Popular Science Monthly/Volume 54/December 1898/A Geological Romance
|A GEOLOGICAL ROMANCE.|
A WESTERN naturalist once said that the geology of Kansas was monotonous. In one sense this remark is certainly justifiable, and the same may be said about the geology of some of the other States on the Western plains. The American continent is built on a comprehensive plan, and many of its formations can be followed for hundreds of miles without presenting much variation in general appearance. Occasionally, however, some feature of special interest crops out from the serene uniformity, and the very nature of its surroundings then makes it appear all the more striking. Minor accidents in the development of our extensive terranes sometimes stand out in bold relief, as it were, from the monotonous background. In their isolation from other details such features occasionally display past events with unusual clearness.
Such is the case with a deposit of volcanic ash which has been discovered in the superficial strata on the plains. It lies scattered in great quantities in a number of localities in Nebraska, Kansas, South Dakota, and Colorado, having been found in no less than twenty counties in the first-mentioned State. It measures from two to fourteen feet in thickness in different localities, and is mostly found imbedded in yellow marl and clay, and has a somewhat striking appearance in the field, due to its snowy whiteness and to the sharpness of the plane which separates it from the underlying darker materials. Many years before its real nature was known it had been noticed and described by Western geologists. Prof. O. T. St. John saw it many years ago in Kansas, where it appeared as "an exceedingly fine, pure white siliceous material," forming a separate layer of several feet, and set off by a sharp line from the buff clay-marl below. His words describe its usual appearance in other places (see Fig. 1).
This ash occurs in several outcrops in McPherson County in the central part of Kansas, where the writer had an opportunity to study it somewhat in detail a few years ago. Some of the features of the dust at this place reveal the conditions under which it was formed with considerable distinctness, and the volcanic episode which produced it appears strikingly different from the dull monotony of the ordinary geological work recorded in the terranes of the plains. It may be said to consist of angular flakes of pumice, averaging one sixteenth of a millimetre in diameter, and having a thickness of about one three-hundredth of a millimetre. The most common shape of the flakes is that of a triangle, or rather of a spherical triangle, since the flakes are apt to be concave on one side and convex on
(From the University Geological Survey of Kansas, vol. ii.)
the other. In thethey sometimes appear like splinters of tiny bubbles of glass, and this is really what they are (Fig. 2).
The explosive eruptions which give rise to showers of this kind of ash, or dust, are due to fusion and superheating of subterranean masses of rocks charged with more or less moisture. A part of this moisture escapes in the form of steam at the time of an eruption. But the viscidity of the ejected material prevents much of the steam from passing off, and such of the lava as cools most rapidly retains a certain quantity in solution, as it were. Obsidian is a rock which has been made in this way. It often contains much of the original water, which will cause it to swell up into a stony froth when fused.
This volcanic dust has the same property. If one small particle of it be heated on a piece of platinum foil it is seen to swell up into a compound bubble of glass (Fig. 3). It is evident that this is due to the expansive force of the heated included moisture, to which the viscid half-molten glass readily yields. At the time of the eruption which produced this dust, subterranean heat was applied to the moisture-bearing rock until this was superheated to such an extent that the weight of the overlying material was insufficient to hold the water from expanding into steam. Then there was a tremendous explosion, and the molten magma was thrown up with such a force that it was shattered into minute droplets, in the same way as water does when it is thrown forcibly into the air. Being thus released from pressure, the steam inside of each little particle of the heated glass caused it to swell out into a tiny bubble. As this kept on expanding it was cooled, the thin glass wall of the bubble congealed, and finally burst from the pressure of the steam within. This is the reason
|Fig. 2.—Flakes of Volcanic Ash. Magnified about 100 diameters. A, flake with a branching rib; B, fragment of a broken hollow sphere of glass; C, fragment with drawn out tubular vesicles; D and E, plain fragments of broken pumice bubbles. (From American Geologist, April, 1893.)|
|Fig. 3.—A Particle of Volcanic Ash swelled up by Fusion. Magnified 100 diameters.|
why the little dust particles are thin, mostly triangular, and often slightly concave flakes with sharp angles. Sometimes the angles appear rounded, as if the fragments had been viscid enough to creep a little after the bubble burst. The study of one single little grain of dust, barely visible to the naked eye, thus makes clear the nature of a catastrophe which must have shaken a whole mountain, and which left its traces over a quarter of a continent.
