Popular Science Monthly/Volume 76/June 1910/Scenery, Soil and the Atmosphere
|SCENERY, SOIL AND THE ATMOSPHERE|
THE atmosphere is commonly considered as a body of gases surrounding the globe, but hardly as a part of our sphere. We must, however, look upon it as being of the very substance of our earth, an integral part of the planet as truly as the waters or the solid crust. The geologist and the geographer, indeed, habitually speak of three envelopes of the globe, the atmosphere, the hydrosphere and the lithosphere. A certain assemblage of gases, all of which may be found in the waters and the rocks, remains in a more attenuated condition as the outer part of the earth. The degree of attenuation increases as we go from the surface of the solid part. Whether the atmosphere actually ceases a few hundred miles, or some hundreds of thousands of miles, from the lithosphere is not important to our present purpose, for its effective work is done within a few miles of altitude.
Looking at the atmosphere as a whole, its calms are exceptional and its movements are the rule. We may find the gentle breeze, the cyclonic wind or the resistless tornado, but always activity. These movements do not tamely confine themselves to horizontal paths, but the gases rise and plunge, pursue broad curves and narrow spirals, and would present, to an eye that could see them from above, a tumult, like the sea in storm. If we add to these mechanical operations the efficient chemical functions of the atmosphere, we shall be ready to agree that it is one of the most powerful agencies that help to mold the form and fashion the quality of the outer parts of our planet.
We well understand that all organic life is dependent on the atmosphere for its existence, and that interchange of materials is constant. The forms of the land are nearly as dependent upon this medium as are those of life. Manhattan Island was once a mountainous tract. The first making of the rocks that composed it was conditioned by an atmosphere. The forces that filed it down to its present forms and heights could not have worked without the gaseous envelope. The channels that invite ships to its water line are an indirect product of atmospheric activity. Indeed, the Palisades Ridge, and the submergence of the coast line are the only features of your inorganic environment that have chiefly been due to underground forces.
The atmosphere is interwoven with all forces operating on or near the surface. Other, or subterranean, energy could produce but a few types of form. We might have great and swelling ridges or domes, or cliffs due to faulting, involving fracture and dislocation, or volcanic cones, streams or sheet outflows. For such initial forms, apart from an atmosphere, gravity would seem to be the only agent of change, and its action would be narrow, for disintegrating forces must operate to make gravity effective.
With so much of introduction we are ready to look at the atmosphere in a threefold way—and consider, first, its indirect work; second, its direct work; and, third, its history.
As a means of changing the face of the earth, and of modifying its rocks to a considerable depth, no agency compares with land waters. But we are to remember that waters, if they could exist, could not move without a gaseous medium. Supply our planet with its outfit of oceans, lakes, rivers and underground waters, and, in the absence of a thermal blanket they would be frozen and silent. If they could be conceived as keeping the liquid condition, no transportation of water vapor could take place, no rainfall, and no rivers or glaciers could accomplish their tasks.
Modern geography has introduced the doctrine of the cycle. We mean by this the period in which a continent or any part of it would be reduced from its initial forms of uplift, to baselevel; in other words, the time necessary to wear out a land, and put its waste under the bordering sea. In the course of this wearing out, many land forms—mountains, plateaus, hills, plains, slopes, valleys—would come into being and disappear, in appropriate stages of youth, maturity and age. A great series of evolutionary forms of the land would characterize the passage of a cycle.
The varying amount and condition of land waters give us three types of the geographic cycle and three typical groups of resulting shapes of the surface; these are the normal, the glacial and the arid. The normal cycle is conditioned by medium temperatures and ample precipitation in the form of rains. The glacial cycle exhibits low temperatures and abundant precipitation in the frozen condition. The arid cycle is marked by higher temperatures and low precipitation—so little rain that lakes can not rise to the rims of closed basins due to warping of the crust, for the simple reason that evaporation and soakage take care of the rainfall and no rivers can reach the ocean.
