The Attack On And Defence Of The Continents

( Originally Published 1915 )

IN a figurative sense, the Earth lives, as we have seen. Just as in the case of all living beings, there are possibilities of organic degradation and of alteration or disturbance of function. In the course of the preceding chapter, we have examined the mechanism of the terrestrial circulation; in the present one, we shall see by what agencies the Earth's surface may be attacked and by what means, in a figurative struggle for life, these destructive causes are counteracted.

The gaseous mass which forms the atmosphere, and the liquid mass constituting the oceans, are in perpetual movement around the solid crust that is the external envelope of the Earth's nucleus. This crust, far from being uniform and level, is furrowed and irregular, traversed by valleys and rugged with mountains. We have to consider what happens to this crust in the presence of the moving fluid masses which unceasingly assail it. In what way, and up to what limit, can the solid material constituting the crust resist the attacks of wind and water? Are there innate means of defence against such incessant activity? We shall endeavour to elucidate this matter in the following chapter.

When masses of air in the equatorial region rise to the upper strata of the atmosphere as a result of the diminution of density caused by the solar heat, they take up with them large quantities of water-vapour, greater in proportion to the elevation of temperature. At 3° C. [86° F.], for example, the maximum pressure of aqueous vapour is measured by 31 millimetres [1.22 in.] of mercury, and consequently represents, in a saturated atmosphere, one twenty-fifth part of the total atmospheric pressure. Carried along with the air by means of the antitrade winds, this vapour arrives over the cooler countries where these air masses descend down to the Earth's surface. Further-more, the masses of warm humid air that accompany the oceanic currents and pass on over the continents also bring considerable quantities of water-vapour to the atmosphere of these. As this vapour thus arrives at a region, the temperature of which is lower than that which corresponds to the pressure of water-vapour actually in the air, some of it rapidly condenses, first in the form of clouds, then rain, and finally snow if the condensation occurs at a sufficiently low temperature, which is always the case on the summits of high mountains. The water so condensed in various forms is the chief agency in the denudation of the land-surface.

The Sun itself, however, begins the action. Under the heating effect of its rays even the hardest rocks become heated during the day. As they are poor conductors of heat, the heating takes place only in the parts directly exposed to the Sun, and consequently the expansion which results from the rise of temperature does not affect the whole mass equally; molecular effects are produced tending to destroy the cohesion which binds the molecules together. Sooner or later, the strain, repeated daily and arrested at night, when there is a corresponding contraction, becomes greater than the molecular cohesion can stand; the rock splits and cracks and instead of a continuous surface the exposed part is fissured and broken.

Then comes the rain. Reversing the Latin proverb, we must here say: post Phoebium nubila. The condensed water falls into the crevices and cracks of the rocks. If the rocks occupy an elevated position, for example the top of a mountain, the water does not remain liquid, but freezes in the interstices of the rocks. In freezing, its volume increases, and, thus, with an irresistible force, it widens the fissures and still further separates the walls of rock on either side. This is the second stage of the destructive action; after having been fissured by the Sun's action the rock is subsequently fractured and disintegrated into larger or smaller fragments.

There now supervenes another destructive agency, that of gravity. At the period of formation of those foldings of the crust which were the origin of mountains, the matter composing them was elevated above the mean level of the seas by effects resulting from the more or less recent manifestations of internal energy. These effects, by thus raising the rocky masses and giving them potential energy, for the time being triumphed over the force of gravity; but this reasserts itself as soon as the rocks begin to disintegrate and lose the cohesion which originally bound the whole mass together. The fragments resulting from the action of freezing water fall down the mountains and form a talus of debris at its base. In the descent they frequently knock against each other, and these shocks, absorbing a part of their energy, end by distributing them in the form of a natural slope from which gravity alone will not suffice to displace them. But, now, a fourth assailant comes into action, viz.: running water derived from the rainfall. Always subject to the action of gravity, water tends to fall to a lower level than that where it originally is. Now solid substances are maintained by friction in more or less steep slopes according to the degree of regularity of the fragments composing the slope. But this is not the case with fluids, which cannot finally remain at rest until they reach a place where the free surface, filling suitable hollows, coincides with a level surface parallel to that of the geoid.

