Asimov's New Guide to Science

by Isaac Asimov

  As time went on other dramatic discoveries were made. The island of Hawaii is the top of an underwater mountain 33,000 feet high, measuring from its undersea base—higher than anything in the Himalayas; thus, Hawaii may fairly be called the tallest mountain on the earth. There are also numerous flat-topped cones, called seamounts or guyots. The latter name honors the Swiss-American geographer Arnold Henry Guyot, who brought scientific geography to the United States when he emigrated to America in 1848. Seamounts were first discovered during the Second World War by the American geologist Harry Hammond Hess, who located 19 in quick succession. At least 10,000 exist, mostly in the Pacific. One of these, discovered in 1964 just south of Wake Island, is over 14,000 feet high.

  Moreover, there are the ocean deeps (trenches), more than 20,000 feet deep, in which the Grand Canyon would be lost. The trenches, all located alongside island archipelagoes, have a total area amounting to nearly 1 percent of the ocean bottom. This may not seem much, but it is actually equal to one-half the area of the United States, and the trenches contain fifteen times as much water as all the rivers and lakes in the world. The deepest of them are in the Pacific; they are found there alongside the Philippines, the Marianas, the Kuriles, the Solomons, and the Aleutians (figure 4.5). There are other great trenches in the Atlantic off the West Indies and the South Sandwich Islands, and there is one in the Indian Ocean off the East Indies.

  Figure 4.5. Profile of the Pacific bottom. The great trenches in the sea floor go deeper below sea level than the height of the Himalayas, and the Hawaiian peak stands higher from the bottom than the tallest land mountain.

  Besides the trenches, oceanographers have traced on the ocean bottom canyons, sometimes thousands of miles long, which look like river channels. Some of them actually seem to be extensions of rivers on land—notably a canyon extending from the Hudson River into the Atlantic. At least twenty such huge gouges have been located in the Bay of Bengal alone, as a result of oceanographic studies of the Indian Ocean during the 1960s. It is tempting to suppose that these were once river beds on land, when the ocean was lower than now. But some of the undersea channels are so far below the present sea level that it seems altogether unlikely they could ever have been above the ocean. In recent years, various oceanographers—notably William Maurice Ewing and Bruce Charles Heezen—have developed another theory: that the undersea canyons were gouged out by turbulent flows (turbidity currents) of soil-laden water in an avalanche down the off-shore continental slopes at speeds of up to 60 miles an hour. One turbidity current, which focused scientific attention on the problem, took place in 1929 after an earthquake off Newfoundland. The current snapped a number of cables, one after the other, and made a great nuisance of itself.

  The Mid-Atlantic Range continued to present surprises. Later soundings elsewhere showed that it was not confined to the Atlantic. At its southern end, it curves around Africa and moves up the western Indian Ocean to Arabia. In mid-Indian Ocean, it branches so that the range continues south of Australia and New Zealand and then works northward in a vast circle all around the Pacific Ocean. What began (in men’s minds) as the Mid-Atlantic Ridge became the Mid-Oceanic Ridge. And in one rather basic fashion, the Mid-Oceanic Ridge is not like the mountain ranges on the continent: the continental highlands are of folded sedimentary rocks, while the vast oceanic ridge is of basalt squeezed up from the hot lower depths.

  After the Second World War, the details of the ocean floor were probed with new energy by Ewing and Heezen. Detailed soundings in 1953 showed, rather to their astonishment, that a deep canyon ran the length of the Ridge and right along its center. This was eventually found to exist in all portions of the Mid-Oceanic Ridge, so that sometimes it is called the Great Global Rift. There are places where the Rift comes quite close to land: it runs up the Red Sea between Africa and Arabia, and it skims the borders of the Pacific through the Gulf of California and up the coast of the state of California.