We may illustrate the three kinds of cycle by three well-known parts of our own land. The southern Appalachians show us what happens in normal conditions. There are indeed at least two cycles whose results are clearly shown in this southern region, but for our purpose we simply observe that here are plentiful rainfall and moderate temperatures. Great initial uplifts have given opportunity for land sculpture on a large scale. Rivers and weathering have done the work. There are practically no closed basins, either dry or wet, no interruptions of drainage, and the soil is residual, having been chiefly made by the decay of the rocks in place.
Let us turn to New England. Here the soils are due to the weathering and organic modifications of the "drift." The drainage has been greatly interrupted, and new channels, waterfalls and lakes abound. Several types of hills of accumulation appear that we can not find in the southern mountains. The tops of hills and mountains of hard rock have a different aspect—it is the product of the glacial cycle which we are contemplating, and the geographer would only need to cast his eye upon maps of the two regions to recognize their character.
Let us go to the Great Basin. Rather is it a group of basins, flanked by the Wasatch on the east and the Sierras on the west. Its broad and arid expanse is intersected by many north and south mountain ridges, which are the up-thrown edges of great crustal blocks. The upper parts of these mountains show normal erosion. The bordering Sierras show normal and glacial types. Within the basin the summits and slopes of the mountain are shedding down their waste and streams of greater or less vigor are carving a normal topography. At the foot of the mountains, the waste, instead of going in the grasp of streams to the sea, is spread out on the inter-montane floors, raising the surface and building plains of sand, clay, salt and gypsum. The rivers become small instead of growing, and lose themselves in the atmosphere, the soil and in shallow lakes which may be permanent and salt, or intermittent and brackish. The soil is formed mechanically and with but a minor amount of those chemical changes which take place in a more humid climate. Vegetation is scant and this condition retards true soil-making and, by lack of cover, aids the action of winds. Monotony characterizes the arid type, as ceaseless variety belongs to the normal and glacial operations. The ridges that separate adjacent arid basins may be cut away, and both be merged in a single featureless plain, and thus we have a growing lake of waste, somewhat akin to closed basins of water. The only means by which such a basin can lose its material is through the winds. Central Persia offers another land of the same order, where severe conditions of aridity have for an unknown period reflected themselves in the life of man and in the very forms of the land which he makes his home.
Hill and mountain, valley and plain, thus own their dependence, in no remote way, upon the gaseous envelope. As it is wet or dry, hot or cold, so evolves the very physiognomy of our globe, and whether we cross the border of an adjacent township or journey to a far country, its handiwork is before our eyes.
No forms are more characteristic or interesting than those which the ocean makes on its borders. These forms in a minor way are due to the tides, in the major way to waves, and thus, at one remove only, are produced by the atmosphere. Likewise ocean currents, involving transfers of heat, moisture, plants and animals, are believed to be chiefly due to atmospheric movements. If we compare New York with Rome and Constantinople, or Labrador and Hudson Bay with Great Britain and the North Sea, or the ice fields of Greenland with the ample life of Scandinavia, if we wonder at the small part which latitude seems to have with opposite shores of the Atlantic, we are at once led back to the atmosphere for light upon our problem.
Likewise for all life, the atmospheric gases are essential as a heat regulator and in the employment of their substances for organic structures. Through these structures, in turn, all soil-making is profoundly conditioned, and those organic accumulations are made possible, which we find in the mineral fuels and in most limestones.
If we turn to the direct work of the atmosphere, we shall find a group of facts, not so conspicuous, perhaps, but worldwide and important. This is one of the newer subjects in geology, and to consult any text-book written twenty years ago would yield a scant result in information.
The atmosphere is the agent of incessant chemical change, the world over, and down to the level of the ground water. Whether the water table be found close to the surface or far down, the atmosphere finds access to some parts of the rocks down to that horizon. The depth below the surface depends on the water supply, on the character of the rocks and on the surface topography, and it is different in the same place at different times, but always and everywhere the gases that surround and surmount us, are pursuing their underground activities as well. And it is scarcely proper to limit them to zones above the water table, for they communicate some measure of their efficiency to the ground waters, which in turn may go far down, and may make long journeys before they emerge, thus giving the atmosphere some share in the segregation of those metallic substances which belong in the inventory of the world's wealth.
