p Two hundred million years is, of course, a very approximate hgure. It was arrived at by estimating the present rate of sedimentation of the Pacific floor and then comparing it with the present thickness of the layer of sediment on the Pacific floor. But we do not yet know whether the rate of marine sedimentation was constant at all periods in the history of our planet. Over the ages the sedimentary layer may have become packed down, consolidated, and we have no reliable data with which to calculate the ratio between consolidation and one or another period of geological time.

p Hence, other methods are used to determine the age of the Pacific, such as, for example, the time it would take to acquire its present degree of salinity. Estimates have shown this to be between 100 million and 300 million years, that is, the average agrees with Menard’s figure. However, we do not know if the oceans grew saline at a constant rate.

p Shephard notes the following interesting circumstance: no matter how many different fossils the dredges bring up from the ocean floor and from seamounts, none are ever older than the Cretaceous period. He believes this shows that the oceans may not be very old. The Cretaceous period began 140 million years ago and lasted 70 million years. This would make the Pacific a good 48 50 million yours younger than Mouunl thinks, that is, about 150 million years old.

p Some scientists put the origin of the Pacific at an even later period. Academician Shcherbakov, for instance, believes the oldest layers of the ocean floor were formed about 100 million years ago. Finally, the Soviet geologist G. Afanasyev offers a new estimate of the sedimentation rate and puts the origin of the Pacific, and of the World Ocean too, in the Tertiary period, which would make the Pacific no more than 70 million years old. In fact, Afanasyev maintains that the Pacific is no more than 50 million years old.

p Other scientists, using the same data on the sedimentary cover, believe that sediment was deposited much more slowly in the past and therefore place the origin of the Pacific at a much more remote period. Professor Leontyev, a Soviet oceanographer, thinks the Pacific is at least 1,000 million (!) years old. He and other scientists in the Soviet Union and abroad consider the oceans to be the same age as the planet, that is, thousands of millions of years old.

p Despite their differences as to the dating, most scientists agree that the Pacific Ocean as we know it today developed as a result of long and intensive processes in the earth’s crust, that it has its own geological history, although its prehistory is shrouded in the mists of time.

p In setting forth this brief history we shall follow, in the main, Menard’s excellent monograph Marine Geology of the Pacific. Here he brings together and summarises the vast amount of factual material that oceanographers and geophysicists have accumulated over recent years. Moreover, he examines all geological, 49 geophysical and biological processes and facts in their interconnection.

p Two hundred million years ago, says Menard, the Pacific basin cannot have been much different from the basin as it is now: it was surrounded by an almost continuous ring of island arcs and submarine ridges. The water depth averaged about the same as now but the distribution of depths was different from the present. Sometime during the Meso/oic era there began, in the eastern and middle Pacific, a great process that led to the rise of a vast underwater land, the highest parts forming islands and archipelagoes, some of which still exist while others have disappeared. Menard calls this the Darwin Rise in honour of the great English naturalist, who first advanced the hypothesis that islands and banks, now submerged, once existed in that part of the Pacific basin.

p This mid-Pacific underwater continent was 10,000 kilometres long and 4,000 kilometres wide, stretching from the Tuamotu Archipelago to the Marshall Islands. The paroxysm of volcanism that produced the Darwin Rise also led to the birth of volcanic islands and islets. Colossal crustal blocks were moved away; on the northwest flank of the Rise, island arcs developed and deep troughs were formed. By the Middle Cretaceous time, about 100 million years ago, large volcanoes had built up from the central part of the Pacific. When closely-spaced volcanoes had grown large enough to overlap along the flanks of the Darwin Rise they built great volcanic ridges such as the Mid-Pacific Mountains and the Tuamotu Ridge. Although other, more widely spaced volcanoes did not overlap, they did break through the water and form separate 50 islands. Among them, in Menard’s opinion, are the Marshall Islands with their numerous volcanic peaks.

