Volcanoes: What They are and What They Teach

by John W. Judd

Over every part of the earth's surface these three grand operations of the disintegration of old rock-masses, the transport of the materials so produced to lower levels, and the accumulation of these materials to form new rocks, is continually going on. It is by the varied action of these denuding agents upon rocks of unequal hardness, occupying different positions in relation to one another, that all the external features of hills, and plains, and mountains owe their origin.

It is a fact, which is capable of mathematical demonstration, that by the action of these denuding forces the surface of all the lands of the globe is being gradually but surely lowered; and this takes place at such a rate that in a few millions of years the whole of the existing continents must be washed away and their materials distributed over the beds of the oceans.

NECESSITY FOR COMPENSATING AGENCIES.

It is evident that there exists some agency by which this levelling action of the denuding forces of the globe is compensated; and a little consideration will show that such compensating agency is found in the subterranean forces ever at work within the earth's crust. The effects of these subterranean forces which most powerfully arrest our attention are volcanic outbursts and earthquake shocks, but a careful study of the subject proves that these are by no means the most important of the results of the action of such forces. Exact observation has proved that almost every part of the earth's surface is either rising or falling, and the striking and destructive phenomena of volcanoes and earthquakes probably bear only the same relation to those grand and useful actions of the subterranean forces, which floods do to the system of circulating waters, and hurricanes to the system of moving air-currents.

If we ride in a well-appointed carriage with good springs, upon a railway which is in excellent order, the movement is almost imperceptible to us; and the rate of speed may be increased indefinitely, without making itself apparent to our senses. The smallest impediment to the evenness of the movement—such as that produced by a small object placed upon the rails—at once makes itself felt by a violent jar and vibration. How perfectly insensible we may be of the grandest and most rapid movements is taught us by the facts demonstrated by the astronomer. By the earth's daily rotation, we are borne along at a rate which in some places amounts to over 1,000 miles an hour; and by its annual revolution we are every hour transported through a distance of 70,000 miles; yet concerning the fact and direction of these movements we are wholly unconscious.

In the case both of the railway train and of our planet, we can only establish the reality of the movement, and its direction and rate, by means of observations upon external objects, which appear to us to have a movement in the opposite direction. In the same way we can only establish the fact of the movement of portions of the earth's crust by noticing the changing positions of parts of the earth's surface in relation to the constant level of the ocean. When this is done we find abundant proof that while some parts of the earth's crust are rising, others are as undoubtedly undergoing depression.

POTENCY OF THE SUBTERRANEAN FORCES.

We shall be able to form some idea of the vastness of the effects produced by the subterranean forces, by a very simple consideration. It is certain that during the enormous periods of time of which the records have been discovered by the geologist, there have always been continents and oceans upon the earth's surface, just as at present, and it is almost equally certain that the proportions of the earth's surface occupied by land and water respectively, have not varied very widely from those which now prevail. But, at the same time, it is an equally well-established bet that the denuding forces ever at work upon the earth's surface would have been competent to the removal of existing continents many times over, in the vast periods covered by geological records. Hence we are driven to conclude that the subterranean movements have in past times entirely compensated for the waste produced by the denuding forces ever at work upon our globe. But this is not all. The subterranean forces not only produce upheaval; in a great many cases the evidences of subsidence are as clear and conclusive as are those of upheaval in others. Hence we are driven to conclude that the forces producing upheaval of portions of the earth's crust are sufficient, not only to balance those producing subsidence, but also to compensate for the destructive action of denuding agents upon the land-masses of the globe.

It is only by a careful and attentive study and calculation of the effects produced by the denuding agents at work all around us, aided by an examination of the enormous thicknesses of strata formed by the action of such causes during past geological times, that we are able to form any idea of the reality and vastness of the agents of change which are ever operating to modify the earth's external features. When we have clearly realised the grand effects produced on the surface of the globe by these external forces, through the action of its investing atmosphere and circulating waters, then, and only then, shall we be in a position to estimate the far greater effects resulting from the internal forces, of which the most striking, but not the most important, results are seen in the production of volcanic eruptions and earthquake-shocks.

Another series of facts which serve to convince the geologist of the reality and potency of the forces ever at work within the earth's crust, and the way in which these have operated during past geological periods, is found in the disturbed condition of many of the stratified rock-masses of which it is composed. Such stratified rock-masses, it is clear, must have been originally deposited in a position of approximate horizontality; but they are now often found in inclined and even vertical positions; they are seen to be bent, crumpled, puckered, and folded in the most remarkable manner, and have not unfrequently been broken across by dislocations—'faults'—which have sometimes displaced masses, originally in contact, to the extent of thousands of feet. The slate-rocks of the globe, moreover, bear witness to the fact that strata have been subjected to the action of lateral compression of enormous violence and vast duration; while in the metamorphic rocks we see the effects of still more extreme mechanical strains, which have been in part transformed into chemical action. No one who has not studied the crushed, crumpled, fractured, and altered condition of many of the sedimentary rocks of the globe, can form the faintest idea of the enormous effects of the internal forces which have been in operation within the earth's crust during earlier geological periods. And it is only by such studies as these that we at last learn to regard the earthquake and volcanic phenomena of our globe, not as the grandest and most important effects of these forces, but as their secondary and accidental accompaniments. 'Volcanoes,' it has been said, 'are the safety-valves of the globe;' and when we come to realise the real extent and nature of the internal forces ceaselessly working in the earth's crust we shall scarcely be disposed to regard the simile as an overstrained one.