That the dust was produced in this way is quite evident from other circumstances. If a handful from the dust of this place be thrown into water and gently stirred, it nearly all will settle after a while. But some rather large particles remain floating on the surface. If these are removed and examined under the microscope, they are seen to be hollow spheres (Fig. 2, b). These are some of the original bubbles that never burst, either because they contained too little steam or else because the steam was cooled before it had time to break the walls open. It is evident that not every droplet of the molten magma would form a single sphere, but that many also would swell up into a compound frothlike mass of pumice. A few such pieces may sometimes be observed in the deposit at this place, and that many more were made and broken is evident from the great number of glass fragments which have riblike edges on their flat sides (Fig. 3, a).
The nature of the force which caused the eruption may thus be understood from the study of one little grain of the dust, but much more extended observations are needed in order to make out the place where the great convulsion took place. It will, perhaps, never be known what particular volcanic vent was the source of this ash. Different deposits may have come from different places. But it seems possible that it all came from the same eruption. There can be no doubt that the volcanic disturbances occurred to the west of the Great Plains. No recent extinct volcanoes are found in any other direction. This conclusion is corroborated by the fact that the dust is finer in eastern localities and coarser nearer the Rocky Mountains. In a bed near Golden, in Colorado, seventy-three per cent, by weight, of the dust consists of particles measuring from one fourth to one thirty-second of a millimetre, while some from Orleans, in Nebraska, contains seventy-four per cent of particles measuring from one six- teenth to one sixty-fourth of a millimetre in diameter. Still finer material comes from the bluffs of the Missouri River near Omaha. Evidently the coarser particles would settle first, and if the dust is finer toward the east, it must be because the wind which brought it blew from the west. Most likely the eruption occurred somewhere in Colorado or in New Mexico.
It may be asked how it can be known that the dust was carried this long distance by the wind. May it not as well have been trans- ported by water ? The answer must be, in the first place, that showers of the same kind of material have been observed in connection with volcanic outbursts in other parts of the world. One such shower is known to have strewn the same kind of dust on the snow in Norway after a volcanic eruption in Iceland, and after the great explosion on Krakatoa, in 1883, such dust was carried by the wind several hundred miles, and scattered over the ocean. If this ash had been transported by water, it would not be found in such a pure state, but it would be mixed with other sediments. There would, no doubt, also be found coarser fragments of the volcanic products. On the contrary, it appears uniformly fine. No particles have been found which measure more than one millimetre in diameter, and less than one per cent of its weight consists of particles exceeding one eighth of a millimetre in diameter. In seven samples taken from different places the proportions of the different sizes of the grains were about as follows:
|Diameter of grains in millimetres.||1—1||1—1||1—1||1—1||1—1||1—1||1—1|
|Percentage of weight of each size||0.1||0.19||19||37||32||9||1|
Flaky particles of this size are easily carried along by a moderate wind. In some places it appears as if the dust were resting on an old land surface where no water could have been standing when it fell. There is really no room for doubt that it was carried several hundred miles by the wind. It must have darkened the sky at the time, and it must have settled slowly and quietly over the wide plains, covering extensive tracts with a white, snowlike mantle several feet in thickness. Fig. 4.—Tracks in the Volcanic Dust, probably made by a Crawfish. Reduced to 2⁄3 diameter. What a desolate landscape after such a shower! What a calamity for the brute inhabitants of the land!
Right here in McPherson County there was either a river or a lake at the time of the catastrophe. This is plainly indicated in several ways. In one place the dust rests on sand and clay, with imbedded shells of fresh-water clams. It is assorted in coarse and fine layers like a water sediment. Lowermost is a seam of very coarse grains. These must have settled promptly through the water, while the finer material was delayed. In another place it lies on higher ground, and here marks of sedges and other vegetation are seen extending up about a foot into the base of the deposit, from an underlying mucky clay. Bog manganese impregnates a thin layer just above the clay, indicating a marshy condition. Here also the material is somewhat sorted, but in a different way. It is ripple-bedded. The water was evidently shallow, if there was any water at all. A burrow like that of a crawfish extended down into the old clay bottom. On a slab of the volcanic ash itself some tracks appeared (Fig. 4). These were probably made by an individual of the same race in an effort to escape from the awful fate of being buried alive like the inhabitants of Herculaneum and Pompeii.
The shower must have lasted for a time of two or three days. I infer this from the nature of the wind changes, which are indicated by the ripples in the dust. These still lie in perfect preservation (Fig. 5), and may be studied by removing, inch by inch, the successive layers from above downward, for it is evident that as the direction of the wind changed, the ripples were also turned. The deciphering of this record must be made backward. The bottom layers were deposited first, and the excavation must begin on top. Otherwise the record is Fig. 5.—Ripple Marks in the Volcanic Dust. Reduced to 1/4 diameter. as perfect as if it had been taken down by an instrument when the shower occurred. It may be only local in its significance, for it shows the direction of the wind at this particular place alone. The wind may have been somewhat deflected from the general direction by local topographic peculiarities, though these appear to have been of small importance. In any case, the old legend is quite interesting to read, being, I believe, the only geological record ever found of the passing of a cyclone over the United States.
