Geology Reference
In-Depth Information
The rate of movement and motion of plates can be
calculated in several ways.
Some type of convective heat system is involved in
plate movement.
Plate movement affects the distribution of natural
resources.
Plate movement affects the distribution of the world's
biota and has infl uenced evolution.
As we stated in Chapter 1,
plate tectonic theory
has had
significant and far-reaching consequences in all fields of
geology because it provides the basis for relating many
seemingly unrelated phenomena. The interactions between
moving plates determines the location of continents, ocean
basins, and mountain systems, all of which, in turn, affect
atmospheric and oceanic circulation patterns that ultimately
determine global climate (see Table 1.3). Plate movements
have also profoundly infl uenced the geographic distribution,
evolution, and extinction of plants and animals. Further-
more, the formation and distribution of many geologic
resources, such as metal ores, are related to plate tectonic
processes, so geologists incorporate plate tectonic theory into
their prospecting efforts.
If you're like most people, you probably have only a vague
notion of what plate tectonic theory is. Yet plate tectonics affects all
of us. Volcanic eruptions, earthquakes, and tsunami are the result
of interactions between plates. Global weather patterns and oce-
anic currents are caused, in part, by the confi guration of the conti-
nents and ocean basins. The formation and distribution of many
natural resources are related to plate movement, and thus have
an impact on the economic well-being and political decisions of
nations. It is therefore important to understand this unifying the-
ory, not only because it affects us as individuals and as citizens of
nation-states, but also because it ties together many aspects of the
geology you will be studying.
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Imagine it is the day after Christmas, December 26, 2004,
and you are vacationing on a beautiful beach in Thailand.
You look up from the topic you're reading to see the sea sud-
denly retreat from the shoreline, exposing a vast expanse
of seafl oor that had moments before been underwater and
teeming with exotic and colorful fi sh. It is hard to believe that
within minutes of this unusual event, a powerful tsunami will
sweep over your resort and everything in its path for several
kilometers inland. Within hours, the coasts of Indonesia, Sri
Lanka, India, Thailand, Somalia, Myanmar, Malaysia, and the
Maldives will be inundated by the deadliest tsunami in his-
tory. More than 220,000 people will die, and the region will
incur billions of dollars in damage.
One year earlier, on December 26, 2003, violent shak-
ing from an earthquake awakened hundreds of thousands
of people in the Bam area of southeastern Iran. When the
magnitude-6.6 earthquake was over, an estimated 43,000
people were dead, at least 30,000 were injured, and approxi-
mately 75,000 survivors were left homeless. At least 85% of
the structures in the Bam area were destroyed or damaged.
Collapsed buildings were everywhere, streets were strewn
with rubble, and all communications were knocked out.
Now go back another 12½ years to June 15, 1991, when
Mount Pinatubo in the Philippines erupted violently, dis-
charging huge quantities of ash and gases into the atmo-
sphere. Fortunately, in this case, warnings of an impending
eruption were broadcast and heeded, resulting in the evacu-
ation of 200,000 people from areas around the volcano. Un-
fortunately, the eruption still caused at least 364 deaths not
only from the eruption, but also from the ensuing mudfl ows.
What do these three recent tragic events have in common?
They are part of the dynamic interactions involving Earth's
plates. When two plates come together, one plate is pushed or
pulled under the other plate, triggering large earthquakes such as
the one that shook India in 2001, Iran in 2003, Pakistan in 2005,
and Indonesia in 2006. If conditions are right, earthquakes can
produce a tsunami such as the one in 2004 or the 1998 Papua
New Guinea tsunami that killed more than 2200 people.
As the descending plate moves downward and is assimi-
lated into Earth's interior, magma is generated. Being less
dense than the surrounding material, the magma rises toward
the surface, where it may erupt as a volcano such as Mount
Pinatubo did in 1991 and others have since. It therefore
should not be surprising that the distribution of volcanoes
and earthquakes closely follows plate boundaries.
CONTINENTAL DRIFT
The idea that Earth's past geography was different from to-
day is not new. The earliest maps showing the east coast of
South America and the west coast of Africa probably pro-
vided people with the fi rst evidence that continents may have
once been joined together, then broken apart and moved
to their present positions. As far back as 1620, Sir Francis
Bacon commented on the similarity of the shorelines of
western Africa and eastern South America. However, he
did not make the connection that the Old and New Worlds
might once have been joined together.
Antonio Snider-Pellegrini's 1858 book
Creation and Its
Mysteries Revealed
is one of the earliest specifi c references to the
idea of continental drift. Snider-Pellegrini suggested that all of
the continents were linked together during the Pennsylvanian
Period and later split apart. He based his conclusions on the re-
semblances between plant fossils in the Pennsylvanian-age coal
beds of Europe and North America.
During the late 19th century, the Austrian geologist
Edward Suess noted the similarities between the Late Pa-
leozoic plant fossils of India, Australia, South Africa, and
South America, as well as evidence of glaciation in the rock
sequences of these continents. The plant fossils comprise
a unique flora that occurs in the coal layers just above the
glacial deposits of these southern continents. This flora is
very different from the contemporaneous coal swamp fl ora
of the northern continents, which Snider-Pellegrini noted
earlier, and is collectively known as the
Glossopteris
flora
after its most conspicuous genus (
◗
Figure 2.1).
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