by Isaac Asimov
And yet evidence in favor of the supposition that the Atlantic Ocean once did not exist, and that the separate continents once formed a single land mass, grew massively impressive. If the continents were matched, not by their actual shoreline (an accident of the current sea level) but by the central point of the continental slope (the shallow floor of the ocean neighboring the continents which is exposed during ages of low sea level), then the fit is excellent all along the Atlantic, in the north as well as the south. Then, too, rock formations in parts of western Africa match the formations in parts of eastern South America in fine detail. Past wanderings of the magnetic poles look less startling if one considers that the continents, not the poles, wandered.
Nor was there only geographic evidence for Pangaea and its breakup. Biological evidence was even stronger. In 1968, for instance, a 2½-inch fossilized bone from an extinct amphibian was found in Antarctica. Such a creature could not possibly have lived so close to the South Pole, so Antarctica must once have been farther from the pole or, at least, milder in temperature. The amphibian could not have crossed even a narrow stretch of salt water, so Antarctica must have been part of a larger body of land, containing warmer areas. The fossil record, generally (which I shall talk about in chapter 16), is quite in tune with the existence at one time, and the subsequent breakup, of Pangaea.
It is important to emphasize here the basis of geologists’ opposition to Wegener. People who pound away at the fringe areas of science frequently justify their dubious theories by insisting that scientists tend to be dogmatic, with their minds closed to new work (true enough in some cases and at some times, though never to the extent the “fringe” theorists claim). They frequently use Wegener and his continental drift as an example, and there they are wrong.
Geologists did not object to the concept of Pangaea and its breakup. Indeed, more radical suggestions to account for the manner in which life was spread over the earth were considered hopefully. What they objected to was the specific mechanism Wegener advanced—the notion of large granite blocks drifting through a basalt “ocean.” There were serious reasons for objecting to it, and those reasons hold even today. The continents do not drift through the basalt.
Some other mechanism, then, must account for the geographic and biologic indications of continental changes in position—a mechanism that is more plausible and for which there is evidence. I shall discuss the evidence later in the chapter; but about 1960, the American geologist Harry Hammond Hess thought it reasonable, on the basis of new findings, to suggest that molten mantle material might be welling up—along certain fracture-lines running the length of the Atlantic Ocean, for instance—and be forced sideways near the top of the mantle, to cool and harden. The ocean floor is, in this way, pulled apart and stretched. It is not, then, that the continents drift, but that they are pushed apart by a spreading sea floor.
As the story seems now, Pangaea did exist, after all, and was intact as recently as 225 million years ago, when the dinosaurs were coming into prominence. To judge from the evolution and distribution of plants and animals, the breakup must have become pronounced about 200 million years ago. Pangaea then broke into three parts: the northern part (North America, Europe, and Asia) is called Laurasia; the southern part (South America, Africa, and India) is called Gondwana, from an Indian province; Antarctica plus Australia formed the third part.
Some 65 million years ago, with the dinosaurs already extinct and the mammals ruling earth, South America separated from Africa on the west, and India on the east separated and moved up toward southern Asia. Finally, North America split off from Europe, India crunched up into Asia (with the Himalayan Mountains folding up at the junction line), Australia moved away from its connection with Antarctica, and the continental arrangement as we have it at present was seen. (For the continental changes, see figure 4.4.)
Figure 4.4. Geologic eras.
THE ORIGIN OF THE MOON
An even more startling suggestion about the changes that may have taken place on the earth over geologic periods dates back to 1879, when the British astronomer George Howard Darwin (a son of Charles Darwin) suggested that the moon was a piece of the earth that had broken loose in early times, leaving the Pacific Ocean as the scar of the separation.
This is an attractive thought, since the moon makes up only a little over 1 percent of the combined earth-moon mass and is small enough for its width to lie within the stretch of the Pacific. If the moon were made up of the outer layers of the earth, it would account for the moon’s having no iron core and being much less dense than the earth, and for the Pacific floor’s being free of continental granite.
The possibility of an earth-moon breakup seems unlikely on various grounds, however; and virtually no astronomer or geologist now thinks that it can have taken place. Nevertheless, the moon seems certainly to have been closer in the past than it is today.
The moon’s gravitational pull produces tides both in the ocean and in the earth’s solid crust. As the earth rotates, ocean water is dragged across sections of shallow floor, while layers of rock rub together as they rise and fall. The friction represents a slow conversion into heat of the earth’s energy of rotation, so that its rotational period gradually increases. The effect is not great in human terms, for the day lengthens by I second in about 62,500 years. As the earth loses rotational energy, the angular momentum must be conserved. What the earth loses, the moon gains. Its speed increases as it revolves about the earth, which means it drifts farther away very slowly.
If one works backward in time toward the far geologic past, we see that the earth’s rotation must speed up, the day be significantly shorter, the moon significantly closer, and the whole effect more rapid. Darwin calculated backward to find out when the moon was close enough to earth to form a single body; but even if we don’t go that far, we ought to find evidence of a shorter day in the past. For instance, about 570 million years ago—the time of the oldest fossils—the day may have been only a little over 20 hours long, and there may have been 428 days in a year.
