Volcanic rock

“ No one who has carefully studied the appearances presented by volcanic mountains in different stages of dissection, by the action of denuding forces, can avoid recognising these great granitic masses as the cooled reservoirs from which volcanoes have in all probability been supplied during earlier periods of the earth's history.
The eruption of these great masses of incandescent rock, impregnated with water and acid gases, through strata of limestone, sandstone, clay, coal, &c., may be expected to produce striking and wonderful chemical changes in the latter. ”

“ On one thing all scientific men seem to be now agreed; namely, that while the rocks inside the earth are hot enough to melt they are hard as steel, owing to the tremendous pressure of the rocks above them, and one theory about volcanic eruptions is that they are caused by the release of the pressure on this rock in one place and a pressing down in another, as the earth's crust settles and crumples around the centre. Some of this rock—that on which the pressure is released—melts and rises under the folds of rising rock, and so makes the granite hearts of the greater mountains. ”

“ A volcano that throws out molten rock, vapor and gases is known as an active volcano. An active volcano, however, is only correctly said to be in a state of eruption when the quantity of the molten rock, lava or vapor it throws out is greatly in excess of the ordinary amount.
Sometimes the volcanic activity so greatly decreases that the molten rock or lava no longer rises in the crater, but, on the contrary, begins to sink, so that the top of the lava in the crater is often at a considerable distance below its edges. ”

“ Not only have vast quantities of detritus of non-volcanic rocks been shot forth from volcanic vents, but sometimes enormous solid masses of rock have been brought up by ascending lavas or have been ejected by explosive vapours. ”

“ When the mass of molten rock or lava that comes out of the crater of a volcano is thrown upwards in the air the condition it assumes by the time it falls back again to the earth depends on the height it reaches. If this height is great the lava chills or hardens before reaching the earth, and assumes various forms according to the size of the fragments. The largest of these fragments are called cinders; the finer particles volcanic dust; while most of those of intermediate particles are known among other things as volcanic ashes. ”

“ Some highly heated rocks, though they never rise to the surface to form lava-flows, are forced up with overwhelming pressure from below, and wedge themselves into 155 the sedimentary rocks that overlie them, thus forming what are known as volcanic dykes, and intrusive masses or sheets of once molten rock. In this category we include such rocks as basalt, felstone, pitchstone, and other rocks of fiery origin that have flowed from volcanoes as lava, as well as those like granite, which have cooled and become solid below the surface, and are Plutonic, or deep-seated, igneous rocks. ”

“ VOLCANIC NECKS. The pipe of a volcano rises from far below the base of the cone,—from the deep reservoir from which its eruptions are supplied. When the volcano has become extinct this great tube remains filled with hardened lava. It forms a cylindrical core of solid rock, except for some distance below the ancient crater, where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air. ”

“ The chief effect of volcanic eruptions on the Geological structure of our Globe consists in the accumulation of cinders and molten rock, either upon the Surface of the Earth, or in the crevices and caverns that abound within its solid Crust. Sometimes, indeed, the operations of an active Volcano are accompanied by a movement of upheaval or of subsidence. ”

“ The three main types of volcanic mountain.—If the eruptions at a volcanic vent are exclusively of the explosive type, the material of the mountain which results is throughout tuff or cinder, and the volcano is described as a cinder cone. If, on the other hand, the vent at every eruption exudes lava, a mountain of solid rock results which is a lava dome. It is, however, the exception for a volcano which has a long history to manifest but a single kind of eruption. ”

“ But it is not only the finely crystallised minerals and gems which we owe to volcanic action. The various metallic minerals have nearly all been brought from deep-seated portions of the earth's crust and deposited upon the sides of rock-fissures by the agency of the same volcanic forces. It is these forces which have, in the first instance, opened the cracks through the solid rock masses ”

“ Our usual impression of a volcano is indicated in the title of “burning mountain,” so often employed, a great fire-spouting cone of volcanic debris, from which steam, lava, rock-masses, cinder-like fragments, and dust, often of extreme fineness, are flung high into the air or flow in river-like torrents of molten rock. ”

“ The second type of neck is that in which the volcanic pipe has been entirely filled up with some massive or crystalline rock. As already remarked, it is not always possible to be certain that bosses of rock, having the external form of necks of this kind, mark the sites of actual volcanic orifices. Eruptive material that has never reached the surface, but has been injected into the crust of the earth, has sometimes solidified there in forms which, when subsequently exposed by denudation, present a deceptive resemblance to true volcanic necks. ”

“ Evidences of remote volcanic action abound in the vicinity, the whole surface of the ground being composed, in fact, of tufa, ashes, and coarse, broken sandstone, mixed with metamorphic rock, ascribed to the cretaceous age, and indicating disturbance by severe earthquakes or volcanic convulsions of a comparatively recent date. None of the trees are perfect—only the trunks and main roots appearing to have been petrified—and all are lying flat upon the ground, or half buried in it, scattered and broken, as if blown down by a sudden gale or whirlwind. ”

“ The other kinds of rocks are known as igneous rocks, which have been molten under the action of heat and become solid on cooling. When in the molten state they have been poured out at the surface as the lava of volcanoes, or have been forced into other rocks and cooled in the cracks and other places of weakness. Much material is also thrown out of volcanoes as volcanic ash and dust, and is piled up on the sides of the volcano. Such ashy material may be arranged in beds, so that it partakes to some extent of the qualities of the two great rock groups. ”