The making of soil is a complicated process, or bundle of processes. Fundamentally it is due to the breaking down of rocks, and this is effected by change of temperature, by organisms, by the wear of running streams, glaciers and ocean waves. But when the rocks are broken down, processes more intimate and essential must be added. These more intimate agencies are the water, the atmosphere and decaying organisms. The water will accomplish solution and thus make certain minerals available for the nutrition of crops. But the water falling as rain has gathered from the atmosphere minute portions of its carbon dioxide, and has become thereby an effective dissolving agent. Most rocks contain more or less iron in a disseminated condition. The oxygen of the air combines readily with this metal, promoting the decay of the rock mass, and coloring not only the rock, but the soils that ultimately come into being. When the farmer selects a field to lie fallow he stirs the soil, gives it all possible exposure to air, water and heat, and thus speeds these silent processes which go on in some measure everywhere, with or without his ken. It is the time required to produce these fine results, that makes wanton destruction of soils criminal and points the rebuke to public authorities and legislative bodies, when they halt at reasonable measures of conservation. We can not grind rocks in a mill and make soil. The operation is at once too large and too delicate, demanding the silent intervention of mechanical and chemical forces, of atmosphere, water, heat and life, through long periods of time.
When the land has suffered a glacial invasion, much of its ancient soil has been lost in the sea, and such as remains is moved from its place and mixed with a large body of drift, mechanically broken from fresh bed rock. This latter material is not soil until it has been subjected to the atmospheric and vital processes which fit it for its function of mediation between the rocky planet and the plant life of the world.
In a non-glacial region, all the soils, save along rivers, or on steep slopes, have been formed by the decay of the bed rocks in place, and this decay does indeed, and fortunately, in favored regions, proceed swiftly. It may be some compensation for people subject to disaster on the slopes of Vesuvius or Etna, that the friable lavas and ash, in that genial climate, speedily become soil. On a lava stream still warm, at the foot of Vesuvius, baskets of earth, suitably spaced for vines, have been deposited, and the lava itself in a year or two will be hospitable to the roots.
Stone from the quarry is popularly thought to be a durable building material, but only the most compact and resistant varieties, and these in a favorable climate, can make any approach to permanence. Granites are regarded as indestructible, but the title of granite to serve as the standard and symbol of strength is clouded when we remember that many beds of soft clay owe their accumulation to the decay of one of the chief mineral constituents of this rock, a decay in which the atmosphere has been a powerful agent. Granite that can be excavated with pick and shovel betokens the ceaseless activity of the gases and waters of the earth's surface.
Some of you will recall the promptness of atmospheric attack upon the obelisk of Central Park after it was transplanted from its arid habitat of millenniums. Many of the beautiful structures of the Oxford colleges, boasting not three centuries of antiquity, are under restoration piece by piece, showing an apparent hoary age through the solvent work of the atmosphere upon their unstable calcareous material. No marble monument has stood in the open air for half a century and retained its polish, and it must have been an exceptional piece of monumental stone if it does not now crack and scale and take on the look of age. Every humid climate with large temperature range introduces a ceaseless struggle with the destructive forces of the atmosphere, whose sum of hostility to the structures of man is far greater than that of flood, fire or earthquake.
The energies of the atmosphere mechanically applied, bring before us a group of results, not perhaps so nearly universal, but even more tangible and conspicuous. Many agents are at work producing mineral materials of such fineness that the winds can carry them. Oriental travelers and explorers know the sand storm as one of the most distressing and sometimes deadly visitations. In the desert mechanical disruption of rocks goes on rapidly and there is little moisture or plant life to hold the fragments down. The winds become factors of transportation in a manner little known by dwellers in moist and verdant lands. Dust storms are not confined to the Sahara, or Persia, or Turkestan. They occur in considerable numbers in our arid regions, where they sweep for hundreds of miles, last for many hours and carry incredible loads. Sand drifts a foot high have gathered in a half hour on railway tracks; thirteen cars of sand were taken from a single depot platform in Colorado; the same careful student who reports these facts estimates from 160 to 126,000 tons of sand carried in a cubic mile of air. This for a single storm may give us hundreds of millions of tons borne for hundreds of miles. Under such conditions the redistribution of surface materials by the atmosphere can no longer be held trivial.