  At first it seemed that the Rift might be continuous, a 40,000-mile crack in the earth’s crust. Closer examination, however, showed that it consists of short, straight sections that are set off from each other as though earthquake shocks had displaced one section from the next. And, indeed, it is along the Rift that the earth’s quakes and volcanoes have tended to occur.

  The Rift was a weak spot up through which heated molten rock (magma) welled slowly from the interior—cooling, piling up to form the Ridge, and spreading out farther still. The spreading can be as rapid as 16 centimeters per year, and the entire Pacific Ocean floor could be covered with a new layer in 100 million years. Indeed, sediment drawn up from the ocean floor is rarely found to be older, which would be remarkable in a planetary life forty-five times as long, were it not for the concept of sea-floor spreading.

  It appeared at once that the earth’s crust was divided into large plates, separated from each other by the Great Global Rift and its offshoots. These were called tectonic plates, tectonic coming from a Greek word for “carpenter,” since the plates seemed to be cleverly joined to make a seemingly unbroken crust. The study of the evolution of the earth’s crust in terms of these plates is referred to by those words in reverse as plate tectonics.

  There are six large tectonic plates and a number of smaller ones, and it quickly became apparent that earthquakes commonly take place along their boundaries. The boundaries of the Pacific plate (which includes most of the Pacific Ocean) include the earthquake zones in the East Indies, in the Japanese islands, in Alaska and California, and so on. The Mediterranean boundary between the Eurasian and African plates is second only to the Pacific rim for its well-remembered earthquakes.

  Then, too, the faults that had been detected in the earth’s crust as deep cracks where the rock on one side could, periodically, slide against the rock on the other to produce earthquakes, were also on the boundaries of the plates and on the offshoots of those boundaries. The most famous of all such faults, the San Andreas, which runs the length of coastal California from San Francisco to Los Angeles, is part of the boundary between the American and the Pacific plates.

  And what about Wegener’s continental drift? If an individual plate is considered, then objects upon it cannot drift or change position. They are locked in place by the stiffness of the basalt (as those who were opposed to Wegener’s notions had pointed out). What’s more, neighboring plates were so tightly wedged together that it was difficult to see what could make them move.

  The answer came from another consideration. The plate boundaries were places where not only earthquakes were common, but volcanoes, too. Indeed, the shores of the Pacific, as one follows the boundary of the Pacific plate, are so marked by volcanoes, both active and inactive, that the whole has been referred to as the circle of fire.

  Could it be, then, that magma might well up from the hot layers deep in the earth through the cracks between the tectonic plates, these cracks representing weaknesses in Earth’s otherwise solid crust? Specifically, magma might be welling up very slowly through the Mid-Atlantic Rift and solidifying on contact with ocean water to form the Mid-Atlantic Range on either side of the Rift.

  We can go farther. Perhaps as the magma welled up and solidified, it pushed the plates apart. If so, it would succeed in pushing Africa and South America apart on the south, and Europe and North America apart on the north, breaking up Pangaea, forming the Atlantic Ocean, and making it ever wider. Europe and Africa would be pushed apart, too, with the Mediterranean and Red seas forming. Because the sea floor would grow wider as a result, this effect was called sea-floor spreading and was first proposed by H. H. Hess and Robert S. Dietz in 1960. The continents were not floating or drifting apart, as Wegener had thought; they were fixed to plates that were being pushed apart.

  How could sea-floor spreading be demonstrated? Beginning in 1963, the rocks obtained from the ocean floor on either side of the Mid-Atlantic Rift were tested for their magnetic properties. The pattern changed with distance from the Rift, and did so in exact correspondence,
but as a mirror image, on either side. There was clear evidence that the rocks were youngest near the Rift and increasingly older as one moved away from it on either side.

  In this way, it could be estimated that the Atlantic sea floor was spreading, at the moment, at the rate of just under an inch a year. On this basis, the time when the Atlantic Ocean first began to open could be roughly determined. In this and other ways, the movement of tectonic plates has completely revolutionized the study of geology in these last two decades.