Nor is this only theory now. Certain corals lay down bands of calcium carbonate more actively at some seasons than others, so that you can count annual bands just as in tree trunks. It is also suggested that some lay down calcium carbonate more actively by day than by night, so that there are very fine daily bands. In 1963, the American paleontologist John West Wells counted the fine bands in fossil corals and reported there were, on the average, 400 daily bands per annual bands in corals dating back 400 million years and 380 daily bands per annual band in corals dating back only 320 million years.
Of course, the question is, If the moon was much closer to the earth then, and the earth rotated more rapidly, what happened in still earlier periods? If Darwin’s theory of an earth-moon separation is not so, what is so?
One suggestion is that the moon was captured at some time in the past. Its capture 600 million years ago, for instance, might account for the fact that we find numerous fossils in rocks dating back to about that time, whereas earlier rocks have nothing but uncertain traces of carbon. Perhaps the earlier rocks were washed clean by the vast tides that accompanied the capture of the moon. (There was no land life at the time; if there had been, it would have been destroyed.) If the moon were captured, it would have been closer then than now, and there would be a lunar recession and a lengthening of the day since, but nothing of the sort before.
Another suggestion is that the moon was formed in the neighborhood of the earth, out of the same gathering dust cloud, and has been receding ever since, but never was actually part of the earth.
The study and analysis of the moon rocks brought back to Earth by astronauts in the 1970s might have settled the problem (many people had thought optimistically that it would), but it did not. For instance, the moon’s surface is covered with bits of glass, which are not to be found on Earth’s surface. The moon’s crust is also entirely free of water and is poor in all substances that melt at relatively low temperatures, p
oorer than Earth is. This is an indication that the moon may at one time have been routinely subjected to high temperatures.
Suppose, then, the moon at the time of its formation had had a highly elliptical orbit with its aphelion at roughly its present distance to the sun and its perihelion in the neighborhood of Mercury’s orbit. It might have circled in this way for a few billion years before a combination of positions of itself, Earth, and perhaps Venus resulted in the moon’s capture by Earth. The moon would abandon its position as a small planet to become a satellite, but its surface would still show the marks of its earlier Mercurylike perihelion.
On the other hand the glasses could be the result of the local heat produced by the meteoric bombardment that had given birth to the moon’s craters. Or, in the very unlikely case of the moon’s having fissioned from the earth, they might be the result of the heat produced by that violent event.
All suggestions about the moon’s origin seem, in fact, to be equally improbable; and scientists have been heard to mutter that if the evidence for the moon’s origin is carefully considered, then the only possible conclusion is that the moon is not really out there—a conclusion, however, that just means they must continue the search for additional evidence. There is an answer, and it will be found.
THE EARTH AS LIQUID
The fact that the earth consists of two chief portions—the silicate mantle and the nickel-iron core (in about the same proportions as the white and yolk of an egg)—has persuaded most geologists that the earth must have been liquid at some time in its early history. It might then have consisted pf two mutually insoluble liquids. The silicate liquid, being the lighter, would float to the top and cool by radiating its heat into space. The underlying iron liquid, insulated from direct exposure to space, would give up its heat far more slowly and would thus remain liquid to the present day.
There are at least three ways in which the earth could have become hot enough to melt, even from a completely cold start as a collection of planetesimals. These bodies, on colliding and coalescing, would give up their energy of motion (kinetic energy) in the form of heat. Then, as the growing planet was compressed by gravitational force, still more energy would be liberated as heat. Third, the radioactive substances of the earth—uranium, thorium, and potassium—have delivered large quantities of heat over the ages as they have broken down; in the early stages, when there was a great deal more radioactive material than now, radioactivity itself might have supplied enough heat to liquefy the earth.
Not all scientists are willing to accept a liquid stage as an absolute necessity. The American chemist Harold Clayton Urey, in particular, maintained that most of the earth was always solid. He argued that in a largely solid earth an iron core could still be formed by a slow separation of iron; and that even now, iron may be migrating from the mantle into the core at the rate of 50,000 tons a second.
The Ocean
The earth is unusual among the planets of the solar system in possessing a surface temperature that permits water to exist in all three states: liquid, solid, and gas. A number of worlds farther from the sun than Earth are essentially icy—Ganymede and Callisto, for instance. Europa has a worldwide surface glacier and may have liquid water beneath, but all such outer worlds can have only insignificant traces of water vapor above the surface.
The earth is the only body in the solar system, as far as we know, to have oceans—vast collections of liquid water (or any liquid at all, for that matter) exposed to the atmosphere above. Actually, I should say ocean, because the Pacific, Atlantic, Indian, Arctic, and Antarctic oceans all comprise one connected body of salt water in which the Europe-Asia-Africa mass, the American continents, and smaller bodies such as Antarctica and Australia can be considered islands.
