Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 30.—Map showing next to the present stage of the Great Lakes history when the land was lower on the north and the upper (Nipissing) lakes drained through the Ottawa River Valley into an arm of the sea (Champlain Sea) which reached through the Champlain and Hudson Valleys. (After Taylor, published by New York State Museum.)

Many lakes, including some remarkable ones, occupy basins which are directly due to movements of the earth’s crust—either faulting or warping. An example of a lake occupying part of a fault basin is the famous Dead Sea of Palestine. This lake lies in the lowest part of the Jordan Valley, which has geologically recently come into existence by the sinking of a long, narrow block of earth for several thousand feet between two great earth fractures (faults). The Dead Sea covers about 500 square miles and its surface lies about 1,300 feet below sea level, which makes it the lowest lake in the world. Almost equally remarkable is the fact that its depth is about 1,300 feet, so that the lowest part of the lake basin is 2,600 feet below sea level. The lake contains approximately 24 per cent salt, mostly common table salt, causing it to be a thick brine in which there is neither plant nor animal life—hence the name “Dead Sea.” At one time, probably just after the Ice Age, the lake was much larger and deeper, when it filled a considerable part of the Jordan Valley and had an outlet to the south. During the high-level stage the water was fresh, but gradually, as the climate became drier, evaporation was greater than intake, the outlet was abandoned, and the mineral matter (mostly chloride of magnesia and common table salt) carried by the streams in solution into the shrinking lake steadily accumulated until the high degree of salinity of the present time has been reached.

Great Salt Lake, Utah, is a remarkable lake whose history has been carefully studied. It occupies the lowest position of an extensive basin which, in turn, forms but part of the whole great district of Utah which has geologically recently sunk thousands of feet on the west side of the great fault already described as occurring along the western base of the Wasatch Mountains. At present the lake covers about 2,000 square miles, but its area fluctuates considerably. It is scarcely believable that this big lake has an average depth of only fifteen feet and a maximum depth of only fifty feet. It lies 4,200 feet above sea level, and it carries about 18 per cent salts in solution. Most abundant by far is common table salt, of which there are no less than 5,000,000,000 tons in solution. The waters also contain about 900,000,000 tons of other salts. Should the lake completely disappear by evaporation, these salts would be deposited. Allowing for cars 40 feet long and of 40 tons capacity, a train more than 1,000,000 miles long would be required to carry the salts. What has been the source of these salts? Great Salt Lake is not, as supposed by some, a remnant of an ocean once covering the region. Briefly, the explanation is as follows: At one time, when the climate was moister, the basin now only in part occupied by the lake was filled to overflowing with an outlet north into the Snake and Columbia rivers. That great body of water (called “Lake Bonneville”) covered nearly 20,000 square miles and its depth was about 1,000 feet deeper than now, the present depth being very small. Because it had an outlet that lake was, of course, fresh. Beaches and shore lines 1,000 feet above the present lake, and at various lower levels, are still wonderfully well preserved. When, due to climatic change, evaporation exceeded intake by streams, the outlet was cut off. But slowly, as the lake shrank, streams (especially the Jordan River) carried a little salt in solution, the percentage of salt increasing until the present stage has been reached. In a real sense, much of the salt was once in the sea, because it has been dissolved out of strata which accumulated under sea water long before the basin of Great Salt Lake came into existence.

Another famous lake, which also occupies part of a basin due to faulting, is Lake Tahoe in the Sierra Nevada Mountains, near Truckee, California. This lake, whose length is 21 miles, and width 12 miles, lies 6,225 feet above sea level. On almost all sides steep mountains rise several thousand feet above its waters. Its great depth of 1,635 feet makes it, so far as known, the second deepest lake in North America, Crater Lake, Oregon, only outranking it. The water is exceedingly clear. An experiment some years ago showed that a white disk eight inches in diameter could actually be seen through a thickness of 216 feet of its water. “The statement sometimes made that “Tahoe is an old volcanic crater” is not true. The region about the lake shows evidences of volcanic activity of various kinds, and the lake waters themselves have probably been dammed at times by outpourings of lava. A lava flow appears to have temporarily filled the outlet channel below Tahoe City. The lake, however, lies in a structural depression—a dropped (fault) block in the earth’s crust.” (U. S. Geological Survey.)

The basin of the largest lake in the world—the Caspian Sea—has resulted from warping of the earth’s crust. It has an area of 170,000 square miles, a maximum depth of 3,200 feet, and its surface is about 90 feet below sea level. The composition of its water and some of its animal life indicate that it was once an arm of the sea. It has been detached or cut off by an upwarp of the land between it and the Black Sea region. If this great lake is a cut-off arm of the sea, with no outlet, how do we explain the fact that its salinity is much less than that of the ocean? Toward the north, where it is shallow and fed by so much river water, it is, in fact, almost fresh water. Even the southern one-half carries not over 1 per cent of salt. The explanation is that a steady current passes through a narrow passageway into a gulf or bay on its eastern side where evaporation is much greater than over the general surface of the Caspian. The salt is, therefore, gradually accumulating at the estimated rate of 350,000 tons per day in this gulf, while the sea itself is becoming fresher.

The basin of Lake Champlain, about 100 miles long, was occupied by tidewater geologically very recently (that is, since the Ice Age), but it has been cut off by uplift of the land on the north, since which time the waters of the lake have been completely rinsed out and freshened.

Many lake basins directly result from volcanic action. In many parts of the world lakes, usually of small size, occupy craters of volcanoes as, for example, in the Eifel region of Germany, the Auvergne district of France, and near Rome and Naples in Italy. Such a lake of exceptional interest fills part of the great crater, several thousand feet deep, which resulted from the explosion of Mt. Katmai, Alaska, in 1912. The water of this lake, more than a mile wide and of unknown depth, is hot.

One of the most unique and beautiful lakes of the world is Crater Lake in the Cascade Mountains of southern Oregon. It partly fills a great, nearly circular hole, six miles in diameter, with a maximum depth of about 4,000 feet, in the top of a mountain ([Plate 11]). The lake is over five miles in diameter and nearly 2,000 feet deep, making it the deepest in North America. Its surface is about 6,200 feet above sea level. Precipitous rock walls rising 500 to 2,000 feet completely encircle the lake, the main body of whose water is of a marvelous deep, sapphire-blue color, while the shallow portions around some of the shore are of emerald-green. Crater Lake has very little intake except direct rainfall and snowfall, and its water is fresh. The great hole was not produced by an explosion like that of Katmai, but rather by the sinking of the top of a once much greater mountain. That the mountain was once about the size and shape of Mt. Shasta is proved by the fact that deep glaciated valleys lead up the slopes and end abruptly at the very rim of the present mountain. Obviously these valleys were scoured out in recent geologic time by glaciers whose sources were several thousand feet up on a former cone-shaped mountain. That the mountain top sank rather than exploded is proved by the absence of volcanic débris over the sides and base of the mountain.

Still another way by which lakes are formed by volcanic action is by streams of lava blocking valleys. The famous Sea of Galilee in Palestine was thus formed by a stream of lava, which geologically recently flowed down from the east into the Jordan Valley and across it, where it cooled to form a dam ponding the waters of the Jordan River. Because the river flows through the lake, its water is fresh. One of the most remarkable facts about this lake is that its surface lies nearly 700 feet below sea level. A number of lava-dam lakes are known in the Sierra Nevada and Cascade Mountains.