist, he was
not a geologist. Second, accepting his ideas would have meant huge
changes in geologic thought. And third, he couldn’t supply
an explanation of how and why continents moved. New discoveries after
his death eventually proved that continents do move. The new areas
of study involved
- changes in the earth’s magnetic field over
time (paleomagnetism)
- changes in the sea floor—the shape of its
surface, types and ages of its rock, heat flow within it, and sea-floor
earthquakes—all of which support the idea of sea-floor spreading
The author spends considerable time developing a historical
context for the theory of plate tectonics. Why? Because
- working through the reasons for accepting new
theories, such as the meaning and significance of paleomagnetism,
provides practice in good scientific thinking
- the gradual acceptance of plate tectonics theory
provides an excellent example of the process by which scientific
knowledge advances as new evidence and better instruments and techniques
are introduced
- plate tectonics was a revolutionary idea that
caused profound changes in the study of geology and thus merits
your thoughtful study and understanding
After establishing that there are good reasons to
believe continents move, the author continues the chapter by explaining
how and why they move. You may find this section of the chapter easier
than the first part. Why? Because understanding why something is
believed can be more demanding than understanding what is believed,
and understanding plate tectonics is beautifully simple. Stripped
to its barest essentials, the theory contends that Earth’s
outer surface is split into pieces, or plates, that slowly shift
around in relation to each other. Plates can move away from each
other (divergence), toward each other (convergence), or past each
other (transform motion). With all these huge masses shifting around,
it’s not surprising that there are huge consequences, and these
usually occur at plate boundaries, where Earth’s surface may
be deformed, built up, or destroyed—all on a grand geologic
scale.
Naturally there are details to be studied as well.
Some of them weren’t recognized and studied until after the
theory was accepted; some were part of the evidence used to develop
the theory. The chapter proceeds with discussions of several of these
details. First is a discussion of the types of plate boundaries and
the features associated with them, including
- earthquake belts
- mid-ocean ridges
- trenches and the subduction that takes place there
- strange and numerous offset segments of the mid-ocean
ridge and the transform faults that bracket them
- the far fewer transform faults that cross land,
such as the infamous San Andreas Fault
Next the author calls attention to two special types
of boundaries, where the action is not caused by the usual plate-against-plate
motion. The first of these is the triple junction, where three plates
meet in a point. The second is the hot-spot phenomenon, illustrated
by many exotic places, like Yellowstone National Park and the Hawaiian
Islands.
This chapter explains that plate boundaries do not
remain unchanged forever. Instead, old boundaries disappear, as illustrated
by India’s collision with Asia to produce the Himalayas, and
new ones appear, as illustrated by the rifting that produced both
the East African Rift Valley and the Basin and Range Province of
the U.S. West. Exactly how does all of this happen? The author concludes
the chapter by discussing the probable explanations for actual mechanisms
of plate motion and by explaining how we are able to determine the
velocity of plates’ motions.
In his closing remarks, the author reminds us that,
directly or indirectly, plate tectonics is the key to understanding
just about everything geologic.
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