In the lowermost foot of the deposit no ripple marks can be seen. But there appear some marks of sedges and other vegetation, and these are inclined to the west, as if the plants had been bent by an east wind. Just above the height to which the imprints of the vegetation extend, ripple marks begin to appear, running on a northeast-southwest course. They were made by a southeast wind, for their northwest slopes are the steeper. A little above this height some peculiar small elevations appear on one of the bedding planes, and slightly raised ridges run for a short distance to the northeast from
each elevation, vanishing in the same direction (Fig. 6). A southwesterly current was unmistakably obstructed by the little elevations, and left the small trails of dust in their lee. Six inches higher up the wind comes more from the south, and for the next foot the ripples continue to gradually turn still more in the same direction so as to at last record a due south wind. At this point it suddenly changed and set in squarely from the west, for the ripples are turned north and south, with the steeper slopes to the east. This direction seems to have prevailed as long as the dust kept on falling. It appears to me that these successive changes are best explained as attendant upon the passage of a cyclone, or of what our daily weather maps call a "low area." Going by from west to east, on the north, it would at first cause an east wind. This would then gradually be turned to the south and then to the west. One such rotation Fig. 7.—Changes in the Wind as recorded by the Ripple Marks. of the wind generally lasts a day or two. The shower must then have kept on at least for the same length of time, if not longer (Fig. 7).
There is reason to believe that this catastrophe a occurred in summer. No crayfish would be out making tracks during the cold months, and the fossil vegetation could hardly have left such plain marks if it had been buried by the dust during the winter. The most conspicuous of these marks are some triangular and V-shaped molds of the stems and leaves 01 sedges, Siliceous skeletons of chara and filamentous algæ were also found upon a close examination in some of these molds.
It is really difficult to appreciate the change such a shower must have produced in the appearance of the landscape, and the effect it must have had on animal and plant life. So far away from the volcanic source, the wind can not have laid down a layer of this dust several feet in thickness without scattering it far and wide all around. It must have covered tens of thousands of square miles. Just imagine, if you can, a whole State, clad in the verdure of summer, suddenly, in two or three days, covered over by a blanket of white volcanic ash! Many species of plants must have found it impossible to grow in such a soil. And what disaster it must have caused in the animal world! Grazing herds had their sustenance buried from their sight, and could save their lives only by traveling long distances in this loose dust. Many a creature must have had its lungs or its gills clogged with the glassy flakes floating in the water and in the air. The sudden disappearance of several mammal species near the beginning of the Quaternary age has been noted by paleontologists. Does it seem unlikely that an event like this, especially if repeated, may have hastened the extermination of some species of land animals? That many individuals must have perished there can be no doubt. Not very far away from that outcrop of the dust which I have described, one of the early settlers in this part of the State once made a deep well that penetrated the ash. Above the deposit, and some sixty feet below the surface of the prairie, he found what he described as "an old bone yard." In digging other wells in this vicinity mammal bones have been taken up by the settlers from about the same horizon. It is to be regretted that, with one exception, none of these fossils have been preserved for study, for it is likely that they were the remains of animals which were killed in the dust shower.
In the absence of fossils definitely known to be connected with the ash, its exact age seems yet uncertain. In McPherson County it is underlaid by clay, gravel, and sand, which contain remains of the horse, of a megalonyx, and of bivalve mollusks of modern aspect. In the bluffs of the Missouri River near Omaha pockets of a similar ash rest on glacial clay under the loess. At the latter place it must belong to the Pleistocene age, and at the former it can not be older than the late Pleiocene. These two deposits may not belong to the same shower, but it appears, at any rate, that the volcanic disturbances which produced them occurred near the beginning of the Pleistocene age.
In comparison with the slow and even tenor of the routine of geological history, the event here sketched appears so unique and so striking that it may well be called a geological romance. Modern science has taught us that the geological forces are slow and largely uniform in their work, and that most of the earth's features must be explained without taking recourse to theories involving any violent revolutions or general terrestrial cataclysms. While the making of this dust is not any real exception to the law of uniformity, we are here reminded that Nature is quite independent in her ways, and that even in her sameness there is room for considerable diversity.
- Dr. Samuel Aughey, Physical Geography of Nebraska, 1880. Prof. J. E. Todd, Science, April 23, 1886, and January 8, 1897. E. H. Barbour, Publication No. V, Nebraska Academy of Sciences. J. A. Udden, The American Geologist, June, 1891, and April, 1893. R. D. Salisbury, Science, December 4, 1896. G. P. Merril, Proceedings of the United States National Museum, 1885.