The Continents Beneath Your Feet—Are They Drifting?
HAVE you ever noticed, when looking at a map of the Atlantic Ocean, how the east coast of South America seems to match the west coast of Africa? If you fit the hump of Brazil into Africa’s Gulf of Guinea, the shoreline all the way from Guyana to Argentina matches amazingly well with the line from Ghana to Capetown. The two continents seem like pieces of a gigantic jigsaw puzzle.
Perhaps when you noticed this, the thought crossed your mind that at one time South America and Africa may have actually been joined, and that somehow they split and drifted apart. If so, you probably dismissed the idea as preposterous, just a curious coincidence.
But do you know that this idea is now considered seriously by most geologists? A theory that proposes that the continents actually move here and there over the fluid mantle inside the earth’s crust has, since 1960, won general acceptance.
Theory of Continental Drift
The theory was first proposed, not by a geologist, but by a meteorologist in Germany, named Alfred Wegener. He suggested that, not only had South America and Africa once been joined, but all the continents had formed part of a single huge landmass. He called this hypothetical ancient continent Pangaea (meaning “all land”). He found that the fit of the continents was better when the outlines of the continental shelves were used, rather than the now-existing shorelines.
Today geologists use computers to slide and turn the continental outlines over a globe to obtain the best fit. In a typical reconstruction of the supposed ancient supercontinent, the southeastern coast of North America lies against the northwest coast of Africa. Eurasia is pivoted about Spain so that the west coast of Europe nuzzles in against Newfoundland and Greenland. Antarctica lies against southeast Africa, with Australia attached to its opposite side.
When Wegener first proposed this revolutionary concept in 1912, it aroused mixed feelings among geologists. Any theory that goes counter to prevailing notions in science is usually received cautiously. Continental drift met with a reception even cooler than usual, perhaps because its author was not a member of the geologists’ circles. Although there were solid bits of evidence to support the theory, it was “proved” mathematically that the earth’s crust is too strong to allow any lateral movement of the continents. And, it was asked, Where would any force originate to push the continents one way or another? No one could suggest anything that stood up under analysis. The idea gradually came to be ignored by reputable scientists.
Evidence for the Theory: Conformity
Why, then, have geologists changed their minds about continental drift? In the first place, there have gradually accumulated several kinds of evidence that they find hard to explain any other way. Among these are the similarity of geological formations and of fossil deposits on continents now widely separated, as well as the wandering of the magnetic poles of the earth.
As an example of geological conformity, we are told of a succession of sedimentary deposits, laid down during what is called the Paleozoic geologic era, and later exposed when they were lifted up into mountain ranges. Deposits of red sandstone, gray shales, and coal beds are found in the Appalachian mountain system in eastern North America, extending to eastern Greenland. They are also found in the highlands of the British Isles. Similar sediments are found in the Kjölen range in Scandinavia, and along the Atlas range in northwest Africa. In the theoretical parent continent of Pangaea, all these rock formations are believed to have been part of a continuous mountain system whose remnants are now widely separated on three continents.
The similarity in fossils found in these strata on both sides of the Atlantic is used as a further argument for the theory. Fish fossils are abundant, also land plants, even forests of tall tree ferns and great scale trees. Another oft-cited example of conformity of the fossil record is that of the mesosaurus, a small dinosaur that lived during the so-called Paleozoic era. Its fossils are found in southwest Africa and in Brazil, but they have not been found in other parts of the earth. If South America and Africa were joined at that time, then the range of the mesosaurus would have been one continuous area.
Wandering Magnetic Poles
More convincing proof has come from study of the mysterious phenomenon of polar wandering. The belief that the magnetic poles of the earth have moved about is based on measurements of the magnetization of igneous rocks. When a hot rock is cooled in a magnetic field, it is left weakly magnetized, because particles of magnetic minerals in the rock line up in the direction of the magnetic field. This shows the direction of the earth’s magnetic field at the time the rock was formed, like a “frozen compass.”
Now you might expect that all such fossil compasses would point north, but, surprisingly, rocks of different geologic ages show magnetization in many different directions. It is as if the magnetic pole were wandering widely and aimlessly all over the earth—hence the expression “polar wandering.”
