Popular Science Monthly/Volume 39/June 1891/The Future of the Dry Land
|THE FUTURE OF THE DRY LAND.|
I PURPOSE to inquire briefly into the probable future of the dry land, to ask if it is not destined to disappear, and to estimate the time that may be required to execute a sentence of extinction against it. It would have been hazardous to touch upon this question a few years ago. Precise data were wanting as to both the value of the relief of the land and the intensity of the actions which are called into play to change it. But the progress of geographical study has now put us in possession of more exact information, enough to permit us to seek a solution of the problem, not in the expectation of getting exact figures, but of calculating approximately the magnitude of the effects which we have to contemplate.
The labors of geographers in later years have given us a much more complete knowledge than we had before of the land relief. Ten years ago we still accepted Humboldt's estimate that, if all the asperities of the land were leveled over its entire surface, the resultant plateau would stand 305 metres above the surface of the sea. This figure began to grow perceptibly about 1880. A German student, Herr Krümmel, raised it to 444 metres. A few years ago, I thought it best, in preparing the chapter in my Traité de Géologie bearing upon this subject, to go into new calculations on the basis of existing hypsometric maps, and I came to the conclusion that the mean altitude of the dry land would be more than 500 metres, and would probably approach 600 metres. I declared this result with some reserve, on account of its novelty. But I had the satisfaction of seeing it immediately accepted by foreign geographers, and my estimates have since been exceeded; for Messrs. John Murray, Penck, Supan, and De Tillo, having been able, by the aid of the cartographic documents accessible to them, to make still more precise calculations, have found that the land relief may be represented by a uniform plateau rising to 700 metres above the level of the sea.
This plateau of 700 metres is the object of incessant attacks by the ocean on one side and atmospheric agents on the other. The rivers never cease carrying to the sea the fine fragments of the rocks which the rain washes into them, after they have been disintegrated by the alternate actions of moisture and drought, cold and heat, freezing and thawing. By observation of what takes place at the mouths of rivers, we may succeed in reaching a clear idea of the measure in which the silent action of atmospheric agents pares away the continental masses. Mr. J. Murray, of Scotland, from the study of all that has been published on this subject, of which he has himself furnished a considerable proportion, has found that the outflow of the nineteen principal rivers of the earth is 3,610 cubic kilometres a year. These 3,610 cubic kilometres bring to the sea a mass of solid matter in suspension equivalent to one cubic kilometre and 385 making a proportion in value of 38 parts per 100,000. On the other hand, meteorological observations have become precise enough to enable us to estimate approximately the annual outflow of all the rivers of the earth. Mr. Murray puts it at 23,000 cubic kilometres. Applying to this figure the same proportion of 38 per 100,000, we get, for the amount of solid matter annually carried mechanically to the sea by rivers, 1,043 cubic kilometres. That is the effect of the mechanical action of the continental waters.
What part do the waves of the ocean take in this action? When we hear the noise of the waves breaking against the bluffs and throwing their grape-shot of pebbles against them, and when we witness the enormous land-slides of which the sea-shores are often the theatre, we are sometimes led to think that the action of the sea is a preponderant factor in the destruction of continents. But the reverse is the case.
England may be regarded as one of the countries in which attacks by the sea upon the coast are most intense; for the waves of the Atlantic are thrown very violently against the shore by the southwest winds. English geologists appear to be agreed in thinking that the waste of the coasts of Britain under the action of the sea is certainly not more than three metres a century. It is true that at certain points of the French littoral, as at Havre, the banks are estimated to lose a quarter of a metre a year. M. Bouquet de la Grye raises the loss to a little more than a metre a year on the limestone coasts of the southwest; but, in compensation, there are seas where the work of the waves may be neglected as null, as well as flat coasts, where the sea, building up littoral bars, adds instead of taking away. I believe, then, till the contrary is proved, that if we allow for the whole earth a waste of three metres in a hundred years, we are above rather than below the truth.
If we suppose that the shore-banks average fifty metres in height, it follows that an annual waste of three centimetres will remove a cubic metre and a half per running metre, or 1,500 cubic metres per kilometre. The extent of the sea-coasts of the earth can be easily calculated with the aid of the figures given in Elisée Reclus's Continents, showing the proportion, in each continental unit, of dry-land surface and extent of coast-line. Applying these figures to those which represent the surface, now well known, of the different countries, we get 200,000 kilometres as the total length of the coast-lines. Hence, the supposed loss of 1,500 cubic metres per kilometre per year would give 300,000,000 cubic metres, or three tenths of a cubic kilometre. Thus, while the running waters take away ten and a half kilometres, the sea does not remove one twentieth of that quantity. Even supposing I have underestimated the height of the coast-banks, and have not given enough importance to the annual waste, let the figures I have used as the base of my calculations be tripled, we still find the effect of sea action a mere fraction, hardly significant, of that which is produced by the silent wash of the rivers. We can say here, as in many other cases, that what does the most work is not that which makes the most noise.
