Volcanic cone

“ In some cases we may find that the sea has encroached on the base of a volcanic hill, till one half of it has been washed away, and the structure of the mass to its very centre is exposed to our view. Thus in fig. 6 (page 43) , it will be seen that there lies in front of Vulcano a peninsula called Vulcanello, consisting of three volcanic cones, united at their base, with the lava-streams which have flowed from them. One half of the cone on the left-hand side of the picture has been completely washed away by the sea, and a perfect section of the internal structure of the cone is exposed. ”

“ Fig. 16.—Effects of denudation on a Vesuvian cone.
Conceive now the effects of prolonged denudation upon such a pile of volcanic rocks. The cone will eventually be worn down, the crater will disappear, and the only relics of the eruptive orifice may be the top of the central lava-column and of any fragmental materials that solidified within the vent (Fig. 16) . The waste will, on the whole, be greater at the cone than on the more level areas beyond. It might, in course of time, reach the original surface of the ground on which the volcano built up its heap of ejected material. ”

“ Volcanic cones. The shape of a volcanic cone depends chiefly on the materials erupted. Cones made of fragments may have sides as steep as the angle of repose, which in the case of coarse [258] scoria is sometimes as high as thirty or forty degrees. About the base of the mountain the finer materials erupted are spread in more gentle slopes, and are also washed forward by rains and streams. The normal profile is thus a symmetric cone with a flaring base.
Fig. 230. Sarcoui, a Trachyte Dome, France
Cones built of lava vary in form according to the liquidity of the lava. ”

“ The materials that are thrown upwards into the air, falling again on the mountain, collect around the crater on all sides, thus giving it the characteristic cone-like shape of the volcanic mountain. Where nothing occurs to disturb the formation of the cone its height above the level of the sea will gradually increase. Very frequently, however, during explosive eruptions, a large part of this cone will be blown away by the force of the eruption only to be again built up during some later eruption. ”

“ It has been suggested that the frequent rending of volcanic cones during eruptions may be connected with the gradual and successive upheaval of the whole mass in such a manner as to increase the inclination of the beds composing the cone; and in accordance with the 380 hypothesis before proposed for the origin of Monte Nuovo, Von Buch supposes that the present cone of Vesuvius was formed in the year 79, not by eruption, but by upheaval. ”

“ Thus Lyell has shown, in the case of Etna, that during the earlier periods in the history of the mountain the piling up of materials went on around a centre which is now situated at a distance of nearly four miles from the present focus of eruptive activity. Some of our old British volcanoes, of which the denuded wrecks exist in the Western Isles of Scotland, show similar evidence of a shifting of the axis of eruption.
One of the most conspicuous features of a volcanic cone is the great depression or crater found at its summit. ”

“ When the height of the cone has grown to a certain extent, it becomes more easy for the volcanic energies below to rend the sides of the cone than to raise the molten materials to its summit. In this way lateral or parasitic cones are thrown up on the flanks of the volcanic mountain, the mass being alternately elevated and strengthened by the ejections from the summit and sides respectively. ”

“ It is evident that in all cases in which eruptions take place beneath the sea-level, and the loose materials are exposed during their accumulation to the beating of the sea-waves, the form of the volcanic cone so produced will be greatly modified by the interaction of the two sets of opposed causes, the eruptive forces from below and the distributive action of the sea-waves. ”

“ On such rents smaller or parasitic cones are often formed, which imitate the operations of the parent cone and, after repeated eruptions, may rise to hills hundreds of feet in height. In course of centuries the result of the constant outpouring of volcanic materials may be to build up a large mountain like Etna, which towers above the sea to a height of 10,840 feet, and has some 200 minor cones along its flanks.
But all volcanic eruptions do not proceed from central orifices. ”

“ Where the actual margin of a neck and its line of vertical junction with the rocks through which it has been drilled can be seen, there is no room for dispute as to the diameter of the original funnel, which must have been that of the actual neck. But in many cases it is impossible to observe the boundary; not merely because of superficial soil or drift, but occasionally because the volcanic detritus extends beyond the actual limits of the funnel. In such cases the necks have retained some portion of the original volcanic cone which accumulated on the surface around the eruptive vent. ”

“ The Volcanoes of the Cantal.—The original [Pg 99] crater-cones of this group have entirely disappeared throughout the long ages which have elapsed since the lava-streams issued forth from their internal reservoirs. The general figure of this group of volcanic mountains is that of a depressed cone, whose sides slope away in all directions from the central heights, which are deeply eroded by streams rising near the apex and flowing downwards in all directions towards the circumference of the mountain, where they enter the Lot, the Dordogne, and the Allier. ”

“ The building up of a volcano may be fairly illustrated by the little cone of sand formed in an hourglass as the sand-grains fall. These settle down at a certain slope, or angle, at which they can remain, instead of falling down to the bottom, as they do directly this slope is exceeded. Some volcanoes are built up almost entirely of volcanic ash and its embedded blocks. Vesuvius, Teneriffe, Jorullo, in Mexico, and Cotopaxi, in the Andes, are examples of steep volcanic cones built up in this way. Others, less steep and more irregular in shape, are chiefly formed of successive lava-flows. ”

