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bedded in the lithosphere. The major
lithospheric plates (there are also a
few smaller ones) are thousands of
miles wide and are 40 to 80 miles
thick. Whole or great segments of
continents and oceans rest upon
them.
Yet, for all their vast size and
weight, these great plates are in
constant motion. Such movement is
imperceptible to human senses, yet
the strains and pressures, or release
of them, on rock structures are great.
We call sudden movements or release
of strain earthquakes or other geo–
logic disruption.
The estimated range of movement
is 1 to
18
centimeters a year. T hat is
just a few inches a year at most. But
the effect of these movements is
enormous.
As people jammed in a subway;
the great tectonic plates of the earth
crowd, jostle and deform each other.
They push against, draw away from,
push down or uplift each other. A
push or pull at one point can set off a
complex series of pushes and pulls
along neighboring plates.
You may ask what force gives
power to the movement of the earth's
gigantic plates. Heat-related forces
in the earth to be sure, say most earth
scientists.
Volcanic and geothermal activity
clearly a ttest to inner earth hea t. Yet
the exact mechanism or detailed flow
patterns of the inner earth heat are
not well understood.
But thc mass of data supporting
the tectonic concept indicates heated
new magma from deep within the
earth's mantle is pushing up through
cracks in thinner oceanic rifts.
Uplifting magma at ridge plate
boundaries slowly but powerfuly
force tedonic plates apart. Conti–
nents, then, do not drift, they are
being pushed.
As plates separate, twist, rub past,
push down or slide over the edges of
one another, bedrock deforms and
bends and breaks.
It
may be surpris–
ing to sorne, but rocks a re somewhat
elastic- they can bend, compress
and stretch. But there is a point
where they give away. Earthquake
and volcanic activity result. But they
are not the only resulting upheavals.
The gigantic pressures between
24
plates over long periods also uplift
mountain ranges, expand sorne
oceans at the expense of others, form
deep ocean trenches and expose new
mineral deposits.
Burlal Ground of Plates
But if new lithosphere is being
formed out of magma in ocean rifts,
where does the old material go?
The tectonic concept explains that
the burial ground of plates are the
deep ocean trenches associated with
advancing continents or island ares.
T hinner ocean plates bend under
thicker continental plates. The sub–
ducting plates are sinking and being
resorbed into the earth's mantle
approximately equal to the amount
of new lithosphere being created at
oceanic rifts.
Many oceanic plates, like the
Pacific plate, are plunging down 45°
under overriding land plates.
The Alaska-Pacific arca gives us
one possible result of this movement.
The great uplift and subsidence of
more than 200,000 square kilometers
( 125,000 square miles) in the great
Alaskan earthquake in 1964 occurred
when a section of the Pacific plate
slipped a few meters deeper into the
mantle beneath southern Alaska.
But continents tend not to be
resorbed. Continental rock for the
most part is considered too buoyant
to be subducted. They are, instead,
moved, broken and deformed.
The downward edge movement of
ocean plates tends to create deep
ocean trenches. But it does much
more. 1mmense friction between the
overriding and down-plunging plates
compresses and melts rock to
hundreds of degrees centigrade,
which often bursts up in volcanic
activity.
The pressure-heat treatment also
increases the bendability of rocks. It
also metamorphisizes (changes the
structure) of sorne rock formations.
Shallow and deep-based earthquakes
are also much more frequent in arcas
of down-plunging plates.
A Fantastlc Scene
Great land features result from jos–
tling plates.
In many places, crunching plates
cause sharply rising, jagged or crum-
pled mountains or island ares to be
pushed up alongside deep ocean
trenches.
Perhaps you can better understand
the features of many land masses
around the world close to subducting
plates. No wonder the Pacific region
is called "the ring of fire"!
Imagine taking a time-lapse cam–
era and snapping a frame of chang–
ing earth movements every century
or so since earth history began-then
replaying it at standard speed. What
a spectacular movie it would make!
Tectonlc' s Effects
on Modern Civllization
Millions flock to scenic wonders all
over the world, yet fail to realize that
many of them are the result of plate
tectonics.
Iceland, with its spectacular geo–
thermal heat sources is really just an
outburst of volcanic rock on top of
the mid-Atlantic ridge. The little
island nation, geologists estímate, is
being pulled apart a centimeter or so
each year.
Without abutting tectonic plates,
the beautiful Sierra Nevadas with
their deep land roots in the Western
United States would not exist. Nor
would one of the world's great agri–
cultura! wonders, the San Joaquín
Yalley in California.
The same holds true for many
other dramatic landscapes. Alaska,
Japan, the Himalayas and the Andes
all owe their breathtaking profiles–
and many problems to human civili–
zation-to the massive powers of
colliding earth plates.
The Himalayas, for instance, are
the result of a head-on collision
between the Indian-Australian plate
and the Asían land plate. And they
are still rising.
The mountain chains of the Alpine
belt of the Mediterranean with their
frequent earthquakes are another
example of a head-on collision of
plates. The Zagros mountains in Iran
have been created by the buckling
between the Arabian and Persian
plates.
The plunging of the Nazca plate
under the South American plate has
produced the Andes, the Chilean
deep-sea trench and sorne of the
world's great earthquakes. Similar
The
PLAIN TRUTH
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