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DIVISION I
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See also:DIVISION I . HYPOGENE OR PLUTONIC See also:ACTION
In the discussion of this See also:branch of the subject we must carry in our minds the conception of a globe still possessing a high See also:internal temperature, radiating See also:heat into space and consequently contracting in bulk. Portions of molten rocks from inside are from See also:time to time poured out at the See also:surface. Sudden shocks are generated by which destructive earthquakes are propagated through the See also:diameter of the globe as well as to and along its surface. Wide See also:geographical areas are pushed up or sink down. In the midst of these movements remarkable changes are produced upon the rocks of the crust; they are plicated, fractured, crushed, rendered crystalline and even fused.
(A) Volcanoes and Volcanic Action.
This subject is discussed in the See also:article See also:VOLCANO, and only a See also:general view of its See also:main features will be given here. Under the See also:term volcanic action (vulcanism, vulcanicity) are embraced all the phenomena connected with the See also:expulsion of heated materials from the interior of the See also:earth to the surface. A volcano may be defined as a conical See also: On such rents smaller or parasitic cones are often formed, which imitate the operations of the See also:parent cone and, after repeated eruptions, may rise to hills hundreds of feet in height. In course of centuries the result of the See also:constant outpouring of volcanic materials may be to build up a large mountain like See also:Etna, which towers above the See also:sea to a height of 10,840 feet, and has some 200 See also:minor cones along its flanks. But all volcanic eruptions do not proceed from central orifices. In See also:Iceland it has been observed that, from fissures opened in the around and extending for See also:long distances, molten material has issued in such abundance as to be spread over the surrounding See also:country for many See also:miles, while along the lines of fissure small cones or hillocks of fragmentary material have accumulated See also:round more active parts pf the See also:rent. There is See also:reason to believe that in the See also:geological past this fissure-type of eruption has repeatedly been See also:developed, as well as the more See also:common See also:form of central cones like See also:Vesuvius or Etna. In the operations of existing volcanoes only the superficial manifestations of volcanic action are observable. But when the rocks of the earth's crust are studied, they are found to enclose the See also:relics of former volcanic eruptions. The roots of See also:ancient volcanoes have thus been laid See also:bare by geological revolutions; and some of thesubterranean phases of volcanic action are thereby revealed which are wholly concealed in an active volcano. Hence to obtain as See also:complete a conception as possible of the nature and See also:history of volcanic action, regard must be had, not merely to See also:modern volcanoes, but to the records of ancient eruptions which have been preserved within the crust. The substances discharged from volcanic vents consist of--(I) Gases and vapours: which, dissolved in the molten magma of the interior, take the See also:chief See also:share in volcanic activity. They include in greatest abundance See also:water-See also:gas, which condenses into the clouds of steam so conspicuous in volcanic eruptions. Hydrochloric See also:acid and sulphuretted See also:hydrogen are likewise plentiful, together with many other substances which, sublimed by the high internal temperature, take a solid form on cooling at the surface. (2) Molten See also:rock or lava: which ranges from the extremely acid type of the obsidians and rhyolites with 7o % or more of See also:silica, to the more basic and heavy varieties such as basalts and See also:leucite-lavas with much See also:iron, and sometimes no more than 45% of silica. The specific gravity of lavas varies between 2.37 and 3.22, and the texture ranges from nearly pure See also:glass, like See also:obsidian, to a coarse granitoid See also:compound, as in some rhyelites. (3) Fragmentary materials, which are sometimes discharged in enormous quantity and dispersed over a wide extent of country, the finer particles being transported by upper See also:air-currents for hundreds of miles. These materials arise either from the See also:explosion of lava by the sudden expansion of the dissolved vapours and gases, as the molten rock rises to the surface, or from the breaking up and expulsion of portions of the walls of the vent, or of the lava, which happens to have solidified within these walls. They vary from the finest impalpable dust and ashes, through increasing stages of coarseness up to huge " bombs " torn from the upper surface of the molten rock in the vent, and large blocks of already solidified lava, or of non-volcanic rock detached from the sides of the pipe up which the eruptions take See also:place. Nothing is yet known as to the determining cause of any particular volcanic eruption. Some vents, like that of Stromboli, in the Mediterranean, are continually active, and have been so ever since See also:man has observed them. Others again have been only intermittently in eruption, with intervals of centuries between their periods of activity. We are equally in the dark as to what has determined the sites on which volcanic action has manifested itself. There is reason, indeed, to believe that extensive fractures of the terrestrial crust have often provided passages up which the