Earth's crust is broken into segments called Tectonic Plates. The major and minor plates move very slowly, carrying along the continents and ocean floor that lie on top of them.


Jigsaw puzzle - that's what may people think of when they look at a map of the world. Push Europe and Africa over across the Atlantic Ocean, and the coastlines fit - fit very well - against the coastlines of the Americas. Move India, Australia, and Antarctica around, and their coastlines also fit together - like piece of a giant jigsaw puzzle.


When the Americans were discovered and mapped some five centuries ago, scientist noticed that the opposing coast along the Atlantic Ocean had shapes that would fit together. They proposed that, early in Earth's history, the continents had been joined, and that later they had been violently torn apart. in the 19th century, this idea was supported by studies of geology and life-forms on  both sides of the Atlantic. The studies revealed many similarities between species, suggesting that they had been intermixing and interbreeding across the continents as recently as 150 million years ago. Studies such as the led Alfred Lothar Wegener, a German meteorologist, to propose in 1912 the theory of CONTINENTAL DRIFT.

CONTINENTAL DRIFT

Wegner believed that the opening of the Atlantic and Indian oceans was not due to an earlier cataclysm, but rather had occurred slowly and gradually. He bolstered his argument with measurements from recent surveys of the distance between Greenland and Europe. The measurement suggested that the two landmasses were moving away from each other at a perceivable rate. Wegner further theorizes that, because Earth is rotating sphere, there exists a force that pushes the continents toward the equator.

The continents, he believed, plow through the rocks of the seafloor like ship through water. As a continents moves, he added, coastal mountain range pile up like low waves along the land's leading edge.

PLATE TECTONICS

By 1965, investigations led to the proposal that Earth's surface was broken into 7 large plates and several small plates. It was further suggested that these plates are rigid, and that their boundaries are marked by earthquakes and volcanic activity. In recent years, satellite pictures have documented the existence of plate boundaries. An especially visible example is the San Andreas Fault in California.

Plates interact with one another at their boundaries by moving toward, away, or along side each other. Faults are examples of boundaries where two plates slide horizontally past each other. Mid-ocean ridges mark boundaries where plates are forced apart as new ocean floor is being created between them. Mountains, volcanic-island arcs, and ocean trenches occur at the boundaries where plates are colliding, causing one plate to slide beneath the other. The network of crustal plates and the geologic activity caused by their movements is referred to as PLATE TECTONICS.

ARCS AND TRENCHES

The theory of plate tectonics may also be used to explain circular island arc and oceanic trenches. Where ocean floor is being carried down freely into the interior, it is likely to do so along circular arcs. Geometry dictates this pattern, a concept illustrated when a person pushes his or her thumb into a dead tennis ball: the resulting depression is circular. This principles may explain the origin of circular volcanic-island arcs, such as the Aleutians.


If, on the other hand, a continental plate pushes past and over an oceanic plate, the ocean floor will be forced down into the interior of the Earth along an offshore trench. The deep trenches of the coasts of Peru and Chile are prime example.

MOUNTAIN BUILDING

The Theory of plate tectonics also explains how and when mountains ranges such as ANDES and the CASCADES arose.

Both this ranges lie at the converging boundaries of 2 plates. As these 2 plates collide, portion of Earth's crust are uplifted and folded, causing a great compressing and thrusting of rock. Earthquakes and volcanoes continue to occur int the area of these new and active mountain ranges.

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EARTHQUOAKES

The crust Earth is constantly moving. Massive forces are continually shaping and reshaping its rocks - twisting and cracking them, thrusting then into enormous folds. These changes are generally slow, and most humans are rarely aware of them - except when they culminate in a major earthquake.

Each year, the world experiences about 200 quakes large enough  to cause major damage. But earthquakes are much more common that this figure indicates. When all shocks are considered, from the greatest to the smallest year. Sizable shocks average about 10,000 each year. Worldwide, about one person in 8,000 will be killed and roughly 10 times as many will be injured by earthquakes.

CAUSES

Throughout humans history, the unpredictability and ruinous effects of earthquakes have provoked some fanciful explanations. In japan, Earth tremors were thought to be caused by the subterranean stirrings of of a giant catfish, normally restrained with a big mallet by a watchful deity. 

This belief was paralleled by similar ideas in China and India-except that a big tortoise and a giant mole were the respective culprits. Around 300 B.C., the Greek philosopher Aristotle theorized that winds from above were drawn into hollow passageways deep inside Earth. Agitated by fire and seeking to escape, these winds caused quakes and sometimes erupted as volcanoes.

Modern geologic research has to led to a much clearer understanding of Earth's structure and the reasons for earthquakes. Scientists now know that deep-seated geologic forces in Earth lead to movements within its uppermost layer.

THE QUAKE ITSELF

Stresses within Earth's rock structure are the driving forces that cause displacements, or strains, int that structure. They take a long time to build, but they finally may reach a point at which even a slight addition of stress, or a small weakening of rock, causes a rupture.