p Today, the central part of the Pacific is an area of enormous waves and considerable depths. Millions of years ago there were groups of islands and extensive shallow banks here. The peaks of underwater ridges, formed when volcanoes merged into a single chain, crowned many of them. This was when the imperceptible but truly titanic work of the corals began. The remains of these tiny marine organisms, which flourish only at moderate depths, built up today’s coral islands and reefs, atolls, submarine plateaus and banks.

p If coral islands are built by shallow-water organisms, how is it that they are found at depths of several miles? Scientists pondered this question for almost 150 years. The great Darwin advanced the theory that a lagoon island is a monument erected by myriads of tiny architects to mark the place where land was buried in the ocean depths.

p Darwin’s hypothesis had its ardent champions and no less fervent detractors. It was debated for more than 100 years, until scientists of our day proved that Darwin was fundamentally right.

p Coral reefs are found in the Atlantic, Pacific and Indian oceans. However, the small reefs of the West Indian and Bermuda islands in the Atlantic, and even the numerous coral islands and islets in the Indian Ocean (the Maldive Islands and the Gocos or Keeling islands) are insignificant compared with the innumerable coral reefs scattered through the tropical part of the Pacific in a gigantic band running 51 northwest to south-east, almost 10,000 kilometres long and some 2,500 kilometres wide!

p The large coral islands and archipelagoes—the Marshall Islands, the Caroline Islands, the Tuamotu Archipelago, the Gilbert Islands and the Ellice Islands—were inhabited long before the beginning of our era. Europeans did not find people on other coral islands in the mid-Pacific, but they came upon many signs that those islands had once been inhabited. Finally, there are hundreds upon hundreds of coral reefs that were never places of human habitation.

p The Great Barrier Reef of northeastern Australia, discovered at the end of the 18th century by the famous Captain James Cook, stretches for almost 2,000 kilometres, the northern part about 100 kilometres from the coast, the middle section coming to within about 15 kilometres, and the southern part being more than 150 kilometres from the coast. Between it and the coast there are a large number of smaller coral reefs.

p The assumption that the Great Barrier Reef and the other, smaller reefs along the coasts of continents and islands are “tombstones” on top of submerged land does not arouse any doubt. We know that the ocean is rising most rapidly in that part of the Pacific, while the South-East Asian coast and the adjacent islands are slowly sinking. But are the numerous coral islands and atolls in the middle of the Pacific also “ tombstones”over sunken land? Darwin and his supporters thought this was so. But their hypothesis was proven only a short time ^ago, by deep drilling on coral islands.

p Coral reefs grow at a rate of about 17 to 37 metres per 1,000 years. The thicker the reef, the 52 older it is. Every 100 metres in ttie depth of a reef corresponds to roughly 50 metres of subsidence of land or shallow-water bank.

p The first boring in a reef, made in 1897-98 on the atoll of Funafuti in the Ellice group, found nothing but coral rock to depths of more than 300 metres. The reef thickness may have been much greater; the drill was able to bore only to that depth. The next boring, on Borodino Island (Smith Island), south of Japan, reached a depth of 432 metres, but here too the researchers were unable to penetrate all the way through the reef.

p Drillers on Bikini Atoll reached a depth of more than 780 metres in the summer of 1947. Geophysical studies showed the thickness at Bikini to be actually 1,300 metres. Later, geophysical investigation of the Eniwetok Atoll revealed its coral thickness to be about 1,500 metres. This means that land in this region has sunk about 1,500 metres, an impressive figure even for the Pacific.

When did the Uarwin Rise begin to subside? Volcanism reached its height in the Pacific between 60 and 100 million years ago, after which the Uarwin Rise started to sink. Many volcanoes became extinct. Their peaks were levelled by the waves, and they turned into shallow-water banks. According to Menard, sinking took place throughout almost the entire area except in the region of the Tokelau group and, possibly, two shallow-water sections in the northwestern ocean. The collapse amounted to almost two kilometres. Despite the subsidence of numerous islands many of them still served as stepping-stones for biological migration because growing coral colonies kept the peaks of the 53 islands at sea level. Volcanoes that disappeared beneath the waves were replaced from time to time by new groups of volcanoes.