Fig. 434.—Map showing the rift valley lakes of east Central Africa.
One of the most striking examples of a rift valley lake is Lake Tanganyika, while Albert Nyanza, Nyassa, and Rudolf in the same region are similar ([Fig. 434]).
Fig. 435.—Earthquake lakes which were formed in the flood plain of the lower Mississippi during the earthquake of 1811 (after Humphreys).
Earthquake lakes.—The complex adjustments in level of the surface of the ground at the time of sensible earthquakes are many of them made apparent in no other way than by the derangements of the surface water. This is at such times impounded either in pools or in broad lakes, which inasmuch as they date from known earthquakes have been called “earthquake lakes”, even though in a strict sense any lake which has originated in earth movements might properly be regarded as an earthquake lake. To avoid unnecessary confusion, the term must, however, be restricted to those lakes which are known to have been formed at the time of definite earthquakes ([Fig. 435]). Reelfoot Lake in Tennessee, which in late years has acquired undesirable notoriety because of the feuds between the fishermen of the district and the constituted authorities, is a lake more than twenty miles across and came into existence during the great earthquake of the lower Mississippi valley in 1811.
Crater lakes.—The craters of volcanic mountains are natural basins in which surface waters are certain to be collected, provided only the supply is sufficient and seepage into the loose materials is not excessive. Some craters, still visibly more or less active, are occupied by lakes ([Fig. 436]).
Fig. 436.—View of lake in Poas Crater in Costa Rica, a volcanic crater more than half a mile across and with walls 800 feet deep. At intervals there is an ejection of steam mixed with mud and ash after the manner of a geyser (after H. Pittier).
In the larger number of cases in which craters become occupied by lakes, the evidence of continued activity is lacking, and it would appear in such cases that the lava of the chimney had consolidated into a volcanic plug, closing the bottom of the crater. Notable groups of crater lakes are the Caldera of the Roman Campagna ([Fig. 437]) and the so-called maare of the Eifel about the Lower Rhine. Crater lakes are easy to recognize by their circular plan, their steep walls of volcanic materials, and their considerable depth with a maximum near the center.
One of the most remarkable of these water-filled basins is Crater Lake in Oregon, which has a diameter of about six miles and is believed to have resulted from the incaving of a great volcanic cone in the latest stage of its activity. This remarkable feature has now been made a national park and will soon be conveniently reached by tourists and counted one of the greatest nature wonders of the Pacific slope.