Most earthquakes seem to be cause by sudden movements along preexisting faults located near plate boundaries. Because of friction and the tremendous pressure between the plates, the rocks on either side of the fault do not slide freely against each other. they are virtually locked together and do not move at first, despite building forces. Great strains build up, and the rocks bends. Eventually the plates reach their elastic limit, or breaking point. They suddenly shift to the fault surface, and "snap" into their new positions - a phenomenon called ELASTIC REBOUND. The resultant sudden jar is felt as an EARTHQUAKE.


As with any tearing or breaking action, an earthquakes begins at a specific point and spreads outward. The point of initial fracture is called the "focus"(or hypocenter) of an earthquake. The point on Earth's surface directly above the focus is called the EPICENTER. The distance from the surface to the focus varies from 3 to 25miles in shallow focus earthquakes, to more than 300miles in the deep-focus quakes characteristics of subduction zones.


The rocks on opposite sides of a fault and offset in proportion to the size of the earthquakes. The displacement varies from a fraction of an inch to 20 feet. In a small quake, the fracturing stops within a few seconds. In the largest quakes, it may continue for more than a minute.


Energy is released from the rocks at the leading edge of the fracture. Most of the released strains is dissipated by the breaking and crushing of rocks, by the moving of adjoining blocks, and by the creation of heat. A small portion of energy radiated outward in the form of seismic waves. The ground motion felt during an earthquake is caused by the arrival of these waves at the surface.

EFFECTS

The motion of Earth's surface caused by earthquakes waves can be so slight that no one feels it. Such minor quakes are detected only by sensitive instruments called SEISMOGRAPHS.


Many larger quakes cause a noticeable swaying of the ground and rattling, but they cause neither serious damage nor visible changes in the surface of Earth. The vast majority of quakes fall into these categories. Truly catastrophic quakes are few in number. When earthquakes of a cataclysmic nature do occur, nearly all human-made structures are destroyed.

THE SEISMOGRAPH

The first effective seismographs were built around 1890. Although today's instruments are much more sophisticated, the basic principle remains unchanged. A pendulum mass mounted on a spring is freely suspended from a frame attached to the ground. This construction allows the mass to be reasonably independent of the frame's motion. When the supporting frame is shaken by earthquakes waves. the inertia of the mass causes it to lag behind the motion is measured electronically by the seismometer, and recorded in a computer or on paper as a seismogram.


Since the motion of the ground at any point is three-dimensional, three directions of movement, perpendicular to one another, are recorded. One vertical and two horizontal seismometers generate three seismograms from which seismologists can calculate the movement in space.


Skilled technicians can interpret the initial arrival times of the P and S waves at the recording station. The distance from the earthquake focus to the station os calculated from the known speed of wave travel between different regions on the globe. The data from three or more recording stations, at different locations, are necessary to pinpoint the source of the earthquake. Today seismographic stations are set up all over the world. More than 20,000 earthquakes are documented every year by this worldwide network. 

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EARTH MAGNETIC FIELD

In the year 1600, William Gilbert, an English physician, set forth the theory that Earth has the properties of a huge magnet, on whose magnetic poles nearly coincide with its geographic poles. He also suggested that Earth's magnetic field originates mainly in the planet's deep interior. These ideas have been confirmed by many investigators. 

Many explanations have been offered to account for the magnetism of our planet. One theory assumed that permanently magnetized iron was present in the deep when it was shown that Earth's core was partly fluid, and therefore could not hold permanent magnetism. It seems likely that permanent magnetism elsewhere in Earth's interior would not provide a sufficiently strong field.

In 1947, Patrick M.S. Blackett of the University of London in Britain suggested that any massive rotating body, such as Earth, generates a magnetic field solely as a consequences of the rotation. Laboratory experiments with large rotating objects, however, did not reveal any such field. A test on Earth itself carried out in deep mines also failed to support Blackett's theory.

It now seems probable that the magnetic field of Earth is generated by ordinary electric currents circulating through the planet's interior.

Sir Harold Lamb pointed out in 1893 that such currents would have to be continuously supplied from some source of energy within Earth. It is natural to suppose that this would take place in the part of Earth where there is the least electrical resistance - that is, in the fluid outer core.

In 1939, Walter Elsasser, a German-born American physicist, suggested that such a current might arise in the core when materials of different electric properties and at slightly different temperatures came into contact. This is called the THERMOELECTRIC HYPOTHESIS. Thermoelectricity is produced by the unequal heating of an electric circuit composed of 2 dissimilar metals. In 1954, Stanley K. Runcorn suggested that there might be a thermoelectric effect at the boundary between the mantle and the outer core.

The most highly developed theory of Earth's magnetism is the Dynamo Theory of Elsasser and Sir Edward Bullard. It holds that a huge natural dynamo deep within Earth converts mechanical energy would be supplied by a special type of fluid motion, called CONVECTION, carrying electric currents inside the outer core. Elsasser calculated that such a motion is possible. The dynamo theory of the origin of Earth's magnetism is considered to be the most promising of all the present time.

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