vapours, imprisoned in the internal magma, have been able to make their way, accompanied by other products. Where chains of volcanoes rise along definite lines, like those of See also:Sumatra, See also:Java, and many other tracts both in the Old and the New See also:World, there appears to be little doubt that their linear See also:distribution should be attributed to - this cause. But where a volcano has appeared by itself, in a region previously exempt from volcanic action, the existence of a contributing fissure cannot be so confidently presumed. The study of certain ancient volcanoes, the roots of which have been exposed by long denudation, has shown an See also:absence of any visible trace of their having availed themselves of fractures in the crust. The inference has been See also:drawn that volcanic See also:energy is capable of itself drilling an orifice through the crust, probably at some weaker See also:part, and ejecting its products at the surface. The source of this energy is to be sought in the enormous expansive force of the vapours and gases dissolved in the magma. They are kept in See also:solution by the enormous,pressure within the earth ; but as the lava approaches the surface and this pressure is relieved these dissolved vapours and gases See also:rush out with explosive violence, blowing the upper part of the lava See also:column into dust, and allowing portions of the liquid See also:mass below to rise and See also:escape, either from the crater or from some fissure which the vigour of explosion has opened on the See also:side of the cone. So gigantic is the energy of these pent-up vapours, that, after a long See also:period of volcanic quiescence, they sometimes burst forth with such violence as to See also:blow off the whole of the upper part or even one side of a large cone. The history of Vesuvius, and the See also:great eruptions of See also:Krakatoa in 1883 and of Bandaizan in 1888 furnish memorable examples of great volcanic See also:convulsions. It has been observed that such stupendous discharges of aeriform and fragmentary See also:matter may be attended with the emission of little or no lava. On the other See also:hand, some of the largest outflows of lava have been accompanied by comparatively little fragmentary material. Thus, the great lava-floods of Iceland in 1783 spread for 40 M. away from their parent fissure, which was marked only by a See also:line of little cones of slag.
The temperature of lava as it issues from underground has been measured more or less satisfactorily, and affords an indication of that existing within the earth. At Vesuvius it has been ascertained to be more than 2000° Fahr. At first the molten rock glows with a See also: Underneath this badly conducting crust, the lava cools so slowly that columns of steam have been noticed rising from its surface more than 8o years after its eruption.
Considerable alteration in the See also:topography of volcanic regions may be produced by successive eruptions. The fragmentary materials are sometimes discharged in such abundance as to See also:cover the ground for many miles around with a See also:deposit of loose ashes, cinders and slag. Such a deposit accumulating to a See also:depth of many
feet may completely See also:bury valleys and water-courses, and thus greatly affect the drainage. The coarsest materials accumulate nearest to the vent that emits them. The finer dust is not infrequently hurled forth with such an impetus as to be carried for thousands of feet into the tracks of upper air-currents, whereby it may be See also:borne for hundreds of miles away from the vent so as ultimately to fall to the ground in countries far removed from any active volcano. Outflows of lava, from their greater solidityand durability, produce still more serious and lasting changes in the See also:external features of the ground over which they flow. As they naturally seek the lowest levels, they find their way into the channels of streams. If they keep along the channels, they See also:seal them up under a mass of compact See also: On surrounding level ground the alternating beds are See also:flat. In course of time, deep gullies are cut on the See also:outer slopes of the cone by See also:rain, and by the heavy showers that arise from the condensation of the copious discharges of steam during eruptions. Along the sides of these ravines instructive sections may be studied of the volcanic strata. The larger See also:rivers of some volcanic regions have likewise eroded vast See also:gorges in the more See also:horizontal lavas and ashes of the flatter country, and have thus laid bare stupendous cliffs, along which the successive volcanic sheets can he seen piled above each other for many See also:hundred feet. On a small See also:scale, some of these features are well displayed among the rivers that drain the volcanic tracts of central See also:France; on a great scale, they are presented in the course of the Snake See also:river, and other streams that See also:traverse the great volcanic country of western See also:North See also:America. Similar volcanic scenery has been produced in western . See also:Europe by the action of denudation in dissecting the flat See also:Tertiary lavas of See also:Scotland, the See also:Faeroe Isles and Iceland. Of See also:special See also:interest to the geologist are those volcanoes which have taken their rise on the sea-bottom; for the volcanic intercalations among the stratified formations of the earth's crust are almost entirely of submarine origin. Many active volcanoes situated on islands have begun their eruptions below sea-level. Both Vesuvius and Etna sprang up on the See also:floor of the Mediterranean sea, and