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Biology 103
2003 First Paper
On Serendip
"Viewed from the distance of the moon, the astonishing thing about the Earth,
catching the breath, is that it is alive. Photographs show the dry, pounded
surface of the moon in the foreground, dead as an old bone. Aloft, floating
free beneath the moist, gleaming membrane of bright blue sky, is the rising
earth, the only exuberant thing in this part of the cosmos. If you could look
long enough, you would see the swirling of the great drifts of white cloud,
covering and uncovering the half-hidden masses of land. And if you had been
looking for a very long, geologic time, you would have seen the continents themselves
in motion, drifting apart on their crustal plates, held afloat by the fire beneath."
(1) These were the words spoken by Lewis Thomas, the U.S. Physician and author.
The story of Plate Tectonics is a fascinating story of continents drifting majestically
from place to place breaking apart, colliding, and grinding against each other;
of terrestrial mountain ranges rising up like rumples in rugs being pushed together;
of oceans opening and closing and undersea mountain chains girdling the planet
like seams on a baseball; of violent earthquakes and fiery volcanoes. Plate
Tectonics describes the intricate design of a complex, living planet in a state
of dynamic flux. (1)
Many forces cause the shape of the Earth to change over long time. However,
the largest force that changes our planet's surface is the movement of Earth's
outer layer through the process of plate tectonics. This process causes mountains
to push higher and oceans to grow wider. The rigid outer layer of the Earth,
the lithosphere, is made up of plates that fit together like a jigsaw puzzle.
These solid but lightweight plates seem to "float" on top of a more dense, fluid
layer underneath. (2)
Motions deep within the Earth carry heat from the hot interior to the cooler
surface. These motions of material under the Earth's surface cause the plates
to move very slowly across the surface of the Earth, at a rate of about two
inches per year. (2) When two plates move apart, rising material from the mantle
pushes the lithosphere aside. Two types of features can form when this happens.
At mid ocean ridges, the bottom of the sea comes apart to make way for new ocean
crust formed from molten rock, or magma, rising from the mantle. Continental
rifts form when a continent begins to split apart (the East African Rift is
an example). If a continental rift continues to split a continent apart it can
eventually form an ocean basin. When two plates move towards each other, several
features can form. Often, one of the plates is forced to go down into the hot
asthenosphere at a subduction zone. Volcanoes may form when a subducted plate
melts and the molten rock comes to the surface. If neither plate is subducted,
the two crash into each other and can form huge mountains like the Himalayas.
(3)
There are several different hypotheses to explain exactly how these motions
allow plates to move. Powered by forces originating in Earth's radioactive,
solid iron inner core, these tectonic plates move ponderously about at varying
speeds and in different directions atop a layer of much hotter, softer, more
malleable rock called the athenosphere. Because of the high temperatures and
immense pressures found here, the uppermost part of the athenosphere is deformed
and flows almost plastically just beneath the Earth's surface. This characteristic
of the athenosphere to flow allows the plates to inch along on their endless
journeys around the surface of the earth, moving no faster than human fingernails
grow. (1)
One idea that might explain the ability of the athenosphere to flow is the idea
of convection currents. When mantle rocks near the radioactive core are heated,
they become less dense than the cooler, upper mantle rocks. These warmer rocks
rise while the cooler rocks sink, creating slow, vertical currents within the
mantle (these convection currents move mantle rocks only a few centimeters a
year). This movement of warmer and cooler mantle rocks, in turn, creates pockets
of circulation within the mantle called convection cells. The circulation of
these convection cells could very well be the driving force behind the movement
of tectonic plates over the athenosphere. (1)
During Earth's 4.6 billion year history, the surface of our planet has undergone
numerous transformations. These transformations have had a profound impact on
the evolution of life on Earth. When plates move they carry living organisms
along with them like passengers on a slow moving ice floe. As a plates' relative
position to the equator changes over time, organisms well adapted to a polar
environment, for example, must either evolve through adaptations or perish as
the plate migrates into a tropical environment. (1)
Did you ever wonder why elephants are only found in Africa and Asia? With plate
tectonics as a guiding principle, the answer becomes moderately clear. As India
broke away from Africa 20 million years ago it very likely ferried some unsuspecting
elephants (along with many other organisms) northward to Asia. The Asian and
African elephants have slight physical variations, but they are clearly cut
from the same genetic mold. (1)
Another very interesting theory to emerge recently concerns, perhaps, the greatest
of all mysteries - the origins of life on earth. The predominant theory held
that life had its origins in warm ponds or similar small bodies of water protected
from the harsh environment of the early earth and far from the escaping heat
of the deep sea-floors. But now scientists have discovered organisms that thrive
in these hellish conditions and appear to have been around long before the earliest
known organisms previously known. Could the hot vents at mid-ocean ridges have
been the incubators of life on this planet? (1)
(1) www.platetectonics.com
(2) www.windows.ucar.edu/tour/link=earth/interior/plate_tectonics.html
(3) www.windows.ucar.edu/tour/link=earth/interior/lithospheric_motion.html&edu=high
(4) www.windows.ucar.edu/tour/links=/earth/interior/how_plates_move.html&edu=high
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