Geology And Geophysics

A Guide to Plate Tectonics how Continents Move



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A GUIDE TO PLATE TECTONICS: HOW CONTINENTS MOVE

The story of Plate Tectonics is a fascinating story of the lighter continents drifting majestically from place to place above the heavier mantle, then breaking apart, colliding, and grinding against each other; of oceanic and continental mountain ranges rising up like ripples and waves in carpets being pushed together; of oceans stretching and contracting and undersea volcanic mountain chains circling the planet like scars; of violent earthquakes and erupting volcanoes. Plate Tectonics describes the intricate design of a complex, living planet in a state of dynamic flux and is one of the core ideas behind the theory of the Earth being a living organism - "Gaia".

It may not seem like it but we are all adrift, the ground beneath us in constant, if imperceptible, motion. Huge slabs of Earth's crust slide over the partially molten mantle, pulled at one end by the continental slab dipping into the mantle at a subduction zone where one continental plate is forced under another continental plate, and pushed by new crust welling up at mid-ocean ridges. The whole upper mantle, at least down to 300 miles, and maybe more, moves with the continent. Thus, Africa is carried like a raft embedded in a larger plate, not propelled as an isolated continent. The continents are really only the bit of the plate that can be seen above sea level, the plate it's self may extend for hundreds of miles below sea level So a continental plate is in many ways similar to an iceberg as it drifts along on the ocean current.

Initially the theory of the continents moving around the surface of the earth was raised in the idea of Continental Drift in 1912, which proposed the theory that continents move slowly about the earth's surface, changing their positions relative to one another and to the poles of the earth. Initially the theory was not generally accepted, most geologists believing the continents were fixed in place and subject only to vertical movements, such as those observed during mountain uplift. In the 1960's, however, a body of evidence in support of a modified form of the drift theory came about. The new ideas were precise and unified, with emphasis on a moving, evolving ocean floor. The new theory was called Plate Tectonics.

About 300 years ago soon after the Atlantic Ocean had been mapped it was noticed that the opposite coasts of South America and Africa had similar interlocking shapes, but it was not until the middle of the 19th century that accurate maps were published demonstrating that the two coasts could be fitted together quite closely. Some geologists even then suggested that the fit of the coasts was not an accident-that the continents were once joined and had subsequently drifted apart but none of the suggestions were taken seriously.

However, In 1912 the German meteorologist Alfred Wegener investigated the fit of the Atlantic coasts more carefully than had his predecessors and grouped all the continents together into one great land mass, which he called Pangaea. He believed that the mass began to break apart about 200 million years ago. He also proved that there were many striking similarities between the fossil plants and reptiles on the opposite coasts. Wegener also pointed out that ancient climatic zones seemed to have lain in different places from the present zones. He demonstrated that where great ice sheets have melted in recent geological times in Scandinavia and North America, the land is rising as fast as a centimeter a year. This vertical uplift, he said, requires horizontal inflow of matter below and implies that flow and motion do take place within the earth.

Wegener's arguments led to heated controversy about continental drift in the 1920's and 1930's. Opponents regarded the idea of the solid rock of continents moving about as so preposterous that they ignored all his other arguments, many of which, it is now clear, were essentially correct. Only a few geologists accepted the theory. Among the first of these was the South African geologist Alex L. Du Toit, Two developments in the 1950's and 1960's added weight to the argument for continental drift, one was the study of the magnetism in ancient rocks [paleomagnatism], carried out from the early 1950's. The other was the discovery, of a continuous mid-ocean ridge, a vast submarine mountain system lying along the middle of most of the oceans. Related to this system is a line of deep trenches, island arcs, and young mountains, where earthquake activity occurs-for example, along the boundaries of the Pacific Ocean which these days as often referred to as the "Rim of Fire".

Although Wegener and Du Toit proposed that the primitive continents began to break up about 200 million years ago, there is much evidence that drift began long before then, and that continental blocks have slowly been moving about the earth's surface throughout much of geological time.

Certain rocks, when they are formed, are magnetized in the direction of the earth's magnetic field. Examination of this paleomagnetism in rocks of various ages revealed the startling fact that the earth' s magnetic field has reversed its direction many times. If a core is drilled through undisturbed rock, one would find that the young rock on top is magnetized in the present normal direction, the older layer underneath is magnetized in the reverse direction, the next layer is again normal, the next layer is again reversed, and so forth. Investigations carried out all over the world show that the earth' s magnetic field has reversed direction every few hundred thousand years.

Such a model of continental drift is confirmed by many lines of evidence in addition to those already mentioned. If the ocean floors are spreading, they will be very young near the mid-ocean ridge and progressively older toward the coasts. Thus, although young sediments and young volcanoes can form anywhere on the ocean floors, older sediments and old volcanic islands should only be found toward the coasts a good example of this is the Hawaiian chain of islands. It has become clear through oceanographic research that no old islands are located near the mid-ocean ridge, but that as the coasts are approached some progressively older islands are found. It has also been shown that the maximum age of sediments decreases toward the mid-ocean ridges, and that the total thickness of sediments increases from zero over the ridges to a few miles near the continents.

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