  Naturally, if two plates are forced apart, each must (in view of the tightness of the fit of all the plates) be jammed into another on the other side. When two plates come together slowly (at a rate of no more than 2 inches or so per year), the crust buckles and bulges both up and down, forming mountains and their roots. Thus, the Himalayan Mountains seem to havc been formed when the plate bearing India made slow contact with the plate bearing the rest of Asia.

  On the other hand, when two plates come together too rapidly to allow buckling, the surface of one plate may gouge its way under the other, forming a deep trench, a line of islands, and a disposition toward volcanic activity. Such trenches and islands are found in the western Pacific, for instance.

  Plates push apart under the influence of sea-floor spreading, as well as come together. The Rift passes right through western Iceland, which is (very slowly) being pushed apart. Another place of division is at the Red Sea, which is rather young and exists only because Africa and Arabia have already pushed apart somewhat. (The opposite shores of the Red Sea fit closely if put together.) This process is continuing, so that the Red Sea is, in a sense, a new ocean in the process of formation. Active upwelling in the Red Sea is indicated by the fact that at the bottom of that body of water there are, as discovered in 1965, sections with a temperature of 56° C and a salt concentration at least five times normal.

  Presumably, there has been a long, very slow cycle of magma welling up to push plates apart in some places, and plates coming together, pushing crust downward, and converting it to magma. In the process, the continents come together into a single land mass and then split up, not once, but many times, with mountains forming and being worn down, ocean deeps forming and being filled in, volcanoes forming and becoming extinct. The earth is geologically, as well as biologically, alive.

  Geologists can now even follow the course of the most recent breakup of Pangaea, though still only in a rough manner: An early break came in an east-west line. The northern half of Pangaea—including what is now North America, Europe, and Asia—is sometimes called Laurasia, because the oldest part of the North American surface rocks, geologically speaking, are those of the Laurentian Highlands north of the St. Lawrence River.

  The southern half—including what is now South America, Africa, India, Australia, and Antarctica—is called Gondwanaland (a name invented in the 1890s by an Austrian geologist, Edward Suess, who derived it from a region in India and based it on a theory of geologic evolution that then seemed reasonable but is now known to be wrong).

  About 200 million years ago, North America began to be pushed away from Eurasia; and 150 million years ago, South America began to be pushed away from Africa—the two continents eventually connecting narrowly at Central America. The land masses were pushed northward as they separated until the two halves of Laurasia clasped the Arctic region between them.

  About 110 million years ago, the eastern portion of Gondwanaland broke into several fragments: Madagascar, India, Antarctica, and Australia. Madagascar stayed fairly close to Africa, but India moved farther than any other land mass in the time since the most recent Pangaea. It moved 5,500 miles northward to push into southern Asia to form the Himalayan Mountains, the Pamirs, and the Tibetan plateau—the youngest, greatest, and most impressive highland area on Earth.

  Antarctica and Australia may have separated only 40 million years ago. Antarctica moved southward to its frozen destiny. Australia is still moving northward today.

  LIFE IN THE DEEP

  After the Second World War, the deeps of the ocean continued to be explored. An underwater-listening device, the hydrophone, has, in recent years, shown that sea creatures click, grunt, snap, moan, and in general make the ocean depths as maddeningly noisy as ever the land is.

  A new Challenger probed the Marianas Trench in the western Pacific in 1951 and found that it (and not one off the Philippine Islands) was the deepest gash in the earth’s crust. The deepest portion is now called the Challenger Deep. It is over 36,000 feet deep: if Mount Everest were placed in it, a mile of water would roll over its topmost peak. Yet the Challenger brought up, from the floor of the abyss, bacteria which look much like bacteria of the surface but cannot live at a pressure of less than 1,000 atmospheres!