have gradually built up their cones into conspicuous parts of the dry See also:land. Examples of a similar history are to be found among the volcanic islands of the Pacific Ocean. In some of these cases a See also:movement of See also:elevation has carried the submarine lavas, tuffs and agglomerates above sea-level, and has furnished opportunities of comparing these materials with those of See also:recent subaerial origin, and also with the ancient records of submarine eruptions which have been preserved among the stratified formations. From the See also:evidence thus supplied, it can be shown that the materials ejected from modern submarine volcanic vents closely resemble those accumulated by subaerial volcanoes; that the dust, ashes and stones become intermingled or interstratified with See also:coral-mud, or other non-volcanic deposit of the sea-bottom, that vesicular lavas may be intercalated among them as on land, and that between the successive sheets of volcanic origin, layers of See also:limestone may be laid down which are composed chiefly, or wholly, of the remains of calcareous marine organisms. Though active volcanoes are widely distributed over the globe, and are especially abundant around the vast See also:basin of the Pacific Ocean, they afford an incomplete picture of the extent to which volcanic action has displayed itself on the surface of our See also:planet. When the rocks of the land are attentively studied they disclose proofs of that action in many districts where there is now no outward sign of it. Not only so, but they reveal that volcanoes have been in eruption in some of these districts during many different periods of the past, back to the beginnings of geological history. The See also:British Islands furnish a remarkable example of such a See also:series of ancient eruptions. From the See also:Cambrian period all through Palaeozoic times there See also:rose at intervals in that country a See also:succession of volcanic centres from some of which thousands of feet of lavas and tuffs were discharged. Again in older Tertiary times the same region witnessed a stupendous outpouring of See also:basalt, the surviving relics of which are more than 3000 ft. thick, and cover many hundreds of square miles. Similar evidence is supplied in other countries both in the Old and the New world. Hence it is proved that, in the geological past, volcanic action has been vigorous at long intervals on the same sites during a vast series of ages, though no active vents are to be seen there now. The volcanoes now active form but a small See also:pro-portion of the See also:total number which has appeared on the surface of the earth. With regard to the. cause of volcanic action much has been speculated, but little can be confidenety affirmed. That water in the form of occluded gas plays the chief part in forcing the lava column up a volcanic See also:chimney, and in the violent explosions that accompany the rise of the molten material, is generally admitted. But opinions differ as to the source of this water. According to some investigators, it should be regarded as in large measure of meteoric origin, derived from the descent of rain into the earth, and its absorption by the molten magma in the interior. Others, See also:con- [HYPOGENE ACTION tending that the See also:supply so furnished, even if it could reach and be dissolved in the magma, would yet be insufficient to furnish the prodigious quantity of aqueous vapour discharged during an eruption, maintain that the water belongs to the magma itself. They point to the admitted fact that many substances, particularly metals in a See also:state of See also:fusion, can absorb large quantities of vapours and gases without chemical See also:combination, and on cooling See also:discharge them with eruptive phenomena somewhat like those of volcanoes. This question must be regarded as one of the still unsolved problems of See also:geology. (B) Movements of the Earth's Crust. Among the hypogene forces in geological See also:dynamics an important place must be assigned to movements of the terrestrial crust. Though the expression " the solid earth " has become proverbial, it appears singularly inappropriate in the light of the results obtained in recent years by the use of delicate See also:instruments of observation. With the facilities supplied by these instruments (see See also:SEISMOMETER), it has been ascertained that the ground beneath our feet is subject to continual slight tremors, and feeble pulsations of longer duration, some of which may be due to daily or seasonal See also:variations of temperature, atmospheric pressure or other meteorological causes. The See also:establishment of self-recording seismometers all over the world has led to the detection of many otherwise imperceptible shocks, over and above the appreciable earth-waves propagated from earth-quake centres of disturbance. Moreover, it has been ascertained that some parts of the surface of the land are slowly rising, while others are falling with reference to the sea-level. From time to time the surface suffers calamitous devastation from earthquakes, when portions of the crust under great See also:strain suddenly give way. Lastly, at intervals, probably separated from each other by vast periods of time, the terrestrial crust undergoes intense plication and fracture, and is consequently ridged up into mountain-chains. No event of this See also:kind has been witnessed since man began to See also:record his experiences. But from the structure of mountains, as laid open by prolonged denudation, it is possible to form a vivid conception of the nature and effects of these most stupendous of all geological revolutions.