We are now to remember that desert conditions furnish but a minor part of the dust that is available. Wherever in all geologic time there have been explosive volcanic eruptions, dust has been expelled, often in prodigious amounts, covering leagues of sea with floating pumice, littering the decks of vessels hundreds of miles away, destroying crops, darkening the atmosphere across wide seas, and enriching the sunset glows, it is believed, around the globe. Every rain storm purges the air of dust, much of local origin, no doubt, but some from remote and subterranean sources. Here is ceaseless accretion for all land surfaces and for all sea bottoms, and we have an impressive illustration of the interdependence and the cosmopolitan efficiency of every part of the earth's machinery.
Man never uncovers a soil surface with the plow or by the passage of hoof or vehicle, without exposing material to atmospheric migration, and it is some years since an expert road maker, in a highway convention, set forth the havoc wrought on macadam roads by winds.
From the point of view of natural scenery the winds' most conspicuous product is the dune. Many have seen a single example of a belt or field of sand hills, but the student of the earth finds in them no phenomenon of small range. He looks for them on the lee side of every river in a desert region and along all sand shores. He finds them invading the olive orchards of Palestine, the vineyards of France, the meadows of Holland, the forests of the Great Lakes and the fields of Cape Cod. The hand of man is put forth to stay the ravages of these flying cohorts and the organized skill of a government department joins in the task. Search is made for sand-binding grasses, in the same spirit in which the agricultural explorer hunts for wheat suited to arid fields or palms for the future orchards of Arizona.
The wind-blown sands are not only materials of accumulation, they are agents of erosion. Deserts abound in bare and unprotected rock surfaces, which occupy thousands of square miles in the Bad Lands, in the ridges of the Great Basin, in northern Africa and in western and central Asia. The impact of sharp-edged grains of quartz, maintained in every wind storm age after age, becomes no small means of wear and destruction, producing a natural sand blast whose principle is now used in many and ingenious ways in the arts. We have interest in Thoreau's quaint story of the clergyman of Cape Cod, frequently setting a fresh pane of glass to preserve the transparency of at least one fragment of window surface, but if we look more widely we find a large and significant phenomenon. Small lake basins have been excavated by the wind, and the sand of desert basins, eroding on its long journey, may come to rest at some remote point, as truly "exported" as if sent across the boundaries of a foreign land.
Before passing from these direct accomplishments of the atmosphere, we must include those peculiar deposits of fine and silty material known to the geologist and the physical geographer as loess. Much has been said of their origin, often in the field of debate, sometimes in the realm of controversy. But these great sheets of material, typically found in the Mississippi Valley, and in the central parts of Asia, have impressed many observers as being in whole or in part the work of winds blowing over vast fields of aridity, or sweeping widely the fine-grained outwash from areas of glaciation.
If we add now the transport of organisms, particularly of seeds, insects and birds, and the influence of winds on the migration of higher animals and man himself, through the medium of ocean currents, we shall see how the face of the organic world gathers its lineaments as broadly and depends on the atmosphere as intimately as the contours of a continent. The organic in turn reacts on the purely physical and we recognize at last that, touch the globe where we will in scientific inquiry we pick up some link in an endless chain.
The climates of the world have not always been what they are to-day. If we go as far back as the records will carry us, we find rocks and fossils that betray the climates of their time. These geologic climates are parts of ancient geographies which, in a long series, lead up to the geography of our own age. Throughout this succession, the atmosphere, its constituents, movements and temperatures held the same influence over the rocks of the crust and the forms of the land, which we now see. The atmosphere has had a history, and its qualities and activities have been among the chief factors in the evolution of the earth's surface.
Geology recognizes many periods of prevailing warmth, in which genial conditions were so wide-spread as almost to amount to a disregard of latitude. These temperate and subtropical conditions in high latitudes belong not only to ancient, but to the middle and modern ages of the world, and geologists long ago surrendered the notion that they could be due to supposed stores of the earth's primal heat.
There have been periods of notable dryness, so that deposits of rock-salt were formed through the evaporation of marine waters. From New York westward occur beds of salt, due to a dry climate, in a region where the rainfall is now abundant and where the basins of the Laurentian lakes are filled to their brims.