However, when the directions are arranged in order according to the apparent successive ages of the rocks, it is found that the pole does follow a definite path from age to age. Furthermore, when the magnetism of rocks in other places on the same continent is measured, it is found that they consistently trace out the same path.
This discovery put the geophysicist in a quandary. Although no one knows what causes the earth’s magnetic field, it seems that it must be in some way related to the earth’s rotation, and it is hard to believe that the magnetic pole can stray very far from the geographic pole, surely not clear across the equator as the rock compasses indicated. Now, of course, the wandering magnetic paths would be explained equally well if the pole stayed fixed while the continents slid around over the globe, but that seemed even harder to believe.
What tipped the balance between two incredible explanations was the discovery that magnetic measurements on different continents usually indicate entirely different paths for the pole. This could not be explained by movements of the pole, because the earth has only one north pole, and it can’t go in several different directions at the same time. This appeared to geologists as a strong indication that the continents had actually moved independently of each other, over many thousands of miles.
Evidence from the Ocean Floors
New evidence that finally converted geologists to belief in continental drift came from the bottom of the sea. Exploration of the ocean floors really got under way in the International Geophysical Year of 1955. Oceanographers used elaborate sounding devices to chart the ocean floors. By timing echoes, they probed, not only the floor of sediment on the bottom, but also the depth of the basement of basalt rock underneath. They came to an astonishing conclusion about the ocean floors: They concluded that these are not fixed, but appear to be forming continuously at definite boundaries and spreading on a global scale.
Let us examine the discoveries that led to this startling hypothesis. The first clue to come to light was a long mountain ridge in the middle of the Atlantic Ocean. Starting there, geologists have mapped a system of mid-ocean ridges that literally encircles the earth. A typical ridge rises from the ocean floor, some three miles (5 kilometers) deep, to a peak about two miles (3 kilometers) above the floor. It is flanked on both sides by a strip of hilly terrain hundreds of miles wide. A striking feature is a valley that runs like a crack right along the crest of the ridge, thus dividing it into a pair of parallel ridges.
The acoustic soundings from the surface have been supplemented by using vessels equipped to drill holes in the bottom of the sea. These have brought up cores of rock for close inspection and analysis, some as long as 1,500 feet (460 meters), from many parts of the ocean. These surveys disclose that the ridges themselves are bare igneous rock, and that there is little or no sediment up to 60 miles (97 kilometers) on either side. Farther away, they show increasingly thicker layers of sediment, up to a mile thick.
Magnetic surveys over the oceans in the vicinity of the ridges resulted in another striking discovery. There are strips of rock lying parallel to the ridges in which the magnetism is reversed. It is as if the north and south poles had been reversed when the rocks formed. This reverse magnetization had been noted earlier in certain volcanic lava flows, but near the oceanic ridges there appears to be a continuous record of normal and reverse magnetic polarities frozen into the ocean bed. There is no explanation for this mysterious change; after all, no one knows why the earth has a magnetic field, much less why it reverses itself. It is just an observed fact of creation.
Geologists explain all three of these observations by a single hypothesis, called sea-floor spreading. They suppose that the mid-ocean ridge is being formed continuously by the upwelling of magma from the earth’s plastic mantle through a crack in the earth’s crust, and that the ocean floor is moving away from both sides of the crack as it is formed. The newly formed rock is clean, and sediment accumulates slowly and becomes noticeable only after the new rock has been exposed for some time and has moved away from the ridge. The parallel bands of normal and reverse magnetic polarity result when the magma oozes out and solidifies for a time while the earth’s poles are normal, and then for a time while they are reversed.
The findings indicate that at the present time the floor of the Atlantic Ocean is spreading a little more than an inch (2.5 centimeters) a year, and the Pacific Ocean about six inches (15 centimeters) a year. But if the earth is forming new crust on the ocean floor on this prodigious scale, it must be getting rid of its old crust somewhere else. After all, the total surface of the earth is not increasing. Geophysicists speculate that this takes place along certain boundaries where one part of the crust slides under another part and descends into the hot interior, where it melts and is consumed into the fluid mantle again. They believe that this is not a smooth process, but is accompanied by earthquakes and volcanic eruptions. It forms deep ocean trenches and high mountain ranges along the consumption boundary lines.