The statistics of this ocean are impressive. It has a total area of 140 million square miles and covers 71 percent of the earth’s surface. Its volume, reckoning the average depth of the oceans as 21⅓ miles, is about 326 million cubic miles. It contains 97.2 percent of all the H2O on the earth and is the source of the earth’s fresh water supply as well, for 80,000 cubic miles of it are evaporated each year to fall again as rain or snow. As a result of such precipitation, there is some 200,000 cubic miles of fresh water under the continents’ surface and about 30,000 cubic miles of fresh water gathered into the open as lakes and rivers.
Viewed in another fashion, the ocean is less impressive. Vast as it is, it makes up only a little over 1/4,000 of the total mass of the earth. If we imagine the earth to be the size of a billiard ball, the ocean would be represented by an unnoticeable film of dampness. If you made your way down to the very deepest part of the ocean, you would only be 1/580 of the distance to the center of the earth—and all the rest of that distance would be first rock and then metal.
And yet that unnoticeable film of dampness means everything to us. The first forms of life originated there; and, from the standpoint of sheer quantity, the oceans still contain most of our planet’s life. On land, life is confined to within a few feet of the surface (though birds and airplanes do make temporary sorties from this base); in the oceans, life permanently occupies the whole of a realm as deep as seven miles or more in some places.
And yet, until recent years, human beings have been as ignorant of the ocean depths and particularly of the ocean floor as if the ocean were located on the planet Venus.
THE CURRENTS
The founder of modern oceanography was an American naval officer named Matthew Fontaine Maury. In his early thirties, he was lamed in an accident that, however unfortunate for himself, brought benefits to humanity. Placed in charge of the depot of charts and instruments (undoubtedly intended as a sinecure), he threw himself into the task of charting ocean currents. In particular, he studied the course of the Gulf Stream, which had first been investigated as early as 1769 by the American scholar Benjamin Franklin. Maury gave it a description that has become a classic remark in oceanography: “There is a river in the ocean.” It is certainly a much larger river than any on land. It transports a thousand times as much water each second as does the Mississippi. It is 50 miles wide at the start, nearly a half mile deep, and moves at speeds of up to 4 miles an hour. Its warming effect is felt even in the far northern island of Spitzbergen.
Maury also initiated international cooperation in studying the ocean; he was the moving figure behind a historic international conference held in Brussels in 1853. In 1855, he published the first textbook in oceanography, entitled Physical Geography of the Sea. The Naval Academy at Annapolis honored his achievements by naming Maury Hall after him.
Since Maury’s time, the ocean currents have been thoroughly mapped. They move in large clockwise circles in the oceans of the Northern Hemisphere and in large counterclockwise circles in those of the Southern, thanks to the Coriolis effect. The Gulf Stream is but the western branch of a clockwise circle of current in the North Atlantic. South of Newfoundland, it heads due east across the Atlantic (the North Atlantic drift). Part of it is deflected by the European coast around the British Isles and up the Norwegian coast; the rest is deflected southward along the northwest shores of Africa. This last part, passing along the Canary Islands, is the Canaries current. The configuration of the African coast combines with the Coriolis effect to send the current westward across the Atlantic (the north equatorial current). It reaches the Caribbean, and the circle starts all over.
A larger counterclockwise swirl moves water along the rims of the Pacific Ocean south of the Equator. There, the current, skirting the continents, moves northward from the Antarctic up the western coast of South America, as far as Peru. This portion of the circle is the cold Peru, or Humboldt, current (named for the German naturalist Alexander von Humboldt, who first described it about 1810).
The configuration of the Peruvian coastline combines with the Coriolis effect to send this current westward across the Pacific just south of the Equator (the south equatorial current). Some of this flow finds its way through the waters of the Indone
sian archipelago into the Indian Ocean. The rest moves southward past the eastern coast of Australia, and then eastward again.
These swirls of water help to even out the temperature of the ocean somewhat and, indirectly, the continental coasts as well. There are still uneven distributions of temperature, but not as much as there would be without the ocean currents.
Most of the ocean currents move slowly, even more slowly than the Gulf Stream. Even at slow speeds, such large areas of the ocean are involved that enormous volumes of water are moved. Off New York City, the Gulf Stream moves water northeastward past some fixed line at the rate of about 45 million tons per second.
There are water currents in the polar regions as well. The clockwise currents in the Northern Hemisphere and the counterclockwise ones in the Southern both succeed in moving water from west to east on the poleward edge of the circle.
South of the continents of South America, Africa, and Australia, a current circles the continent of Antarctica from west to east across unbroken ocean (the only place on Earth where water can drift from west to east without ever meeting land). This west-wind drift in the Antarctic is the largest ocean current on Earth, moving nearly 100 million tons of water eastward past any given line each second.
The west wind drift in the arctic regions is interrupted by land masses, so that there is a North Pacific drift and a North Atlantic drift. The North Atlantic drift is deflected southward by the western coast of Greenland, and the frigid polar water passes Labrador and Newfoundland, so that that portion is the Labrador current. The Labrador current meets the Gulf Stream south of Newfoundland, producing a region of frequent fogs and storms.