One of the greatest stock piles of minerals in the world came from the evaporation of a great inland sea in the western United States. This is Searles Lake in the Mohave Desert of California. An arm of the sea that overlay this region was cut off from the ocean by the thrusting up of a range of mountains; as the lake evaporated away, the water that remained became ever more salty through the inwash of materials from all the surrounding land. Perhaps Searles Lake began its slow transformation from a landlocked sea to a ‘frozen’ lake—a lake of solid minerals—only a few thousand years ago; now its surface is a hard crust of salts over which a car may be driven. The crystals of salts form a layer 50 to 70 feet deep. Below that is mud. Engineers have recently discovered a second layer of salts and brine, probably at least as thick as the upper layer, underlying the mud. Searles Lake was first worked in the 1870’s for borax; then teams of 20 mules each carried the borax across desert and mountains to the railroads. In the 1930’s the recovery of other substances from the lake began—bromine, lithium, and salts of potassium and sodium. Now Searles Lake yields 40 per cent of the production of potassium chloride in the United States and a large share of all the borax and lithium salts produced in the world.
In some future era the Dead Sea will probably repeat the history of Searles Lake, as the centuries pass and evaporation continues. The Dead Sea as we know it is all that remains of a much larger inland sea that once filled the entire Jordan Valley and was about 190 miles long; now it has shrunk to about a fourth of this length and a fourth of its former volume. And with the shrinkage and the evaporation in the hot dry climate has come the concentration of salts that makes the Dead Sea a great reservoir of minerals. No animal life can exist in its brine; such luckless fish as are brought down by the River Jordan die and provide food for the sea birds. It is 1300 feet below the Mediterranean, lying farther below sea level than any other body of water in the world. It occupies the lowest part of the rift valley of the Jordan, which was created by a down-slipping of a block of the earth’s crust. The water of the Dead Sea is warmer than the air, a condition favoring evaporation, and clouds of its vapor float, nebulous and half formed, above it, while its brine grows more bitter and the salts accumulate.
Of all legacies of the ancient seas the most valuable is petroleum. Exactly what geologic processes have created the precious pools of liquid deep within the earth no one knows with enough certainty to describe the whole sequence of events. But this much seems to be true: Petroleum is a result of fundamental earth processes that have been operating ever since an abundant and varied life was developed in the sea—at least since the beginning of Paleozoic time, probably longer. Exceptional and catastrophic occurrences may now and then aid its formation but they are not essential; the mechanism that regularly generates petroleum consists of the normal processes of earth and sea—the living and dying of creatures, the deposit of sediments, the advance and retreat of the seas over the continents, the upward and downward foldings of the earth’s crust.
The old inorganic theory that linked petroleum formation with volcanic action has been abandoned by most geologists. The origin of petroleum is most likely to be found in the bodies of plants and animals buried under the fine-grained sediments of former seas and there subjected to slow decomposition.
Perhaps the essence of conditions favoring petroleum production is represented by the stagnant waters of the Black Sea or of certain Norwegian fiords. The surprisingly abundant life of the Black Sea is confined to the upper layers; the deeper and especially the bottom waters are devoid of oxygen and are often permeated with hydrogen sulphide. In these poisoned waters there can be no bottom scavengers to devour the bodies of marine animals that drift down from above, so they are entombed in the fine sediments. In many Norwegian fiords the deep layers are foul and oxygenless because the mouth of the fiord is cut off from the circulation of the open sea by a shallow sill. The bottom layers of such fiords are poisoned by the hydrogen sulphide from decomposing organic matter. Sometimes storms drive in unusual quantities of oceanic water and through turbulence of waves stir deeply the waters of these lethal pools; the mixing of the water layers that follows brings death to hordes of fishes and invertebrates living near the surface. Such a catastrophe leads to the deposit of a rich layer of organic material on the bottom.
Wherever great oil fields are found, they are related to past or present seas. This is true of the inland fields as well as of those near the present seacoast. The great quantities of oil that have been obtained from the Oklahoma fields, for example, were trapped in spaces within sedimentary rocks laid down under seas that invaded this part of North America in Paleozoic time.
The search for petroleum has also led geologists repeatedly to those ‘unstable belts, covered much of the time by shallow seas, which lie around the margins of the main continental platforms, between them and the great oceanic deeps.’
An example of such a depressed segment of crust lying between continental masses is the one between Europe and the Near East, occupied in part by the Persian Gulf, the Red, Black, and Caspian seas, and the Mediterranean Sea. The Gulf of Mexico and the Caribbean Sea lie in another basin or shallow sea between the Americas. A shallow, island-studded sea lies between the continents of Asia and Australia. Lastly, there is the nearly landlocked sea of the Arctic. In past ages all of these areas have been alternately raised and depressed, belonging at one time to the land, at another to the encroaching sea. During their periods of submersion they have received thick deposits of sediments, and in their waters a rich marine fauna has lived, died, and drifted down into the soft sediment carpet.
There are vast oil deposits in all these areas. In the Near East are the great fields of Saudi Arabia, Iran, and Iraq. The shallow depression between Asia and Australia yields the oil of Java, Sumatra, Borneo, and New Guinea. The American mediterranean is the center of oil production in the Western Hemisphere—half the proved resources of the United States come from the northern shore of the Gulf of Mexico, and Colombia, Venezuela, and Mexico have rich oil fields along the western and southern margins of the Gulf. The Arctic is one of the unproved frontiers of the petroleum industry, but oil seepages in northern Alaska, on islands north of the Canadian mainland, and along the Arctic coast of Siberia hint that this land recently raised from the sea may be one of the great oil fields of the future.
In recent years, the speculations of petroleum geologists have been focused in a new direction—under sea. By no means all of the land resources of petroleum have been discovered, but probably the richest and most easily worked fields are being tapped, and their possible production is known. The ancient seas gave us the oil that is now being drawn out of the earth. Can the ocean today be induced to give up some of the oil that must be trapped in sedimentary rocks under its floor, covered by water scores or hundreds of fathoms deep?
Oil is already being produced from offshore wells, on the continental shelf. Off California, Texas, and Louisiana, oil companies have drilled into the sediments of the shelf and are obtaining oil. In the United States the most active exploration has been centered in the Gulf of Mexico. Judging from its geologic history, this area has rich promise. For eons of time it was either dry land or a very shallow sea basin, receiving the sediments that washed into it from high lands to the north. Finally, about the middle of the Cretaceous period, the floor of the Gulf began to sink under the load of sediments and in time it acquired its present deep central basin.