Popular Science Monthly/Volume 71/August 1907/Some Little-Known Mexican Volcanoes
| SOME LITTLE-KNOWN MEXICAN VOLCANOES |
By Professor HERDMAN F. CLELAND
WILLIAMS COLLEGE
WHILE the inhabitants of the United States were suffering from the heat of an unusual summer, the member-s of the International Geological Congress were making a study of the geology of the plateau region of Mexico and were enjoying the delightful climate of that country. Few regions of the world are more fascinating; the combination of vast volcanic peaks, broad arid plains, with their curious desert flora and a brilliant tropical sky, leaves an impression never to be forgotten.
It was in the midst of such interesting surroundings that during August, September and October, 1906, the International Congress of Geologists met, and, as a member, the writer had an opportunity to visit several of the Mexican volcanoes under especially favorable circumstances. In the first place, the guides to the various volcanoes were trained geologists of the Mexican Geological Survey, who had previously made a study of the regions to which they conducted the party. Moreover, the personnel of the visitors embraced geologists from Europe and America, who had investigated volcanoes in many parts of the world, and consequently, by way of comparison, were able to add a great deal of interesting information.
If you will look at a map of Mexico, you will notice that the names Volcano Colima, Nevado de Toluca and Valle de Santiago form the vertices of an obtuse triangle west of the City of Mexico. With these three points we will concern ourselves.
Volcano Colima
Volcano Colima, the most recently active volcano in Mexico, whose cloud-crowned summit can be seen for many miles along the Pacific coast, is situated almost due west of Mexico city and about fifty miles from the Pacific Ocean. It can be reached without much difficulty from the village of Zapotlan by a horseback journey of ten hours. On the ride one winds along the sharp divides which separate deep ravines, around the high Nevado, and finally reaches the foot of the cone from which the climb on foot must begin.
A more beautifully symmetrical volcanic cone than that of Colima, as viewed from the north, can hardly be imagined; the only feature breaking the symmetry being a secondary cone which arises from the northeast slope. The beauty of the cone is enhanced by the great clouds of steam which. continually arising from the crater, either envelop the summit or, blown by the wind, stretch out into long white clouds.
The altitude of the principal cone is a little less than 12,600 feet above sea-level, while the top of the secondary cone is 780 feet lower. The Lava Flow of 1869.—One of the chief difficulties in ascending Colima from the north side is the necessity of crossing the rough lava flow which was poured out from the secondary cone in 1869. The north side of this flow is very precipitous, as its highest point rises considerably above that of the central portion of the flow, thus forming a kind of wall on the north side. A more rapid cooling of the outer edge of
the molten lava stream than that of the center formed this wall. As a result the sides hardened rapidly and consequently have an altitude about equal to that of the stream at its greatest height. The central portion, remaining hot for a longer time, flowed on after the lava had ceased to flow from the cone, and thus lowered its surface. In August, 1869, a month after the principal eruption, the lava is said to have flowed a little more than nine feet per day. The surface of this lava is as scoriaceous, irregular and crumbly (aa), as one can well imagine, but does not differ greatly from the mal pais seen in other parts of Mexico. Because of this character, one is obliged to walk with the greatest care, stepping over or descending-into fissures, climbing up or over irregular masses of scoriæ. Indeed, the roughness of the lava hurts the hands and tears the shoes, and its treacherous character compels one to be on the alert at all times, which makes the work very exhausting.
Secondary Cone.—The secondary cone is composed of a compact though somewhat vesicular andesite with a steep slope. From this cone, as has been said, lava poured forth in 1869. There is no crater in the cone, although the summit is broken by three parallel fissures. This fissuring may have been the result of shrinkage by cooling or it may have been due to a sinking of the lava in the cone. It seems probable that the secondary cone was formed as follows: Previous to the eruption of 1869, the pressure from below fissured the main cone. 
Fig. 4. Crater Colima September, 1906. Through this fissure the lava welled up, flowing away as a lava stream. The stiffer and cooler lava, which was later forced up, failed to flow, and hardened to form the mound.