We have, in addition to this, to consider the solvent action of continental waters. They partially dissolve all the rocks, aided, as they are in the action, by carbonic acid; and they come to the sea charged with a considerably larger proportion of matter in solution than one would at first be liable to suppose. According to the labors of the English, American, and International commissions, which have especially studied the composition of the waters of rivers, particularly of the Mississippi, Danube, and Thames, the quantity of solid matters brought down in solution from the continents is not less than five cubic kilometres a year. This, added to the matter carried down mechanically, gives about 151 cubic kilometres, or, including the results of marine action, 16 cubic kilometres. This, then, is about what the continental masses lose each year.
Let us consider this supposed uniform plateau standing up 700 metres above the level of the sea. By the operation of the circumstances of which I have spoken, 16 cubic kilometres are taken from this mass every year. The continental surfaces covering 146,000,000 square kilometres, we calculate that a waste of 16 cubic kilometres will remove, each year, a layer 11 of a millimetre thick. The débris from this layer will settle on the bottom of the sea and assume the form of sedimentary deposits; they will take the place, then, of a corresponding quantity of water, in consequence of which the sea will rise to a certain extent. The ratio of the continental surface being to that of the seas about as 100 to 252, the total result will be a lowering of the height of the plateau of about 155 of a millimetre every year.
As many times as this 155 of a millimetre is contained in 700 metres, or 700,000 millimetres, so many years will be required to bring about the disappearance of the dry land. Make the calculation, supposing the present intensity in the phenomena of destruction to continue, and you will find that it will take 4,500,000 years to wear the surface of the earth entirely away. This may be a reassuring figure to us. But the geologist, who looks at the past as well as at the future, far beyond existing generations, can draw more than one lesson from it. First, the whole history of the globe not being included in a space of time relatively so short, the result teaches us that its equilibrium has more than once been troubled by great phenomena of dislocation, too rare, however, for it to be possible for man to have been a witness of them, which, building up new reliefs as barriers to destruction, have given new impulses to the action of natural forces.
On the other hand, geological observations furnish a tolerably approximate measure of the maximum thickness of the deposits that are made in the bottom of the sea. The total thickness amounts, according to Dana, to 45,000 metres. To learn how long a time may have been occupied with the formation of such deposits, let us seek to represent to ourselves what now becomes of the products of the destruction of the continents.
These deposits, it is now known, do not extend, by a great deal, over the whole surface of the sea bottom; but they form a zone of strata which the deep-sea sounding expeditions have enabled us to define fairly well. According to Mr. John Murray's estimate, the sediments formed by the destruction of the continents spread themselves over about a fifth of the oceanic surface. Thus, although the oceanic area is superior to the surface of the land, the mass of the deposits distributing themselves over only a fraction of the extent, there may result, at the end of 5,000,000 years, an accumulation of sediments capable of forming a body 750 metres thick. But this thickness would certainly be very unevenly distributed; almost null at the finishing point of the deposits in breadth, its thickness would be much greater near the coasts, and it would not be hazardous to suppose that it might rise there to 2,000 or 3,000 metres. To realize the total thickness of 45,000 metres—that is, to explain geological history—it would be sufficient to suppose that the life of the globe has included some fifteen or twenty periods of 4,500,000 years, or from 67,000,000 to 90,000,000 years, a number a little less than the 100,000,000 years which Sir William Thomson has calculated upon estimates of the loss of internal heat.
The objection may be brought up that I have neglected in this calculation the contributions of volcanic action to the land relief, which it is thought should be counted in attenuation of the destructive effect of running waters. We owe to Cordier a calculation that the lavas which have been thrown up during the historical period represent at most 500 cubic kilometres, or, counting that period at 3,000 years, a sixth of a cubic kilometre per year. This is a very little affair compared with the amount of the waste which I have pointed out. We should likewise recollect that besides eruptions there are also volcanic explosions like those which our generation has witnessed at Krakatoa in 1883, Bangtaisom in 1887, and which our fathers observed at Temboro in 1832. If we reflect that the explosion of Krakatoa threw 16 cubic kilometres of matter into the air, and that of Temboro was still more considerable, we may be permitted to say that volcanic action, instead of diminishing, adds to the constant degradation of the continental relief.
I have not assumed to give precise figures on this subject. My object is less to exhibit numerical results than to present a view of the relative magnitude of the effects under analysis. It is evident that these effects can not be neglected, and that they permit us to assign to the geological history of our globe a duration less than the somewhat fantastic figures to which we have been occasionally asked to give credit.
It is nevertheless true that the disappearance of the continental relief, while it may receive the attention of the geologist and thinker, is not one of those events concerning which present generations need trouble themselves. Neither our children nor our great-grandchildren will have a visible prospect of it presented to them as an actual danger.—Translated for the Popular Science Monthly from Ciel et Terre.
- Address before the Geographical Society of Paris.