In the course of its descent towards this final level, the water falls with more or less swiftness and vigour according to the steepness of the slopes over which it passes. In proportion as it descends, it takes up and carries along particles that external physical agencies have detached from the land surface; furthermore, in virtue of its force it hollows out a channel which marks its course and so continuously wears away the solid earth. When a drop of water, which represents a liquid projectile, unites with other similar drops the effect of the added mass is multiplied by the square of the velocity. In consequence of this, the stream that has so taken birth, and which gradually becomes a torrent, eats deeper and deeper into the initial groove in which it flows, and it carries down the material so removed. There are many obstacles in its path however; for example, it may arrive at a hollow or depression in the ground that has no outlet, in which case after transforming this into a lake it has to wear down the edges. Also it may have to fall, in the form of cascades and waterfalls, over any rocks that are too hard to be worn away immediately and so open a passage to its waters. It is only able to wear away such a surface very gradually, but this it does by the constant friction of its waters, aided by the continual knocking of the stones and pieces of rocks previously broken off and carried along by the water.

In all cases, the stream becomes smoother and more gentle little by little; it retains its original impetuosity only among mountains where there are high rocky masses which intercept the waters. In regions of average altitude, and plains, the violence of its descent gradually slackens. The constant friction exercised by the bed of the stream on the water, which always grows larger in volume by the contributions of affluent streams, tends to retard its movement. Also, the slope diminishes gradually in proportion as the mass of water in-creases, reducing the velocity and force of the stream. The wearing away of solid matter from the two shores also influences the stream; the rocks, sand, and stones carried along knock against and exert friction on the irregularities of the bottom of the stream and gradually wear them smooth, all the time decreasing the slope of the river in proportion to the distance from the mountains where it had its source that is, in proportion as it nears the sea.

We thus see that stones, gravel, and sand or, generally, alluvial matter which water has deposited on lands that it covered at a certain period represent, together with mud, the products of destruction of portions of the crust over which a river formerly flowed. But this is not the only way in which running water wears down the relief of the land-surfaces. There is another means, equally effective, which, instead of acting in a continuous manner like the flowing of a stream of water, acts intermittently. This is infiltration. On very inclined slopes, great masses of rock and earth soaked with water may over-hang valleys hollowed by the stream. If the layer which supports them is, for example, a clay capable of sliding, the whole mass gives way and falls down into the valley. In such a way millions of cubic metres [or yards] of debris of all sorts may be thrown down in a few minutes; this is of frequent occurrence in Switzerland.

The tributaries of a river also carry on a similar work of slow and continuous destruction; from the smallest steamlet to the mightiest of rivers every one is producing disintegration of the solid earth over which it flows. The result of this incessant wearing down, prolonged during numerous centuries, should be the complete destruction of all the continents. Under the action of gravity all the materials resulting from their demolition, carried to the seas by running water, would ac-cumulate in the form of sediments at rest on the oceanic bottoms.

We thus arrive at the seeming possibility of a complete levelling of the continental relief by the action of running water, after the elapsing of a sufficiently great length of time. And there is also still another form which the attack upon the solid land by water may take, viz.: the action of glaciers.

