Popular Science Monthly/Volume 15/October 1879/The Age of Ice
|THE AGE OF ICE.|
ANCIENT moraines, striations denoting the action of vanished glaciers, the lost rocks, clay-beds evidently of glacial origin—all these are evidences testifying that at some period not very remote, as we count geological periods, the whole northern hemisphere down to the southern limit of 40° was submerged and covered with vast glaciers and ice-floes. There is not a living scientist of any eminence who questions the truth of this assertion.
When we come to study the cause of these phenomena, we find many perplexing and contradictory theories in the field. A favorite one is that of vertical elevation. But it seems impossible to admit that the circle inclosed within the parallel of 40°—some 7,000 miles in diameter—could have been elevated to such a height as to produce this remarkable result. This would be a supposition hard to reconcile with the present proportion of land and water on the surface of the globe and with the phenomena of terrestrial contraction and gravitation. Moreover, it seems evident that an extensive submergence was one of the features of the glacial age. The frozen archipelago called Greenland is a fair picture of what northern America and Europe must have been at that time; and, of course, this precludes the idea of elevation.
If it were not true that submergence and a great lowering of temperature occurred simultaneously, we might imagine that a sort of undulation in the earth's crust, alternately raising and lowering each portion of it, could have caused this result. However, there is no evidence that such an undulatory motion has ever occurred, and we can not conceive of any force likely to produce it.
For the past fifty years, the relation of the inclination of the earth's axis to the plane of the ecliptic and its varying angle with the line of the apsides, has been the subject of careful study, from the impression that herein was a key to the mystery. Astronomical and geological works abound with hints and suggestions of this sort, but I have never yet seen any satisfactory analysis of the question. St. Pierre, Adhemar, and others have presented theories which seem strangely illogical in many of their conclusions. I have been striving to analyze the question, and will present a few of my conclusions.
The orbit of the earth is an ellipse, of which the sun occupies one of the foci. The major axis, or line of the apsides, is the longest diameter of the ellipse, passing through its two foci and through the points of perihelion and aphelion. This line is not fixed with respect to the other bodies in space. Owing to the attractive force of the other planets, it is slowly revolving in the same direction that the earth moves in its orbit. The rate of revolution of this line of the apsides or major axis is a little over 11·6′ a year.
We ordinarily speak of the earth's axis as being always parallel to itself. It is, in fact, however, subject to a very slow gyratory motion, caused by the attraction of the sun upon the meniscus, or ring of matter bulging at the equator, which motion is retrograde, or in an opposite direction to the revolution of the earth. This retrograde motion causes the phenomenon known as the precession of the equinoxes. Its rate is about 50·3′ a year.
It will thus appear that a vast period of time will elapse before the earth's axis will occupy the same position with respect to the line of the apsides that it occupies to-day. This period is sometimes called the Great Year. Its length may be ascertained by reducing three hundred and sixty degrees to seconds, and dividing this quotient by the annual amount of the two motions described above:
1,296,000" (50·3" 11·6') 61·9" 20,937.
The Great Year is thus seen to be about 21,000 years in length.
I have here drawn a diagram illustrating the present position of the earth's orbit with respect to the line of the apsides, purposely exaggerating the eccentricity of the ellipse in order to better present it to the eye of the observer. In the diagram, A B is the line of the apsides; C and D are, respectively, the vernal and autumnal equinoxes; E F is the section of a plane passing through the poles of the earth at right angles with the plane of its orbit. It is evident that this plane will coincide with the line of the apsides once every 10,500 years; E is the winter solstice; F the summer solstice; C is the vernal equinox. While the earth is passing through B to D the north pole is inclined toward the sun. This period is the summer of the northern hemisphere.
D is the autumnal equinox. While the earth is passing from D through A to C, the north pole is inclined from the sun. This period is the winter of the northern hemisphere. In the southern hemisphere these seasons are reversed.
By consulting the diagram it will appear that the arc C B D is greater than the arc DAC. Therefore, the summer of the northern hemisphere is now longer than its winter. On the contrary, the winter of the southern hemisphere is now longer than its summer. At present, this difference is about eight days. It has been greater, but is gradually diminishing.
The southern hemisphere has at present a winter of 187 days, and a summer of 179 days. We may justly infer that during this winter more snow and ice accumulated at the south pole than the heat of the shorter summer is able to melt. The amount of this increase is very slight in a single year, but it accumulates a large aggregate in the course of ages.
This accumulation of ice at the south pole is continually increased and thickened by the deposition of moisture from the atmosphere. Every wind from warmer regions that passes over it adds to its mass.
The "Antarctic Continent" is an ice-cap, nearly circular in form, and about 3,000 miles in diameter, unexplored and uninhabitable. We can not easily ascertain its thickness. The arctic ice-cap is much smaller, and is honeycombed by the Kuro Siwa, or Japan Current, and the Gulf Stream. Nevertheless, the Greenland Archipelago seems covered with glaciers often several thousand feet in depth. If we could assign to the Antarctic ice-cap a thickness of 15,000 feet, we should have a mass of ice large enough to displace the earth's center of gravity nearly a mile to the southward of its center. A gradual displacement of this sort, caused by the slow accumulation of ice, would produce an imperceptible drainage of the oceans from the north to the south, and the gradual emergence of northern and submergence of southern continents.
