Information for Plate Tectonics
"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...