Volcanic region

“ On the floor of the plain itself the effects of volcanic action are visible in the forms of hummock and rounded hill. Volcanic emissions have produced the lap-like enclosures which are the reservoirs of the lonely lakes. Their waters are fed by springs from beneath the surface, and by copious rains from the clouds of the Pontic region, which fly the topmost bulwarks of the tableland and distil on the western slopes of the meridional volcanic barrier, the limit on the east of the even ground. ”

“ In the case of the Roman volcanic district, a bay seems to have been formed about the close of the Miocene period, bounded on all sides but the west by hills of limestone, over whose bed strata of marl, sandstone, and conglomerate were deposited. This tract was converted by subsequent movements into a fresh-water lake, and contemporaneously volcanic operations commenced over the whole region, and beds of tuff, often containing blocks of rock ejected from neighbouring craters, were deposited over those of marine origin. Meanwhile numerous crater-cones were thrown up ”

“ The great group of islands lying to the south-east of the Asiatic continent is at the present time the grandest focus of volcanic activity upon the globe. No less than fifty active volcanoes occur here.
Farther south, the same chain is probably continued by the four active volcanoes of New Guinea, one or more submarine volcanoes, and several vents in New Britain, the Solomon Isles, and the New Hebrides, the three active volcanoes of New Zealand, and possibly by Mount Erebus and Mount Terror in the Antarctic region. ”

“ What geological forces created these striking and peculiar pinnacles and balanced rocks? Geologists explain that millions of years ago volcanic activity was extensive throughout this region. A series of explosive eruptions covered the level area with layers of volcanic rock fragments. Many years elapsed between such periods of activity, resulting in a series of blankets, layer upon layer. Since the eruptions varied in magnitude, the resulting deposits were of different thickness. ”

“ Even when every other trace of the volcano has been swept away, erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark. In volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills. ”

“ Again, the extinct volcanoes of Central France, which appear to form an isolated group, indicate, when viewed in detail, a linear arrangement ranging from north to south. [13] Another region over which extinct craters are distributed lies along the banks of the Rhine, above Bonn and the Moselle; a fourth in Hungary; a fifth in Asia Minor and Northern Palestine; and a sixth in Central Asia around Lake Balkash. These are all continental, and the linear distribution is not apparent.
[1] For an interesting account of this range of volcanic islands see Kingsley's At Last. ”

“ In many cases volcanoes themselves are left, though they may have been for ages extinct. In some volcanic regions where no great central cones have existed the vast floods of lava that were poured forth extend to-day as vast black plains of naked rock, mottled with shifting sand-hills, or as undulating tablelands carved by running water into valleys and ravines, between which the successive sheets of lava are exposed in terraced hills. ”

“ The heated, quaking soil was covered with thick deposits of silica, sulphur, oxide of iron, pumice, obsidian, scoria, and other volcanic products, and, with its sulphurous atmosphere, fierce heat, and shrieking sounds, it appeared as we entered it like a short cut to Pandemonium. The high hills on each side of the gorge rose up in quaint, fantastic shape, and their rugged sides, composed of shattered volcanic rock, sent forth water and jets of steam from a thousand fissures. ”

“ A thick ash deposit sandwiched between layers of sandstone in Nebraska, the massive granite peaks of the Sierra Nevada mountain range, and a variety of volcanic layers found in eastern Maine are but a few of the striking clues of past volcanism. With this perspective, an erupting volcano is not only an exciting and awesome spectacle in its own right but a window into a natural process that has happened over and over again throughout Earth’s history. ”

“ There are two distinct types of vulcanism, the one manifested by an active volcano where clouds of smoke and steam are forced from a volcanic vent, with showers of ashes, cinders, bombs, electrical displays and great streams of molten lava pour out of the crater down the sides of the volcano. ”

“ It has frequently been asserted that in the great districts covered by basaltic lavas which we find in the - 270 - Rocky Mountains of North America, in the Deccan of India, in Abyssinia, and even in the Western Isles of Scotland, we have proofs of the occurrence, during earlier geological periods, of volcanic action very different in character from that which at present takes place on our globe. ”

“ As the strata dip from west to east across the course of the river, it follows that those on the western banks are below those on the opposite side; and thus the marine sands and marls which underlie the volcanic tuff, and are concealed by it along the eastern side of the valley, emerge on the west, and form the range of hills on that side. Such being the structure of the formations under Rome, it is evident that it is not "built on a volcano." ”

“ No tract of equal size yet known on the face of the globe furnishes so ample a record of volcanic activity from the earliest geological periods down into Tertiary time. Every degree of energy may be signalized in that record, from colossal eruptions which piled up thousands of feet of rock down to the feeblest discharge of dust and stones. Every known type of volcano is represented—great central cones like Etna or Vesuvius, scattered groups of small cones like the puys of France, and fissure- or dyke-eruptions like those of recent times in Iceland. ”