Not many years ago, the ice invasion of the Pleistocene was regarded as simple in character and unique in time. We now accept, among the commonplaces of glacial geology, that the late ice invasion was composite, marked by great advances and recessions, and by interglacial times of genial climate. And we recognize further, among the accepted facts of the science, the existence of vast glacial sheets in Permo-Carboniferous times, in India, Australia and South Africa, in regions which are now either warm or warm-temperate, and in lands of no great elevation.
Yet more remote, in the Cambrian, in an age of early life forms, an age recognized by the older geology as having almost ubiquitous warmth, the evidences of extensive glaciation have been brought to light. We must remember that humidity, precipitation, great or slight, and variations of temperature, are intimate questions of meteorology, whether we raise them in Cambrian or Miocene or present times. The meteorology of the passing age is related to geologic climates precisely as the physiography of existing lands is related to the rocks and rock structures of the past. The atmosphere has undergone a prolonged evolution in close association with the progress of the solid earth. As a part of the earth's history the study of the atmosphere is somewhat belated, but its importance is now recognized, and it will fill a large place in the geology of the future.
The present atmosphere therefore has not always existed and is but the latest term in an evolutionary series. We find two leading assumptions concerning this history.
There is a widely prevalent geological doctrine that our atmosphere is a residuum from a more dense or a more extensive body of gases. On this view it once contained all, or much, of the carbon dioxide whose carbon is now wrapt up in the coal and the limestones of the earth's crust. Thus Dana refers to the "purification of the air and waters through the making of limestone" as commencing in later Archean time and continuing through the Cambrian. Accepting, as he does, the idea that all the carbon of the coal and of many rocks was originally in the air and the waters, he still finds difficulty, for in early Paleozoic time life shows an atmosphere not too heavily charged with this gas, notwithstanding the fact that great coal beds and many great limestones had not yet been formed.
Geikie recognizes a continual abstraction of carbon dioxide since land plants began to live, but only allows that the amount in the air in Paleozoic time may have been somewhat greater than now. Davis thus expresses the view which has been long current:
These views, it will be seen, follow naturally upon the nebular hypothesis, with its mass of heated gases undergoing consolidation.
We find under discussion at the present time the view that the atmosphere is not greatly different from that of early geological periods, but has been subject to important fluctuations in the amounts and proportions of its constituents. These changes are believed to be due to many causes, some effecting loss and some bringing about renewal. Various interchanges are postulated, on the one hand, between the earth and outside spaces, and on the other between the atmosphere and the crust or the interior of the globe. This line of investigation has been recently pursued by Chamberlin and others, particularly with reference to its bearing on glacial climates, and has involved new conceptions of the origin of the earth. But entirely apart from the possible validity of these reasonings, the researches have value in setting forth the changes to which the atmosphere is subject. These changes have so much to do with the earth's crust that they are germane to our theme.
It is shown that the atmosphere loses carbon dioxide in several ways; as through carbonation, that is, by the decomposition of silicates and the formation of carbonates of calcium and magnesium, in limestone and dolomites of great extent. This decomposition is extensive in times of elevation of the lands, such as have occurred widely in some geological periods. When the lands are high, the water table is farther below the surface, and the air pierces deeply, with its chemical activities, and the ground waters also have much more vigorous circulation. The carbon dioxide thus employed in making limestone is extracted from the atmosphere.
There is loss of carbon dioxide through fixation in coal, oil and in all organic matter, diffused through the sedimentary rocks. There is temporary loss of this gas through the ordinary feeding of plants, and the view has been held, that plant growth would exhaust the CO2 of the atmosphere in one hundred years, but for the renewal through plant decay and from other sources. All these processes would take from the air in geological time, many thousand times as much CO2 as it now holds.
