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. Thus for instance, we have seen that a portion of the Italian coast was elevated when Monte Nuovo was thrown up, that the town of Tomboro was submerged on the occasion of the eruption of Sumbawa, and that the bottom of the sea was notably upheaved by the last outbreak of the volcanic fires of Santorini. Nevertheless it appears to be generally the case that when the Crust of the Earth is once burst open, and a means of escape thus afforded to the fiery agent below,—in other words, when the active volcano is established,—the process of upheaval gives place to that of eruption. But when, as is often the case, no such safety-valve is offered to the surplus energies of the subterranean fires, then the giant power of heat, in its struggle to escape, shakes the foundation of the hills, and uplifts the superincumbent mass of solid rocks.
This theory which ascribes the phenomena of Earthquakes and Volcanos to the same common cause, acting under different circumstances, is now almost universally adopted by Geologists; and it may be briefly enforced by the following considerations. First, though Earthquakes have sometimes occurred far away from any known volcanic region, yet they are more frequent in the neighborhood of active or extinct Volcanos. Secondly, almost all volcanic eruptions are preceded by Earthquakes; and the Earthquakes generally cease, or, at least become less violent, when the subterranean fire breaks out in the form of a Volcano. And, Thirdly, it is plain that the condensed steam which is generated by internal heat, and the expansive power of the heat itself, must, of necessity, when pent up in the caverns of the Earth, tend to produce those very phenomena by which Earthquakes are distinguished.
Let it be observed, however, that while we explain the phenomena in question by the agency of subterranean heat, this doctrine is by no means necessary for the main purpose of our present argument. Whatever may be the cause from which the Earthquake shock proceeds, it is enough for us to show that the Crust of the Earth has been from time to time upraised, and dislocated, and rent asunder in modern times, just as it is supposed in Geological theory, to have been upraised, and dislocated, and rent asunder from time to time in by-gone ages. We will set down a few out of the many examples observed and recorded during the last hundred and twenty years.
When the English colonists settled in New Zealand, about fifty years ago, they were told by the natives that they might expect a great Earthquake every seven years. This alarming prediction has not been literally fulfilled; but it is fully admitted that the total number of such disturbances within the last half century has not fallen short of what it should have been according to the above estimate. During the years 1826 and 1827 several shocks were felt in the neighborhood of Cook Strait, after which it was observed that the sea-shore had been uplifted on the north side of Dusky Bay. So transformed was the outline of the coast that its former features could no longer be recognized; and a small cove called the Jail, which had previously afforded a commodious harbor to vessels, engaged in seal fishing, was completely dried up.
But the most memorable convulsion took place on the night of January the twenty-third, 1855. A tract of land, about as large as Yorkshire, on the southwest coast of the North Island, was permanently upraised from one to nine feet. The harbor of Port Nicholson, together with the valley of the Hutt, was elevated four to five feet; and a sunken rock, regarded before as dangerous to navigators, has remained since the Earthquake three feet above the level of the water. The shock was felt by ships at sea a hundred and fifty miles from the coast; and it is estimated that the whole area affected was not less than three times the extent of the British Islands.
The whole coast of Chili has been subject to great disturbances and changes of level during the present century. In November, 1837, the town of Valdivia was destroyed by an Earthquake, and at the same moment, a whaling vessel, a short distance out at sea, was violently shaken, and lost her masts. The bottom of the sea was afterward found to have been raised in some places more than eight feet; and several rocks appeared high above the water which had previously been covered at all times by the sea. Two years before, in 1835, the town of Conception and several others were reduced to ruins by a like visitation. After the first great convulsion the Earth remained for many days in a state of commotion. More than three hundred lesser shocks were counted from the twentieth of February to the fourth of March. On this occasion, too, the bed of the sea was upheaved; and the whole island of Santa Maria, seven miles in length, was lifted up from eight to ten feet above its former level.
The Earthquake of 1822 was more violent, perhaps, and more striking in its effects, than either of those just mentioned. On the nineteenth of November in that year a sudden convulsive shock was simultaneously felt over a space 1200 miles in length. At Valparaiso, and on either side for a considerable distance, the coast was permanently upheaved. When Mrs. Graham, who was then living on the spot, and who has left us an account of the Earthquake, went down to the shore on the following day, she “found the ancient bed of the sea laid bare and dry, with beds of oysters, mussels, and other shells adhering to the rocks on which they grew, the fish being all dead, and exhaling most offensive effluvia.” Some idea may be formed of the gigantic power here in operation, when it is remembered that to uplift the coast of Chili, it was necessary to move the mighty chain of the Andes, and, amongst the rest, the colossal mass of Aconcagua, 24,000 feet in height. How far this process of upheaval extended out to sea, beneath the bed of the ocean, has not been accurately ascertained: but certain it is that, for a considerable distance, the soundings were found to be shallower than before the Earthquake. It is roughly estimated that the Crust of the Earth was elevated over an extent of 100,000 square miles, or about half the area of France.
On the western coast of India, near the mouth of the river Indus, is the well-known district of Cutch. In the month of June, 1819, this extensive territory, not less than half the size of Ireland, was violently shaken by an Earthquake, several hundred people were killed, and many towns and villages were laid in ruins. The shocks continued for some days, and ceased only when the outburst of a Volcano seemed to open a vent for the troubled spirit within. But what is particularly worthy of note is that when the Earthquake had passed away, a permanent change was found to have been effected in the level of the surrounding country. The town and fort of Sindree, situated on the eastern arm of the Indus, together with a tract of land 2,000 square miles in extent, were submerged beneath the waters. The principal buildings, however, still remained standing, with their upper parts above the surface; and many of the inhabitants, who had taken refuge in one of the towers attached to the fort, were saved in boats when the Earthquake had ceased. On the other hand, within five miles and a half of this very spot, the level surface of the Earth was upheaved, so as to form a long elevated bank, fifty miles in length and sixteen in breadth, which has been called the Ullah Bund, or the Mound of God. Nine years after this event, Sir Alexander Burnes went out in a boat to the ruins of Sindree, and standing on the summit of the tower, which still rose two or three feet above the surface of the water, he could see nothing around him but a wide expanse of sea, save where a blue streak of land on the edge of the horizon marked the outline of the Ullah Bund. Here was a striking illustration, on a small scale, of those changes which Geologists suppose to have been going on since the world first began; the dry land had been converted into the bed of the sea, and the level plain had been elevated into a mountain ridge.
Toward the close of the last century the province of Calabria, in Southern Italy, was the scene of an Earthquake which offers a very apposite illustration of our present argument. This celebrated convulsion is not, however, chiefly remarkable for its violence, or for its duration, or for the extent of the territory moved. In all these respects it has been surpassed by many Earthquakes, experienced in other countries, within the last hundred and fifty years. But the Calabrian Earthquake has an especial claim on our attention, mainly from this unusual circumstance, that the region of disturbance was visited, as Sir Charles Lyell tells us, “both during and after the convulsions, by men possessing sufficient leisure, zeal, and scientific information, to enable them to collect and describe with accuracy such physical facts as throw light on geological questions.”