On the summits of high mountains rain does not fall: on account of the low temperature, the result of the altitude, the drops of water condense and then solidify, taking a crystalline form and falling as snow. Snow is very light, because of the numerous spaces separating the branches of its beautiful crystals, and so it is easily driven by the wind, and accumulates in masses which fall as avalanches, carrying down with them stones and pieces of rock detached from the mountains. These consequently fall into the valleys and ravines. The snow also accumulates in these hollows, and under the pressure exerted by the superimposed layers it exhibits the phenomenon of congelation by pressure. It becomes transformed into compact ice, so that a kind of river of ice occupies the bottom of the ravine, a river which slowly descends under the thrust of the upper layers of snow and ice. The velocity is small, of the order of one metre [or yard] daily. Subsequent avalanches fall on to the glacier and deposit large rocks thereon; thus the frozen river or glacier, to give it its proper name, carries along huge blocks that have broken off the mountains from which came the snow that gave rise to it. These blocks become aligned along the edges of the glacier in the form of very characteristic trains which are called moraines. When, at the end of its course, the glacier enters regions of lower altitude or more elevated temperature, so that the ice permanently melts, these rocks are deposited just beyond its termination where they constitute a kind of embankment or barrier which has received the name of the frontal moraine. This frontal moraine is incessantly traversed by the rapid streams of water produced by the fusion of the ice at the end of the glacier. The constituent rocks are thus rolled one against another when the action of these torrents has, after a time, destroyed their equilibrium and washed out the stones and gravel which supported them in position.

When the glacier is formed of great thickness in the cold regions in the neighbourhood of the poles, its front end may reach down to the sea. There, enormous blocks break from it, and these are carried by ocean currents towards warmer regions. In this way, the icebergs are formed, large masses of ice the sunken part of which is eight or nine times greater than the emergent part. Some icebergs weigh as much as millions of tons. Icebergs may destroy the largest ships, in consequence of their vast size. When they melt completely, any rocky masses that they have torn from the land-surface and have carried with them, fall to the bottom of the sea.

During winters which are characterised by abundant snowfalls, the glacier descends farther, and so encroaches on the valley, while, on the other hand, after unusually dry years the ice recedes, laying bare the lowest part of its bed. It is then possible to see the destructive action that its descent has produced; it seems as if a gigantic plane had been passed over the ground that was covered by the ice. All the rocks and stones are rounded as if they had been turned, and on their polished surfaces may be seen grooves and scratches produced by the action of small harder stones that the ice has caused to rub against them. Thus, while plains and low valleys are subjected to river erosion, the mountains, in spite of their height and consequently apparent immunity from attack by water, are assailed and disintegrated by the action of snow and ice. Glaciers carry the resulting debris to the end of their course and finally the fragments are subjected to the action of the torrents of water arising from the melting of the ice which finally carry them to still lower regions. Thus, low-lying country becomes covered with the debris of the highest summits. To recapitulate, watercourses constitute a slow but sure agency for the disintegration of the continents, the debris of which they ultimately convey to the sea.

Not only does water wear away the exposed surface of the land over which it flows, but it also has another and more insidious destructive action.

It infiltrates into the mass of the Earth and traverses it in the form of subterranean streams and channels. In the course of this it dissolves a quantity of material from the strata and as finally such water re-enters the ocean, it brings to the latter substances in solution derived from the rocks through which it has flowed. The sea is thus the vast reservoir in which accumulate little by little the materials arising from the demolition of dry land.

It might be thought that there are at any rate certain regions of the Earth's surface immune from destruction by any form of aqueous agency, viz: those where no rain ever falls, in other words the deserts. Sublatá causâ, tollitur effectus.

Unfortunately as regards the relief of such regions, although rain takes no part in their disintegration, the wind plays an equally destructive rôle. The rocks of these regions are subjected to the breaking-up influence of the alternate expansions and contractions caused by the solar heat during the day and the subsequent cooler night. The results of disintegration, fragments of rocks which mutual friction has ground to the condition of grains of sand, are carried by the wind and forced into hollows where they accumulate, or frequently they may be piled up in the form of dunes. There hard sand grains are flung by the wind against the emergent rocks and so grind them down little by little, producing the curious phenomena of wind erosion so well described by Professor Vélain. Thus, no part of the land surface escapes from the destructive effects of subaerial denudation, which if not due to water, derived from the condensation of atmospheric water vapour, forming rain, watercourses or glaciers, acts through the instrumentality of the winds which, aided by the grains of sand, gradually wear away crags and mountains.