“ A more beautifully symmetrical volcanic cone than that of Colima, as viewed from the north, can hardly be imagined; the only feature breaking the symmetry being a secondary cone which arises from the northeast slope. The beauty of the cone is enhanced by the great clouds of steam which. continually arising from the crater, either envelop the summit or, blown by the wind, stretch out into long white clouds.
Fig. 1. Secondary Cone and Lava Flow of 1860.
Fig 2. Volcano Colima with Secondary Cone. ”

“ Many hundreds of these cones are seen in central France, in the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well understood, the growth of many having been watched during volcanic eruptions. ”

“ For a long distance up the mountain its rugged sides glittered with icicles, which clustered about the enormous masses of trachytic lava which cropped up everywhere around, while the ground was covered in every direction with a thick coating of frost and frozen snow. At a height of 7000 feet the whole aspect of the cone had a very bare and desolate look, and, besides the enormous boulders we encountered, we passed over a steep slope covered with volcanic conglomerate, which was very treacherous and slippery with sheets of ice. ”

“ As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world. ”

“ In 245 the centre of this crater a very small cone ("minor cone") is frequently found; and it is interesting to find that many of the moon's volcanic craters possess these "minor cones." A number of cracks or fissures intersect the volcano. These frequently spread out from the centre of the mountain in all directions, like the spokes of a wheel. They generally get filled with lava that wells up from below, thus forming "dykes," which may be regarded as so many sheets of igneous rock, such as basalt, that have forced their way while still liquid in among the layers of lava and ashes. ”

“ The rock material ejected from submarine vents is probably less widely distributed than that from vents on land, and so far forth, the volcanic cones in the oceans are steeper than those on land. Where volcanic cones are built up near the surface of the sea, they often furnish a home for shallow-water life, such as polyps. Wherever built up so as to be within the reach of waves, gradational processes are stimulated.
The processes of vulcanism do not commonly influence coasts of continents directly, for few volcanoes lie immediately on coasts. ”

“ When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface—a telltale remnant of the vanished volcano.
Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.
An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. ”

“ The shape of the cone may undergo changes during an eruption, as in the accompanying outlines of Vesuvius (Fig. 9) . Most of the great volcanic mountains belong to the class of "composite cones," and are built up by alternate ejections of fluid lava and fragmentary materials. The slopes of their sides are subject to a wide range of variation, corresponding with the varying character and degree of liquidity of these materials. The sides of the cones are liable to be rent asunder and traversed with fissures, through which liquid lava forces its way and ”

“ It is often of large dimensions, and sometimes of vast depth. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on the top of a mountain, which in some instances towers to an enormous height. The part of the mountain which terminates in the principal crater is usually of a conical form—much like a glass-house chimney, and is therefore named the cone. It is generally composed of loose ashes and cinders, with here and there masses of stone, which have been tossed into the air by the volcanic forces. ”

“ On the whole, it may be said that the general tendency of recent soundings is to extend the area with depths greater than one thousand fathoms, and to show that numerous volcanic cones rise from the general level of the floor of the ocean basins up to various levels beneath the sea surface. Considerably more than half of the sea floor lies at a depth exceeding two thousand fathoms, or more than two geographical miles. On the Challenger charts all areas where the depth exceeds three thousand fathoms have been called "deeps," and distinctive names have been conferred upon them. ”

“ The lava-streams do not form a continuous sheet surrounding the cone—see diagram, where they are seen cut through in transverse section. When a mountain of some height has been formed, it then becomes liable to fracturing, and the formation of so-called volcanic dikes. Mr. Mallet, in a communication to the Geological Society, [1] thoroughly explained this condition of things. As we have seen, the cone may form an embankment around the column of lava occupying the chimney and crater, consequently there is an enormous pressure put upon the supporting wall of loose material. ”

“ The sides of such a cone may remain tolerably uniform slopes so long as they are always being renewed by deposits from fresh eruptions. But when the volcanic activity ceases, and the declivities undergo no such reparation, they are rapidly channelled by the descent of rain-water, until the furrows grow by degrees wide and deep ravines, with only narrow and continually-diminishing - 101 - crests between them. If unchecked by any fresh discharge of volcanic material, the degradation will at last have removed the whole cone. ”

“ Clarke tells us that millions of red-hot stones were propelled into the air to at least half the height of the cone itself; then turning, they fell all around in noble curves. They covered nearly half the cone of Vesuvius with fire. Huge masses of white smoke were vomited forth by the disturbed mountain, and formed themselves, at a height of many thousands of feet above the crater, into a huge, ever-moving canopy, through which, from time to time, were hurled pitch-black jets of volcanic dust, and dense vapours, mixed with cascades of red-hot rocks and scoriæ. ”

“ The volcanic forces are now laboring, therefore, to repair the breach which subsidence has caused on one side of the great cone; and unless their energy should decline, or a new sinking take place, they may in time efface this inequality. In that event, the restored portion will always be unconformable to the more ancient part, yet it will consist, like it, of alternating beds of lava, scoriæ;, and conglomerates, which, with all their irregularities, will have a general slope from the centre and summit of Etna towards the sea. ”