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Understanding the Land
By Kay Davis
Imagine the Lake Chapala landscape 40,000 years ago. Fossil evidence shows that ancestral mammoths, mastodons, camels and horses roamed the area before the basins were filled by water. At night the landscape was illuminated by the glow of scores of volcanoes in a zone 150 miles wide.
This volcanic belt crosses Mexico between the 19th and 21st parallels of latitude. For 10,000,000 years this area was a fiery inferno of constant volcanic activity for such magnificent giants as Popocatepetl near Mexico City and Volcan de Colima near Manzanillo and not far from Lake Chapala. Along with these were thousands of other volcanic cones. The two Colima volcanoes, which lie on the 19th parallel, formed one of the most active areas in Mexico. Even today there is plenty of activity, including boiling springs, fumarolas (steam vents), geysers and actual eruption of the Volcan de Colima, primarily in the form of steam.
Our wrinkled land twisted itself. North of the 21st parallel the continent moved slowly but inexorably east, and south of the 19th parallel, it moved west. Lakeside, on the 20th parallel, was distorted by the counter movements between the two fault zones, and here the crust is broken into massive blocks varying in elevation.
Huge volumes of lava poured onto the surface separating the great plateau into basins. One of these is the Jalisco basin, inside of which are smaller basins. One is now the city of Guadalajara.
As mentioned, this occurred 40,000 years ago, and climatic evidence suggests very heavy precipitation during that period. Over 30,000 years the basin reached its maximum capacity until a huge inland sea covered 8500 square miles, about one fifth of what is now Jalisco. The average depth was 820 feet while 700 feet of water covered the area known as Guadalajara.
The age of this ancient lake was established by means of Carbon 14 dating of wood samples found in sediment. Aged 38,000 years they establish the lake, identified as Lake Jalisco, in a geological epoch that spanned from 1,000,000 years ago to as recent as 25,000 years ago.
But where did the precipitation come from that formed this huge lake? It was a remarkable time in Earth history during which four great glacial invasions of ice occurred in the Northern Hemisphere, glaciers which advanced as far south as St. Louis in North America and Berlin in Europe. Most of the mountains were covered by ice, which eventually melted to form large lakes on the continents. Great Salt Lake in the state of Utah is a descendant of glacial Lake Bonneville, which covered 19,000 square miles and was fed by glaciation in the Wasatch Mountains.
Naturally, in the vicinity of the 19th parallel, Lake Jalisco was not fed by glaciers, but like the rest of the Northern Hemisphere, the precipitation was much greater than at present. Samples found throughout the area imply a cool climate during that period, which would be in keeping with the extremely frigid conditions that existed in North America during that time.
The Santiago River cut a fantastic gorge known as the Barranca de Oblatos, a gorge, or barranca, which is frequently admired by visitors who see it from the road that runs alongside it in Guadalajara. The barranca is 2000 feet deep. Proof of faulting lies in trying to match layers of rock on both sides of the gorge. They do not align. Within 5000 years, plus or minus, erosion occurred caused by immense water volumes and velocities pouring into the river. Imagine standing on the rim of the Barranca de Oblatos, observing the spectacle of a mighty river roaring through the canyon below, carving that tremendous gorge!
Lake levels appear to have been stable for long periods of time, wave action creating terraces similar to those at the beach along the seashore. Layered deposits are visible throughout the area once covered by the lake. These distinctive bands are exposed in many locations and can be observed where cuts were made for the Guadalajara-Chapala highway, and for the Ajijic bypass. The terraces are particularly prominent at Chapala. Lakeside communities are located on the lower terrace, and three others are visible on the slopes of the mountains behind.
The largest remnant of ancient Lake Jalisco is Lake Chapala, with only a very small area of 825 square miles compared to its predecessor’s 8500 square miles. A tremendous reduction in area and volume has taken place over time, due to ruptures on the basin walls and eventual drainage.
So, can the future of Lake Jalisco/Lake Chapala be predicted? Yes. All lakes are subject to the same fate. They are doomed as soon as they come into existence. A lake basin is a natural depression for deposits, and given enough time, they will be filled by sediments from streams and rivers, plus the material washed in from slopes surrounding it.
Will the rivers continue in their erosion and eventual destruction of the plateau, or will volcanic activity restore the lake and plateau? ¿Quien sabe? (Who knows?)