The Theory of Tectonic Plates
From a world map of the mid-ocean ridges and the consumption boundaries, geologists have divided up the whole earth’s surface into six large (and several smaller) plates of rigid rock. These plates, they postulate, are being formed at the ridges and move like a conveyor belt toward boundaries with other plates, where one of them is thrust underneath into the mantle and is dissolved. The continents are carried on these plates, like an Eskimo’s igloo on an ice floe.
This is called the tectonic-plate theory, from the Greek word for “builder.” Both the continental drift and the sea-floor spreading are included as parts of the broader theory.
Let us look at a few examples of how this theory is used to explain observed features of the earth’s crust. The American plate, which carries both North and South America, as well as the western half of the Atlantic Ocean, theoretically is being formed at the mid-Atlantic ridge and moving west. Along the western coast of South America, a smaller plate arising in the eastern Pacific collides with and plunges under the American plate. This supposedly causes a deep trench in the ocean off the coast of South America, and lifts the Andes mountains to the highest peaks in the Americas. The crumpling of the oceanic plate causes frequent earthquakes all along the Pacific coast. When, according to the theory, the lighter rock carried down into the mantle melts, it rises through cracks in the continental crust above it to form the volcanoes in the Andean Cordillera.
A detailed map of the mid-oceanic ridge shows that it is not really continuous, but it is offset by numerous faults at right angles. Along these transform faults, as they are called, the two theoretical plates slide horizontally. Geologists suggest that the friction from this movement is another cause of earthquakes. One of the longest of these transform faults lies between the American plate and the Pacific plate along the west coast of North America. Along this line, well known to Californians as the San Andreas fault, the Pacific plate is moving northwest against the American plate at about two inches (5 centimeters) per year. The resulting strains cause frequent earthquakes.
The city of San Francisco lies athwart this fault, and the coast of California to the south lies west of it, on the Pacific plate. So if the present movement is not interrupted, it is predicted that at some far-distant time the site of Los Angeles will lie close to where San Francisco is today.
Evidences that some places once had a climate very different from the present one also are viewed by geologists as fitting the theory of continental drift. In the postulated Pangaea, the present-day continents were all much farther south than now, excepting Antarctica. North America and the Spanish peninsula were on the equator. South America, Africa, India, and Australia were all clustered around Antarctica in the south polar regions.
Will the Theory Stand?
Scientists take satisfaction in finding a theory that apparently brings many disparate kinds of information together into a unified picture. That is what they believe the tectonic-plate theory has done for the science of geology. But does that mean that it is therefore the final and correct answer? Not necessarily.
In spite of seeming wide-ranging successes of the theory, there are still many bits of information that do not fit into it. Geologists argue over the interpretation of details. As research continues, some of these questions may be answered in a way that harmonizes with the theory. On the other hand, there may remain stubborn facts that cannot be reconciled with it.
One major shortcoming is acknowledged in the present state of the theory. The forces that cause the upwelling magma along the ridges are not explained. Some geologists have been content with the general statement that convection currents inside the earth’s mantle are responsible. But what generates the convection, and why does its pattern change? When this idea is examined in detail, it breaks down. A convection current in air or water rises around a central axis, not in a long slender sheet that would form a ridge. It is even more difficult to imagine how the displacements along the transform faults can result from convection currents.
Professors Flint and Skinner of Yale University offer this word of caution in their book Physical Geology:
“The theory of plate tectonics seems to provide answers for so many questions that we are tempted to believe it is the long-sought unifying theory that explains the lithosphere [the land areas of the earth, from its surface to the center of the earth]. But we must be careful. Other theories, too, have seemed overwhelming in their promise, yet in the long run have proved incorrect. The theory of plate tectonics is still only a theory.”
Whether the tectonic-plate theory survives the test of time and proves correct or not, we have abundant evidence of the great power and wisdom of earth’s Creator. Of him the psalmist wrote: “Long ago you laid the foundations of the earth itself, and the heavens are the work of your hands.” (Ps. 102:25) The questions Jehovah put to Job thousands of years ago still remain unanswerable by modern geologists: “Where did you happen to be when I founded the earth? Tell me, if you do know understanding. Who set its measurements, in case you know, or who stretched out upon it the measuring line? Into what have its socket pedestals been sunk down, or who laid its cornerstone?”—Job 38:4-6.