On the other hand, there have been gains through various sources of supply. These are, from the ocean, and from the interior of the earth by volcanic action, and in escape in connection with earthquake movements. Any changes involving deformation and fracture open the way for supplies of this gas from below. Van Hise lays emphasis on CO2 as derived from volcanoes and in the same passage refers to emanations from hot springs and mine waters. He quotes Lecoq to the effect that the mineral springs of the Auvergne alone give off nearly one tenth as much of this gas as is freed by the entire coal burning of Europe. The same author includes meteorites as a source of CO2 but regards this means of gain as unimportant in later geological eras. Possible supplies may have been received from the sun, if the projection of gases from that body is sometimes so energetic as to shoot them within the orbit of our planet; and, as has been implied, there is a steady restoration of this gas through decomposition of organic matter, and through organic processes.
As oxygen combines actively with some substances, notably iron, we are to expect large losses of this gas from the atmosphere, when we remember that oxidation has been a world-wide process, throughout the history of land surfaces, down to the lowest level of atmospheric penetration. Thus Smyth concludes a brief essay on this subject by saying, "The abstraction of oxygen by iron is a factor that can not be disregarded in any attempt to work out the geological history of the atmosphere."
There have also been compensating supplies of oxygen. The fixing of carbon in the crust involves the freeing of oxygen. There have been times of predominant plant life, leading to the abstraction of CO2, and the release of O, thus notably changing the atmosphere until animal life enlarged its province, using and freeing CO2. Chamberlin thinks that organisms have freed more oxygen than the rocks have absorbed and that this gas therefore has had a growing part in the atmosphere.
Nitrogen, as is well known, is the inert part of our atmosphere, but is absorbed through certain bacteria, leading to modern effort to utilize it in restoring the fertility of the soil. On the other hand, some nitrogen is supplied to the atmosphere through volcanic eruptions.
These views of atmospheric inconstancy involve general doctrines of climate and earth history which are in the crucible of discussion. The efficiency of the atmosphere as a thermal blanket is held to be so dependent on the amount of carbon dioxide present, that moderate fluctuations in the quantity of this gas may go far to explain the noteworthy glacial episodes of later and earlier geological times, and those warm climates which at other periods have spread so widely over the earth.
Thus we have geographic hypotheses, and astronomic hypotheses, so it seems appropriate that we should have atmospheric hypotheses, in the laudable effort to understand and explain that great series of geologic climates, which may indeed seem remote, but with the latest of which, we should remember, we ourselves have to do.
Our interest in the evolution of the atmosphere and of climate is of no theoretical sort. We are not in the grip of forces which are in a despotic way our masters. We have a large control of organic life on the earth and of the disposition and character of all land waters. Through these means we also largely regulate the processes of denudation and we may thus in some measure modify the very constitution of the atmosphere. Van Hise, on what he regards as a moderate estimate of the coal the human race will burn per annum during the present century, estimates that in 812 years the amount of carbon dioxide in the atmosphere would be doubled. According to the view of Arrhenius such a change would greatly ameliorate the climate of the world. This view of the heat-holding effects of an increase of CO2 is not undisputed, but so large a change in the constitution of the atmosphere, by the hand of man himself, may well cause him to investigate, with serious persistence, the terrestrial consequences of his own deeds.
Van Hise has impressively set forth the work of man in lowering the level of the ground water. We do this by deforestation, by cultivation, by irrigation, by the sinking of artesian wells and by mining. For so many purposes man needs water, or needs to get rid of water, that actual and serious lowering of the water table has taken place, and will be brought about more and more with growing density of population. Lowering the ground water, as we have seen, increases the contact of the atmosphere with the rocks, and sets in motion a chain of actions which may have consequences for good or ill, quite outside our present knowledge and inviting expert investigation for many years to come. These considerations have a bearing, possibly an imminent bearing, upon all our conservation enterprises in the United States.
Scientific truth often gives us no inkling of astonishing practical results which are about to flow from it. Thus meteorology is no restricted theme for the curious. It is not merely a science of climate, though this would give it the highest interest for science and for life: it is profoundly related to the history of our planet and it is an essential part of physical, biological and human geography.
- "Manual of Geology," fourth ed., p. 484.
- "Elementary Meteorology," W. M. Davis, p. 3.
- "Treatise on Metamorphism," pp. 969–970.
- "The Abstraction of Oxygen from the Atmosphere by Iron," C. H. Smyth, Jr., Jour. Geol., 13, 319–323.
- "Treatise on Metamorphism," p. 464.