The Main Cone.—The ascent of the main cone is difficult because of the insecure footing afforded by the rolling ash and cinders and the steepness of the lava wherever it outcrops, as well as because of the altitude. The slope of the cone is between 35 and 39 degrees, although from a distance it appears to be much greater.
The Crater.—The rim of the crater is entire with the exception of a depression through which a lava stream flowed in 1885 and again in 19(13. The view into the crater from The higher portions of its rim is very impressive, even awe-inspiring. The slope on the outside of the rim is that of the volcano, but the inside drops precipitously to the bottom of the crater, a depth of more than 100 feet in many places. On account of the great quantities of steam which arc continually rising.
it is only when an occasional gust of wind partially lifts the steam that one can get a glimpse of the floor of the crater. Descent into the crater by way of the breach in the side is comparatively easy and is attended with less danger than the view from the rim prophesied. The floor is covered with scoriaceous lava equaling, if not exceeding, in ruggedness that of the lava flow at the foot of the volcano. Steam with a temperature of about 130 degrees Fahrenheit and sulphur dioxide are issuing from numerous fumaroles, some of which are lined with sulphur crystals. The crater is comparatively small, having a diameter of little over half a mile.
Recent Eruptions.—In 1877, 1884 and 1885 minor eruptions occurred. The last eruption of the volcano commenced in the month of February, 1903, and practically ceased in May of the same year. Since that time the only evidences of activity are the fumaroles from which issue large quantities of steam and other gases. During this eruption
(1903) a lava stream flowed down the slope of the volcano in a northwest direction, but barely reached the foot of the volcano (see diagram), where it dammed a small stream, thus forming a shallow pond.
The accompanying diagrammatic cross-section of the volcano shows the relations of the secondary cone to the main cone, the position of the lava flow of 1869, the edge of this flow (a), the rim of the old crater (b), the lava flow of 1903, and the position and relative heights of Colima and the Nevado of Colima (which may have been the remnant of the rim of a great volcano long since destroyed). In the construction of this diagram, no attempt was made to draw the distances or heights to scale, but to bring out the salient points as clearly as possible,
Volcano Toluca
feet above the sea. It is called the Nevado, because usually its summit is white with snow. This volcano is isolated, being surrounded at some distance by volcanoes which have formed by the accumulation of their ash and lava an almost enclosed basin. It is one of the few high volcanoes of the world that can be ascended with ease, since it is possible to make the journey to and into the crater on horseback in four or five hours. Because of the ease with which it may be climbed the ascent has been made by a number of persons, the first of whom was the great geographer and traveler Humboldt, who reached the crater in 1803.
General Description.—Volcano Toluca is underlaid by calcareous rocks of Cretaceous age. The great mass of the volcano is composed of many layers of ash of varying degrees of thickness which conform quite closely to the slope. These layers of ash were apparently formed partly by the ash which rained down during the eruptions and partly by that which was carried down by streamlets and to a considerable extent in sheets during heavy rains. The accompanying photograph shows the stratified character of the slope and also a stratum of fossil soil, which in several of the "barrancas" or dry ravines is seen to be of considerable thickness. From this evidence it is fair to conclude that the last eruptions were preceded by a long period of inactivity, during which a large quantity of organic material was mixed with the weathered ash. Toluca has not been in eruption within historic times and at present there are no signs of activity, even secondary effects, such as fumaroles of steam and sulphur dioxide, being absent. To watch the change in vegetation from the plain to the summit of the mountain is a constant pleasure. On the dry plain cactus and other desert plants are common, hut on the flanks of the mountain pines begin and many bright-colored flowers. These, as one continues the ascent, become shorter and more stunted, until in the crater the flower blossoms an inch or thereabouts from the ground instead of one or two feet from the ground, as is the case lower down. On the highest portions of the rim vegetation is almost lacking.