If we examine the globe, we seem to discover an actual result of this sort. The greatest mass of the ocean is gathered about the south pole. The northern hemisphere includes about five sixths of the land surface of the globe.
Moreover, geologists affirm, that this inequality is increasing; they assert that the northern continents are slowly rising, and that the islands of the south Pacific are sinking. It is more probable that the water is slowly draining away from the northern hemisphere and accumulating in the southern. If our hypothesis he true, this process will continue for some thousand of years to come; until, through the submergence of southern lands and the extension of the Antarctic icecap, the condition of the southern hemisphere will approximate to that of the northern continents during their glacial age. Indeed, if the present position of the earth's axis with respect to the line of the apsides were invariable, we might expect to see a final complete submergence and refrigeration of the southern hemisphere.
But this position is not invariable. As I remarked above, it is slowly changing, making a complete revolution in 21,000 years. Refer to the diagram, and you will see that when F coincides with B, the winter of the southern hemisphere is longest; when F coincides with C, winter and summer in each hemisphere are of equal length; when F coincides with A, the summer of the southern hemisphere is longer than its winter. The earth occupies successively all these situations during its great cycle of 21,000 years. Each hemisphere passes through alternate periods of a preponderance of summer or winter; each period being 10,500 years.
There is one more factor in this problem which must be considered, and that is the periodical variation in the ellipticity of the earth's orbit. Sometimes the line of the apsides is longer than at other times. This variation occupies regular but vast periods of time. It is evident that when the variation is greatest, the accumulation of ice at the winter pole must be most rapid.
The 10,500 years during which the pole is refrigerated and deluged, may be called its great winter; the other half of the cycle, its great summer.
Unless astronomical calculations fail, the last great summer of the northern hemisphere commenced some 6,500 years ago. When it began, northern America, Europe, and Asia were frozen and deluged. The Arctic Ocean extended to a line south of the present bed of the Great Lakes. The Alps and the Alta were also southern boundaries of this ocean. Europe was the home of a swarthy, dwarfish race, who hunted the aurochs and great hairy mastodon at the foot of the glaciers that then half overflowed the continent.
But the intenser suns of many ages have done their work. The glaciers have melted, dwindled, retreated to the high Alpine valleys, and to the high northern latitudes. The swarthy troglodytes and lake dwellers have followed them. Under the name of Esquimaux, Lapps, and Finns, a feeble remnant of these preadamite people still lingers within the Arctic Circle; but their doom as a living race seems near and sure. The Aryans have come marching across the Eastern steppes, for the building of society, civilization, human history.
In the outset of this argument, I assumed an Arctic ice-cap 15,000 feet in thickness, and a displacement of the earth's center of gravity one mile toward the north at the height of the glacial age. In fact, it is not necessary to assume any such amount of displacement. If the earth's center of gravity coincided with its center, so as to equalize the amount of water in the northern and southern hemispheres, Itasca Lake would not be more than 600 feet above sea-level. Now push the center of gravity 2,000 feet toward the north, and the Arctic Ocean would be so much deeper over the pole, and the water would be about 1,000 feet deeper at the latitude of 45°. To accomplish this result, we must calculate that the space within the Arctic Circle was covered by an ice-cap averaging perhaps 8,000 feet in thickness—an entirely supposable case. Such an amount of displacement would flood all the low lands of North America down to the line of 40°, and fully satisfy all the conditions of the problem.
It thus seems probable that there have been many glacial periods in each hemisphere, and that the ocean, like a mighty pendulum, vibrates from pole to pole through vast but regular periods. It is not necessary to suppose a cataclysm at the end of each period, as some of the earlier writers did; but rather, an insensible drainage of waters, which so gradually submerges the lands and pushes the human race before it, as hardly to be perceptible in the course of generations; ever uncovering new continents, and opening up fresh fields and pastures new to human industry, when the old are exhausted.
The southern hemisphere is now undergoing the slow refrigeration of its long winter. This began about 6,500 years ago; it will end about the year 4,870. It has passed its middle, but not its culmination, even as the greatest average cold of our ordinary winter is nearer the vernal equinox than the winter solstice. It is probable that, 2,000 years from now, the southern continents will be still more deeply deluged; the Antarctic ice-cap will have extended several hundred miles to the northward, and the glaciers which have already appeared among the Andes will have covered the plateaus of Patagonia and Chili. Nevertheless, we need not expect that mankind will then witness the utmost possible degree of refrigeration, because the ellipticity of the earth's orbit is now less than it has been at certain periods in the past, and will be again in the remote future.
I feel that, in this discourse, I have ventured upon doubtful and perilous ground. Nevertheless, however illogical and imperfect my conclusions may have been, I feel certain that herein is the key to the mystery. I leave the question, trusting that abler minds may be directed to its consideration and solution.
- Abstract of a lecture delivered before the Kansas Academy of Science.