We have just considered the action of water flowing over the Earth's surface, thus forming an immense circulatory system. But the sea is not merely a passive receptacle for the waste of the land brought down to it by running water. The sea also directly attacks the land; raised by the tides, agitated by waves and endowed with a progressive motion by marine currents, it exerts a destructive influence along the shores of the land surface. The force of the liquid masses so projected against rocks and cliffs is enormous and reaches thirty tons per square metre [or yard]. It is estimated that the whole Earth contains about 250,000 kilometres [155,000 miles] of marine shores, so it is obvious that there is a large space over which the sea's destructive action can be exerted. This action is manifest in the erosion of the hardest granites, as seen on the Brittany coasts, by the rounding of enormous blocks that the waves pile one on the other, by the piercing of great arches, and the falling of whole cliff surfaces, as may be observed on the Normandy coasts. Although very formidable, yet the attack made by the ocean is confined to the region along the shores, while the action of running water is felt over the whole area of an extent of land so that the destructive effect of the latter is eight or nine times greater than that of the sea.

As regards the statistical expression of the work of destruction of the continents, geologists have calculated that the sum total of solid matter yearly removed from the land-surface is about 25 cubic kilometres [6 cub. miles]. We have seen, on the other hand, that the total continental mass elevated above the level of the geoid is about 100 million cubic kilometres [24 million cubic miles]. If the rate of denudation be assumed constant, it would, thus, require four million years to erode the whole of the dry land, and carry to the sea the debris resulting therefrom. The time would actually be less than this, for, in proportion as the solid matter accumulated in the ocean, so would the level of the latter rise, consequently diminishing the volume of the remaining land. We may, therefore, follow Lapparent in estimating the time necessary for the complete destruction of the continental relief as three and a half million years.

We must now inquire whether the continents have no power of defence against these indefatigable adversaries, and whether they are not able to make some kind of a struggle for life, just as men and all living beings do, instinctively resisting the germs of disease and disintegration.

Such is the case. Since the debris from land destruction accumulates in the sea it is built up in layers on the bottom and so by the slow upward growth of the sediment new strata are added to those which originally formed the solid crust of the terrestrial globe. The way in which this building-up process occurs is simple.

When the sea has sapped a cliff at the base the fragments resulting from its fall become rounded by the action of the waves and so are transformed into shingle. This shingle collects in a line at the foot of the cliff, forming an enbankment which extends parallel to the line of the coast. Beyond the shingle, and washed incessantly by the waves which alternately advance and retreat, are the fine sands and then the ooze which is gradually deposited at the bottom of the sea, where the violent agitation of the surface is felt less and less in proportion to the depth.

When a large river enters into a tideless sea the water bringing down the sand and mud deposits these materials little by little and the river mouth becomes gradually blocked by the continual arrival of new alluvial matter. A direct passage for the water is all that remains in the midst of the sands so deposited by the rivers. These sands extend seawards more and more, gradually gaining on the sea and forming the characteristic regions that may be seen at the mouths of the Rhone, the Po, the Nile, and Mississippi, and which are called deltas. The vegetable remains carried down by the river accumulate in these deltas. Subsequent deposits bury them under new layers of mud and so protect them from atmospheric action, transforming them into abundant reserves of fuel for future exploitation. If a river enters an ocean that exhibits marked tides, instead of a tideless one such as the Mediterranean or Adriatic, the formation of a delta is difficult, but in such a case a bar is gradually formed by means of the periodical conflict between the river current and the rising tide. A bar is a submarine embankment of mud and sand which the variations of the tides and the vicissitudes to which the river is subjected may slowly displace, and which always constitutes an obstacle, and sometimes a danger, to navigation on account of its submerged character and the breaking of waves upon it.