“ But when we realize that the rock is an unstratified agglomerate, probably marking the place of a volcanic vent, and therefore rising transgressively through the surrounding strata, the necessity for a fault is removed, or if a fault is inserted its existence should be justified on other evidence than the relations of the igneous rock to the surrounding strata. ”

“ It is succeeded by a thin band of greenish limestone (3) containing an admixture of fine volcanic detritus, and partially cut out by an irregular bed, four to eight feet thick, of a highly slaggy, greenish, decomposing, spheroidal and amygdaloidal diabase (4) . This unmistakable lava-sheet is followed by a bed of green granular tuff (5) , which in some places reaches a thickness of three feet, but rapidly dies out. Over a space several yards in breadth, the succeeding strata are concealed, and the next visible rock is a dark, compact dolerite which weathers spheroidally (6) . ”

“ Of the eruptive bosses of massive rock outside the limits of the Old Red Sandstone which may be plausibly referred to the volcanic phenomena of the period, though they cannot be proved to be actually part of them, the most - 290 - notable are the bosses of granite and other acid material which rise through the Silurian strata of the Southern Uplands of Scotland. ”

“ The most elevated of an extensive system of volcanic rocks, resting partly on granite or gneiss, and in part on the Jurassic formation, which by their position and constitution prove themselves to be the remains of a single and powerful volcano, of the same character as those in the Mont Dore and Cantal. Its products, however, are disposed in a somewhat different manner, being spread over an almost equally extensive surface without accumulating into such mountainous masses around their centre of eruption. ”

“ It can only endure so long as it is continually renewed by fresh eruptions, or where it is carried down by subsidence under water and is there buried under a cover of protecting sediments. When, therefore, we meet with volcanic rocks of ancient date exposed at the surface, we may be quite certain that their present contours are not those of the original volcano, but have been brought about by the processes of denudation. ”

“ The greater or less pressure under which merely softened or wholly liquid fluids are solidified, appears to constitute the main difference in the formation of Plutonic and volcanic rocks. The mineral mass which flows in narrow, elongated streams from a volcanic opening (an earth-spring) , is called lava. where many such currents meet and are arrested in their course, they expand in width, filling large basins, in which they become solidified in superimposed strata. These few sentences describe the general character of the products of volcanic activity. ”

“ The Plutonic Rocks are chiefly granite of some kind or another; and though they now often appear at the surface, they are supposed to have been produced originally at a considerable depth within the crust of the Earth, “or sometimes, perhaps, under a certain weight of incumbent ocean.”14 The Volcanic Rocks have been formed at or near the surface of the Earth, and, as the name implies, they are usually ejected, in a state of fusion, from the fissures of an active volcano ”

“ The loftiest volcanic cones must be as insignificant, when contrasted to the products of fire in the nether regions, as are the deposits formed in shallow estuaries when compared to submarine formations accumulating in the abysses of the ocean. In regard to the characters of these volcanic rocks, formed in our own times in the bowels of the earth, whether in rents and caverns, or by the cooling of lakes of melted lava, we may safely infer that the rocks are heavier and less porous than ordinary lavas, and more crystalline, although composed of the same mineral ingredients. ”

“ Magnificent sections of the rocks are laid open to view along the sides of the valleys, which are steep and rock-bound. Except towards the south-west, about Aurillac, where lacustrine strata overlie the granite, the platform from which rises the volcanic dome is composed of granitic or gneissose rocks. Accompanying the lava-streams are great beds of volcanic agglomerate, which Mr. ”

“ Not only are volcanic rocks of many different ages, and lie, therefore, on many successive platforms within the crust of the earth: their places have been still further dependent upon changes in the arrangement of that crust. Fracture, upheaval, depression, curvature, unconformable deposition of strata, have contributed to protect some portions, while leaving others exposed to attack. Hence it happens that the volcanic record varies greatly in its fulness of detail from one geological system or one district to another. ”

“ Intermediate between the extruded volcanic rocks, and the highly crystalline varieties of the internal reservoirs of molten material, are the dyke or hypabyssal rocks, which as narrow vertical or nearly vertical intrusions pass in all probability from the deep-seated source towards the surface of the earth, when they reach it giving rise to volcanic effects. ”

“ These considerations lead the geologist to conclude that the evidence afforded by the ancient volcanic rocks is clear and positive in support of the view that the manifestations of the subterranean forces in the past agree precisely in their nature and in their products with those taking place around us at the present time. On the question of great secular changes having occurred in the amount of volcanic energy in past geological periods, the evidence must be pronounced negative, or at the best doubtful. ”

“ The structure of a volcano is very different from that of other mountains, really consisting of layers of lava and volcanic ashes, alternating with each other and all sloping away from the center. These elevations, in fact, are formed in a different manner from ordinary mountains. The latter have been uplifted by the influence of pressure in the interior of the earth, but the volcano is an immediate result of the explosive force of which we have spoken, the mountain being gradually built up by the lava and other materials which it has flung up from below. ”

“ The prolonged ascent of hot vapours, stones, dust and lava, in the funnel of a volcano must necessarily affect the rocks through which the funnel has been driven. We may therefore expect some signs of alteration in the material forming the walls of a volcanic neck. The nature of the metamorphism will no doubt depend, in the first place, on the character and duration of the agents producing it, and in the second, on the susceptibility of the rocks to undergo change. Mere heat will indurate rocks, baking sandstone, for instance, into quartzite, and shales into porcellanite. ”