The Crater.—The crater of the volcano is somewhat elliptical in form, being a little more than a mile in its longest diameter and about a third of a mile in its shortest. The crater rim is complete on all sides, but is low on the side through which entrance is made. In the bottom of the crater and 1,000 feet below the highest portion of the rim are two beautifully clear lakes, the larger of which is almost one fifth of a mile in diameter and has a maximum depth of thirty feet. These two lakes are separated by a dome of compact andesite of considerable height (see illustration). This dome is of especial interest, because of its bearings upon the origin of the Mt. Pelée spike. There seems to be little doubt, as T. Flores points out, that it is composed of the lava which was forced up and out of the vent after the last eruption and which now closes it and stands above the floor of the crater.
Comparison with Mt. Pelée.—It was suggested by Dr. E. O. Hovey that the Pelée plug was formed in this way also, i. e., that instead of a solid mass of lava being pushed up bodily, as Heilprin believed, very
stiff lava, being forced from the vent after the last eruption, hardened into a high mound. In the case of Pelée the shape of the mound was modified by a splitting off of the lava along vertical planes, which produced the unique "spike" of that volcano.
Age.—A comparison of this volcano with others in Mexico has led Ordoñez to state that it probably made its appearance during Pliocene times.
Cinder Cones of Valle de Santiago
Cinder cones a few hundred feet in height are common objects in the central volcanic plateau of Mexico. Many of these may be seen in the basin in which the City of Mexico is situated, where the lower flanks of the higher volcanoes meet the plain. Near Toluca excellent examples occur. Because of the smallness of these cones as compared with the volcanoes near whose base they rise they are likely to be overlooked on
account of the overshadowing effect of the former. This is not true of the group of cinder cones, situated near the city Valle de Santiago, which are scattered about the valley some distance from the higher volcanoes, and which are, consequently, very conspicuous, their symmetrical truncated cones being the most marked features of the landscape.
This group of eleven craters occupies an area roughly circular in outline, one diameter of which is about six miles. Because of the fact that the valley of Santiago is a dry plain, the presence of lakes of pure water in four of the craters is unexpected. The clear blue water of the lakes with their settings of green cultivated fields which cover the inner slopes of the craters are most beautiful objects.
The existence of these lakes is due to the fact that their bottoms are below the levels of underground water. All these crater lakes are at practically the same level, a condition which is due to the fact that the volcanic material in which they rest and of which the plain is composed is extremely porous, which permits the free circulation of the water. The craters of the majority of the cones were partially filled with lava which poured out quietly after the explosions which formed them had ceased. In some cases they were filled until their bottoms were above the level of underground water and are consequently dry; in others there was either no subsequent outpouring of lava or the quantity was very limited, in which case the cavity remained below the level of underground water and a lake resulted. The diameter of the craters vary in size from that of Solis (1,500 feet)—which was apparently produced by the sinking of the crust—to the largest, which is more than a mile in diameter. The craters are not all perfect; some are entire, while others are broken by one or two subsequent craters of explosion. In one of these breeched craters three small cones rise from the bottom, the material of which is apparently being used in the city for constructional purposes.
The plain upon which the craters rest is underlaid by one or more strata of basaltic lava which evidently flowed from the neighboring mountains and which may be seen near the water level of the lakes and in ravines which have been deeply cut by streams. Since neither this stratum nor the strata of basaltic lava are disturbed by being domed up or bent to any extent, it seems safe to conclude that the explosions forming the craters must have been near the surface and very local, otherwise the strata overlying the plain at that place would have been more or less bent.
The cones are made up in some cases of volcanic ash of various degrees of fineness, in others of volcanic breccia. The slopes are those which are normally made by such materials.
Because of the fact that craters of explosion in other parts of Mexico—Puebla, Mexico City, here in Valle de Santiago, and elsewhere in the republic—arise from a plain or a more or less enclosed basin which is full of water at a comparatively shallow depth, Ordoñez suggests that superficial water may have had a share in the production of the explosions.
Such are a few of the points of interest on the volcanic plateau of Mexico, a region which, interesting because of its scenery and climate, fascinating because of its romantic history, is to the geologist a volume which which studied will explain many points that are now a matter of speculation.