“ When now we inquire into the origin and history of the topography which has so influenced the population around it, we find that its prominences are relics of ancient volcanoes. The feudal towers are based on sills and dykes and necks. The fields and gardens, monuments and roadways, overlie sheets of lava or beds of volcanic ashes. Not only is every conspicuous eminence immediately around of volcanic origin, but even the ranges of blue hills that close in the distant view to south and north and east and west are mainly built up of lavas and tuffs. ”

“ In the basal wrecks of volcanoes, of which we have spoken, we usually find only small and fragmentary remains of the great accumulations of loose and scoriaceous materials which originally constituted the bulk of the mountain mass. In the centre of the ground-plan - 142 - of such a denuded volcano we find great masses of highly crystalline or granitic rock, which evidently occupy vast fissures broken through the sedimentary or other rocks upon which the volcanic pile has been reared. ”

“ These occur in the form of extinct craters, sometimes filled with water, and thus converted into circular lakes; or of extensive sheets and conical hills of tuff; or, finally, of old necks and masses of trachyte and basalt, sometimes exhibiting the columnar structure. The Eternal City itself is built on hills of volcanic material which some observers have supposed to be the crater of a great volcano ”

“ 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. ”

“ Indeed, on both shores of the Red Sea there are a few volcanoes that are still active, while in Sinai, and in the districts of the south, there are several extinct craters.
But it is in the New World, especially on the Pacific coast of North America, that volcanic activity was especially great during the geological past. There is a district containing volcanic rocks that extends through various parts of western North America, from New Mexico and North California, to Oregon and British Columbia. This district has a width of from eighty to 200 miles, and a length of not quite 800 miles. ”

“ 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. ”

“ In others, the work of destruction has proceeded still further, and slowly wearing rivers or the waves of the sea may have cut perfect, vertical sections of the mountain-mass. Sometimes the removal of the materials of the volcanic mountain has gone on to such an extent that its base and ground-plan are fully exposed. ”

“ At the period when the most violent mountain-forming movements occurred, these fissures were in their most active condition, and at this time two great volcanic belts stretched east and west, on either side of, and parallel to, the great Alpine chain. The Northern volcanic band was formed by the numerous vents, now all extinct, in Auvergne, Central Germany, Bohemia, and Hungary, and was probably continued in the volcanoes of the Thian Shan and Mantchouria. ”

“ It is by the constant accumulation of these ejected materials that such great mountain masses as Etna, Teneriffe, Fusiyama, and Chimborazo have been gradually built up around centres of volcanic action.
There are cases in which the formation of volcanic mountains on a small scale has actually been observed by trustworthy witnesses. There are other cases in which volcanic mountains of larger size can be shown to have increased in height and bulk by the fall upon their sides and summits of fragmentary materials ejected from the volcanic vent. ”

“ Where the growth of a volcanic mound takes place from within, as in certain steep-sided trachytic cones, there may be no perceptible crater or external outlet. Again, there are many volcanoes which have no crater at the summit, because the eruptions always take place from lateral outlets. Even when a terminal pit is present, the lava may issue from the body of the mountain, and in some cases it exudes from so many vents or cracks that the volcano has been described as “sweating fire.” ”

“ The latter is the great centre of volcanic action, and has a large crater on its summit, sending forth smoke and steam, and from which eruptions of lava not unfrequently take place. Pumice, obsidian, and porphyry are abundantly distributed about this district. Hot springs (pui) are numerous; some have an argillaceous, others a sulphureous taste, and often a boiling temperature, accompanied by continuous subterranean sounds. Cold saline springs occur near the hot ones. Shocks of earthquakes, termed by the natives "Wiringa O te Wenua," or, trembling of the land, are occasionally felt. ”

“ 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. ”

“ One step further back in time from today’s active volcanoes are people who picnic in the White Mountains of New Hampshire, enjoy the autumn colors in the Blue Ridge of Shenandoah National Park, and hike in the rugged Big Bend National Park of Texas. Many of the rocks in these areas were formed by eruptions or by intrusion of magma into the Earth’s crust many millions of years ago. Because volcanic activity has been so important in shaping the Earth, watching active volcanoes today provides a window through which we can glimpse and reconstruct the early volcanic history of our planet. ”

“ 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. ”

“ Again, if we turn to other districts we have been considering, we find that in the Indian Peninsula, in Arabia, in Syria and the Holy Land, in Persia, in Abyssinia and Asia Minor—regions where volcanic operations were exhibited on a grand scale throughout the Tertiary period, and in some cases almost down into recent times—we are met by similar evidences either of decaying volcanic energy, or of an energy which, as far as surface phenomena are concerned, is a thing of the past. ”

“ We have no evidence that any volcanic eruption has drawn its material from zones or magmas especially rich in metals or their ores, and on the contrary, volcanic districts, like those mentioned, and regions, such as the Sandwich Islands, where the greatest, eruptions have taken place, are poorest in metalliferous deposits. ”