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Biology 103
2003 First Paper
On Serendip

Information for Plate Tectonics

Vanessa Herrera

"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)


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