The flood-tide adds daily to the bar what is brought down by the river, and the ebb-tide carries the rest of the material into the open sea where it is deposited on the bottom as sand and ooze. Thus, a layer of deposits is formed around the continental coasts, material being laid down over an average breadth of 200 kilometres [124 miles]; near the coasts it is formed of shingle and sand, but farther seawards it is exclusively composed of very fine particles of pure clay which constitutes marine ooze. In this way a sedimentary deposit is slowly built up, gaining more than a millimetre [.039 in.] annually in height.

This describes what occurs along marine shores where important rivers carry down materials arising from the disintegration of solid earth. In the midst of the vast marine expanses which separate the continents from each other, the building up of solid earth occurs by other means. Tiny living creatures work indefatigably at the task of reconstructing new land, finding the necessary materials for their work in the dissolved mineral matters that the water of the sea has received from the land. They use these substances in the construction of their skeleton or their shell. These microscopic sea-workers are the polyzoa which live in colonies in the tropical seas, when the temperature of these does not fall below 2° C. [68° F.], in the superficial regions of the sea down to a depth of 30 or 40 metres [loo to 130 ft.]. These little beings constituting a veritable vegetation, for their colonies take arborescent forms, construct with the lime extracted from the sea the bases on which they build up their edifice. As this increases in height, the lower portions cease to live, and only the chalk structure remains, the solidity of which increases by reason of the debris accumulated within the interstices of the base as a result of the waves breaking off the emergent branches of the coral. Little by little, the whole is trans-formed into a coral reef emerging just above the level of the sea at its lowest, and so constituting a great danger to navigation. In front of the north-east of Australia a veritable rampart, a coral barrier, exists.

Often, especially in the Pacific, where the manifestations of internal energy are intense and numerous, submarine eruptions have taken place which have lifted up volcanic cones to the surface of the sea. These would have been quickly washed away by the agency of the waves, but the polyzoa have gathered round the emergent crater and formed coral girdles or rings, the debris from which, washed off by the sea, accumulated towards the centre and so formed annular islands, the atolls, several of which have become habitable and are actually inhabited by human beings, who thus benefit by the work of these microscopic animals.

Throughout the whole of the sea live animalculae which are often phosphorescent, and which also extract and solidify the chalky matter that the rivers have dissolved from the land and carried to the oceans. After their death, the chalky envelope of these minute creatures falls to the bottom of the sea in the form of foraminiferal ooze which entirely covers the bottom. Other small animals act similarly with regard to silica, and their debris also adds to the accumulating sediment on the oceanic bottoms.

However, in spite of all, the continents must end by disappearing, for the marine shores become bordered little by little with debris; the level of the sea rises because of the intrusion of solid matter and the surviving parts of the continents always continue to suffer disintegration by atmospheric agencies while the sea rises higher and higher. Finally, therefore, the continents must completely disappear, all being covered by the water of the sea. Yet there is one way in which this process is retarded, viz : the action of the internal energy manifested as volcanic eruptions and seismic phenomena. We have seen in the course of the preceding chapter that active craters eject an enormous amount of solid matter on to the Earth's crust. That in the Sandwich Islands alone has restored to the crust a quantity of material equal to that which 12,000 years of erosion remove from the land surface. Taking account therefore of the total number of existing volcanoes and also those that have existed in the past we can see they constitute a considerable, if not a complete, compensation for continental diminution due to erosion.

Nevertheless, it must be remembered that the material that is thus ejected above the superficial crust disappears from the central magma whence it comes. Consequently the crust, ceasing to be sustained at every point must sink down and a sinking even to the extent of only a few milli-metres [few hundredths of an inch] all over the globe would be a greater loss to the dry land than the entire gain brought to it by eruptions. We have seen that such sinkings occur, they produce seismic phenomena and earth convulsions. These sinkings may be partially counterbalanced by risings, either sudden or gradual ones, and these always, so to speak, rejuvenate the exterior surface of the crust where they occur, forming new relief, the opposite process to the continual wearing down of the irregularities of the crust by denudation.