In considering this See also:department of geological inquiry it will be convenient to treat it under the following heads: (I) Slow depression and upheaval,; (2) Earthquakes; (3) Mountain-making; (4) See also:Metamorphism of rocks.
i. Slow Depression and Upheaval.—On the See also:west side of See also:Japan the land is believed to be sinking below the sea, for See also:fields are replaced by beaches of See also:sand or See also:shingle, while the depth of the sea off See also:shore has perceptibly increased. A subsidence of the See also:south of See also:Sweden has taken place in comparatively recent times, for streets and See also:foundations of houses at successive levels are found below high-water See also:mark. The west See also:coast of See also:Greenland over an extent of more than 600 m. is sinking, and old settlements are now submerged. Proofs of submergence of land are furnished by submerged forests, and beds of terrestrial See also:peat now lying at various depths below the level of the sea, of which many examples have been collected along the shores of the British Isles, See also: One of the most celebrated instances is that of the shores of the Gulf of See also:Bothnia, where, at See also:Stockholm, the elevation, between the years 1794 and 1875, appears to have been 48 centimetres (182 in.) in a See also:century. But on the west side of Sweden, fronting the Skager Rak, the coast, between the years 182o and 187o, rose 30 centimetres, which is at the See also:rate of 6o centimetres, or nearly 2 ft. in a century. In the region of the Great Lakes in the interior of See also:Canada and the See also:United States it has been ascertained that the land is undergoing a slow tilt towards the south-west, of which the mean rate appears to be rather less than 6 in. in a century. If this rate of See also:change should continue the See also:waters of Lake See also:Michigan, owing to the progress of the tilt, will, in some 50o or 600 years, submerge the See also:city of See also:Chicago, and eventually the drainage of the lakes will be diverted into the basin of the See also:Mississippi. See also:Proof of recent emergence of land is supplied by what are called " raised beaches " or " strand-lines," that is, lines of former shores marked by sheets of littoral deposits, or platforms cut by shore-waves in rock and flanked by old sea-cliffs and lines of sea-worn caves. Admirable examples of these features are to be seen along the west coast of Europe from the south of See also:England to the north of See also:Norway. These lines of old shores become fainter in proportion to their antiquity. In See also:Britain they occur at various heights, the platforms at 25, 50 and zoo ft. being well marked. The cause of these slow upward and downward movements of the crust of the earth is still imperfectly understood. Upheaval might conceivably be produced by an ascent of the internal magma, and the consequent expansion of the overlying crust by heat; while depression might follow any subsidence of the magma, or its displacement .EPIGENE 'ACTION] to another See also:district. If, as is generally believed, the globe is still contracting, the shrinkage of the surface may cause both these movements. Subsidence will be in excess, but between subsiding tracts lateral thrust may suffice to push upward intervening more solid and See also:stable ground; but no solution of the problem yet proposed is wholly satisfactory. 2. Earthquakes.—As this subject is discussed in a See also:separate article it will be sufficient here to take See also:note of its more important geological See also:bearings. It was for many centuries taken for granted that earth-quakes and volcanoes are due to a common cause. We have seen that in classical antiquity they were looked on as the results of the movements of See also:wind imprisoned within the earth. Long after this notion was discarded, and a more scientific appreciation of volcanic action was reached, it was still thought that earthquakes should be regarded as manifestations of the same source of energy as that which displays itself. in volcanic eruptions. It is true that earth-quakes are frequent in districts of active volcanoes, and they may undoubtedly be often due there to the explosions of the magma, or to the rupture of rocks caused by its ascent towards the surface. But such shocks are comparatively See also:local in their range and feeble in their effects. There is now a general agreement that between the great world-shaking earthquakes and volcanic phenomena, no immediate and intimate relationship can he traced, though they may be connected in ways which are not yet perceived. Some of the more recent great earthquakes on land have proved that the waves of See also:shock are produced by the sudden rupture or collapse of rocks under great strain, either along lines of previous fracture or of new rents in the terrestrial crust; and that such ruptures may occur at a remote distance from any volcano. Thus the recent disastrous See also:San Francisco See also:earthquake has been recognized to have resulted from a slipping of ground along the line of an old See also:fault, which has been traced for a long distance in See also:California generally parallel to the coast. The position of this fault at the surface has long been clearly followed by its characteristic topography. After the earthquake these superficial features were found to have been removed by the same cause that had originated them. For some 300 M. on the track of this old fault-line a renewed slipping was seen to have taken place along one or both sides, and the ground at the surface was ruptured as well as displaced horizontally. Obviously, the See also:jar occasioned by the sudden and simultaneous subsidence of a portion of the earth's crust several hundred miles