  The creatures of the trenches are so adapted to the great pressures of these bottoms that they are unable to rise out of their trench; in effect, they are imprisoned in an island. They have experienced a segregated evolution. Yet they are in many respects related to other organisms closely enough that it seems their evolution in the abyss has not gone on for a very long time. One can visualize some groups of ocean creatures being forced into ever lower depths by the pressure of competition, just as other groups were forced ever higher up the continental shelf until they emerged onto the land. The first group had to become adjusted to higher pressures; the second, to the absence of water. On the whole, the latter adjustment was probably the more difficult, so we should not be amazed that life exists in the abyss.

  To be sure, life is not as rich in the depths as nearer the surface. The mass of living matter below 4½ miles is only one-tenth as great per unit volume of ocean as it is estimated to be at 2 miles. Furthermore, there are few, if any, carnivores below 4½ miles, since there is insufficient prey to support them. They are scavengers instead, eating anything organic that they can find. The recentness with which the abyss has been colonized is brought out by the disclosure that no species of creature found there has been developed earlier than 200 million years ago, and most have histories of no more than 50 million years. It is only at the beginning of the age of the dinosaurs that the deep sea, hitherto bare of organisms, was finally invaded by life.

  Nevertheless, some of the organisms that invaded the deep survived there, whereas their relatives nearer the surface died out—as was demonstrated, most dramatically, in the late 1930s. On 25 December 1938, a trawler fishing off South Africa brought up an odd fish about 5 feet long. What was odd about it was that its fins were attached to fleshy lobes rather than directly to the body. A South-African zoologist, J. L. B. Smith, who had the chance of examining it, recognized it as a matchless Christmas present. It was a coelacanth, a primitive fish that zoologists had thought extinct for 70 million years. Here was a living specimen of an animal that was supposed to have disappeared from the earth before the dinosaurs reached their prime.

  The Second World War halted the hunt for more coelacanths; but in 1952, another of a different genus was fished up off Madagascar. By now, numbers have been found. Because it is adapted to fairly deep waters, the coelacanth dies soon after being brought to the surface.

  Evolutionists have been particularly interested in studying the coelacanth specimens because it was from this fish that the first amphibians developed; in other words, the coelacanth is a direct descendant of our fishy ancestors.

  An even more exciting find was made in the late 1970s. There are hot spots in the ocean floor, where the hot magma of the mantle rises unusually near the upper boundary of the crust and heats the water above it.

  Beginning in 1977, a deep-sea submarine carried scientists 40wn to investigate the sea floor near hot spots east of the Galapagos Islands and at the mouth of the Gulf of California. In the latter hot spot, they found chimneys, through which hot gushes of smoky mud surge upward, filling the surrounding sea water with minerals.

  The minerals are rich in sulfur, and the neighborhood of these hot spots is also rich in species of bacteria that obtain their energy from
chemical reactions involving sulfur plus heat, instead of from sunlight. Small animals feed on these bacteria, and larger animals feed on the smaller ones.

  This was a whole new chain of life forms that did not depend upon the plan t cells in the uppermost layers of the sea. Even if sunlight did not exist at all anywhere, this chain can exist provided heat and minerals continue to gush 177 upward from the earth’s interior; hence, it can exist only near the hot spots.

  Clams, crabs, and various kinds of worms, some quite large, were retrieved and studied from these sea-floor areas. All of these flourished in water that would be poisonous to species not adapted to the chemical peculiarities of the region.

  DEEP-SEA DIVING

  This is an example of the fact that the ideal way to study the ocean deeps is to send human observers down into them. Water is not a suitable environment for us, of course. Since ancient times, divers have practiced their skills and learned to dive down for 60 feet or so and remain underwater for up to 2 minutes. But the unaided body cannot much improve this performance.

  In the 1930s, goggles, rubber foot fins, and snorkels (short pipes, one end in the mouth and the other sticking up above the surface of the water, from a German word for “snout”) made it possible for swimmers to move underwater for longer periods of time and with more efficiency than otherwise. This was skin diving, immediately below the surface, or “skin,” of the ocean.