long, must be far more serious than could be produced by an earthquake radiating from a single local volcanic See also:focus. From their disastrous effects on buildings and human lives, an exaggerated importance has been imputed to earthquakes as agents of geological change. Experience shows that even after a severe shock which may have destroyed numerous towns and villages, together with thousands of their inhabitants, the See also:face of the country has suffered scarcely any perceptible change, and that, in the course of a See also:year or two, when the ruined houses and prostrate trees have been cleared away, little or no obvious trace of the See also:catastrophe may remain. Among the more enduring records of a great earthquake may be enumerated (s) landslips, which See also:lay bare hillsides, and some-times See also:pond back the drainage of valleys so as to give rise to lakes; (h) alterations of the topography, as in fissuring of the ground, or in the See also:production of inequalities whereby the drainage is affected; new valleys and new lakes may thus be formed, while previously existing lakes may be emptied; (c) permanent changes of level, either in an upward or downward direction. 3. Mountain-snaking.—This subject may be referred to here for the striking evidence which it supplies of the importance of movements of the earth's crust among geological processes. The structure of a great mountain-See also:chain such as the See also:Alps proves that the crust of the earth has been intensely plicated, crumpled and fractured. Vast piles of sedimentary strata have been folded to such an extent as to occupy now only See also:half of their See also:original horizontal extent. This See also:compression in the See also:case of the Alps has been computed to amount to as much as 12o,000 metres or 74 See also:English miles, so that two points on the opposite sides of that chain have been brought by so much nearer to each other than they were originally before the movements. Besides such intense plication, extensive rupturing of the crust has taken place in the same range of mountains. Not only have the most ancient rocks been squeezed up into the central See also:axis of the chain, but huge slices of them have been torn away from the main See also:body, and thrust forward for many miles, so as now actually to form the summits of mountains, which are almost entirely composed of much younger formations. If these See also:colossal disturbances occurred rapidly, they would give rise to cataclysms of inconceivable magnitude over the surface of the globe. No 'record has been discovered of such accompanying devastation. But whether sudden and violent, or prolonged and See also:gradual, such stupendous upturning's of the crust did undoubtedly take place, as is clearly revealed in innumerable natural sections, which have been laid open by the denudation of the crests and sides of the mountains. 4. Metamorphism of Rocks (see METAMORPHISM).—During the movements to which the crust of the earth has been subject, not only have the rocks been folded and fractured, but they have like-See also:wise, in many regions, acquired new internal structures,' and have thus undergone a See also:process of " regional metamorphism." This rearrangement of their substance has been governed by conditions65 which are probably not yet all recognized, but among them we should doubtless include a high temperature, intense pressure, See also:mechanical movement resulting in crushing, shearing and foliation, and the presence of water in their pares. It is among igneous rocks that the progressive stages of metamorphism can be most easily traced. Their definite original. structure and See also:mineral See also:composition afford a starting-point from which the investigation may be begun and pursued. Where an igneous rock has been invaded by metamorphic changes, it may be observed to have been first broken down into separate lenticles, the cores of which may still retain, with little or no alteration, the original characteristic minerals and crystalline structure of the rock. Between these lenticles, the intervening portions have been crushed down into a See also:powder or See also:paste, which seems to have been squeezed round and past them, and shows a laminated arrangement that resembles the flow-structure in. lavas. As the degree of metamorphism increases, the lenticles diminish in See also:size, and the intervening crushed and foliated See also:matrix increases in amount, until at last it may form the entire mass of the rock. While the original minerals are thus broken down, new varieties make their See also:appearance. Of these, among the earliest to See also:present themselves are usually the micas, that impart their characteristic silvery sheen to the surfaces of the folic along which, they spread. Younger felspars, as well as See also:mica, are developed, and there arise also See also:sillimanite, See also:garnet, See also:andalusite and many others. The texture becomes more coarsely crystalline; and the segregation of the constituent minerals more definite along the lines of foliation. From the finest silky phyllites a See also:graduation may be traced through successively coarser mica-See also:schists, until, we reach the almost granitic texture of the coarsest gneisses. Regional metamorphism has arisen in the See also:heart of mountain-chains, and in any other district where the deformation of the crust has been sufficiently intense. There is another type of alteration termed " contact-metamorphism," which is developed around masses of igneous rock, especially where these have been intruded in large bosses among stratified formations. Additional information and CommentsThere are no comments yet for this article.
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