The Romance of Modern Geology / Describing in simple but exact language the making of the earth with some account of prehistoric animal life

by Edwin Sharpe Grew

People living on the earth will only see the results of the earthquake on the ground just immediately below their feet; and there these results are often very destructive to life and property; yet if they were all that happened, we should expect them to be covered up in time, and the "geological record" of an earthquake would not be a very important or even discernible thing a million years after it had happened. But are these things, which the eye of man can perceive, the only things that are happening during an earthquake? Is nothing happening underneath the earth which will leave its mark thousands of years after man has left the spot where the earthquake took place? May it not be that the earthquake is the outcome of some mighty force deep down in the earth; and may not this force cause both the earthquake and the geological "fault" which remains as the witness of its occurrence? If this be the case then the earthquake may be of enormous importance in geology.

We regard an earthquake, as we see it, as a destructive force. That is because it destroys the works of man. But earthquakes are doing constructive work as well; or, at any rate, they are usually present when constructive work is being done. Destructive forces, such as erosion, are wearing down the structure of the globe, while earthquakes are the only known forces that are building it up. It is true that when an earthquake occurs rocks often fall, loose sediment is shaken down, and other settlements occur, but the real constructive work consists in upheavals, little by little, as it may be, of beaches, islands, coasts, plateaux, and perhaps larger areas. These elevations are actually witnessed in certain earthquakes.

Many islands in the sea have been raised from time to time within even living memory.

The south-western part of the island of Crete has been elevated within the historical period.

The region about Pozzuoli and the Bay of Naples has suffered both elevation and depression. There is the famous instance cited by Sir Charles Lyell nearly eighty years ago of the Temple of Jupiter Serapis. This temple had many columns; and they are now situated on dry land. The pillars are forty-two feet in height, and for twelve feet upwards they are of smooth undisfigured marble. Then for another twelve feet they are pitted with the holes made by a marine shell-fish called Lithodomus, the stone-dweller. What are we to judge from this? The temple was first built on dry land. Then the land sank taking the temple with it, and the columns were submerged in sea sediment to a depth of some thirty feet above their pedestals. The lower portions of these pedestals were preserved intact, but the marine shell-fish found a home in the upper part of the marble columns, and pierced them with the channels and grooves. After this had gone on for a number of years the elevation of the land lifted the temple and its columns clean out of the sea again, and the marine shell-fish could no longer live in the columns. But the traces of their habitation remain.

The elevation of this coast was actually witnessed at the time of the eruption of Monte Nuovo in 1538. Moreover, the raising of the land was perceived on a larger scale round the whole of the Bay of Naples during the eruption of Vesuvius in April, 1706. Professor Lorenzo found the elevation of the land at Pozzuoli to be six inches, and at Portici one foot.[14] The foundations of both Etna and Vesuvius were ages ago laid in the sea.

[14] The coast about Pozzuoli is now sinking again.

In almost every part of the world there are raised beaches, such as we have already mentioned in the neighbourhood of Valparaiso, on the Chilian coast. The idea has been put forward by Dr. T. J. See that the same cause which produces earthquakes produces these elevations of the land and produces also volcanoes. There are many circumstances which favour this idea. Let us consider what is happening at the bed of the sea. Some years ago, when certain officers of the United States Navy were making ocean surveys, it was found that if hollow balls of thick glass were sunk to great depths in the ocean, they came up more and more completely filled with water in proportion as the depth increased, though no breakage or cracking of the glass had occurred, and no holes in it could be discovered even by the best microscopes. In other words, it became evident that the water had been slowly but bodily forced through the thick walls of the glass (under a pressure of less than 15,000 lb. to the square inch) in less than an hour's time. Evidently, then, even such a substance as glass will be penetrated by water if the pressure is great enough.

To make a practical application of these principles, what shall we now say with respect to the ocean bottoms? In deep places the pressure of the sea-water on them is very great, sufficient to force water through glass. Obviously most of these bottoms will leak, and leak at a rapid rate under the enormous pressure operating in the greatest depths of the sea. The bed of the ocean will not leak with equal rapidity in all places; but almost universal leakage will certainly develop, and the water will be driven back into the earth at various rates. Where the rock is volcanic and badly fractured, or sandy, the leakage will be most rapid; and where the bed is made of clay or unbroken granite the leakage will be much more gradual. It will also depend on the depth of the sea, and will be greatest where the ocean is deepest, and quite insignificant in shallow water. A rapid rate of leakage would mean that large quantities of water quickly come in contact with the heated rock, and develop correspondingly great steam pressure in the crust which underlies that part of the ocean. One case in which we may suppose a rapid leakage to be taking place is in the case of volcanoes near the sea. In the case of lava pouring from a volcano, it is observed that the molten rock emits vast quantities of vapour, of which, according to Sir Archibald Geikie, 999 parts in 1000 are steam. The enormous volume of these has been brought home to us in recent years by the behaviour of the volcano Mount Pelée, from which for several years after the great eruption which devastated Port au Prince the vapours rose in clouds that were to be measured in cubic miles. Similar observations about the quantities of vapour ejected by volcanoes have been made in Japan.

While speaking of Mount Pelée we may recall another phenomenon connected with it, which also appears to bear out the supposition that in the volcano's activity the action of steam takes a very large share. After its first outburst Mount Pelée continued to pour out lava and great quantities of vapour, as if like some gigantic cauldron it were being fed with fresh supplies of water; and there in the early March of the following year a most amazing thing took place, under the very eyes of a celebrated investigator of volcanoes, now dead, Professor Angelo Heilprin, who was remaining on the island. A great obelisk of andesite (a stone not unlike basalt) was forced up from the crater. It rose rapidly, as much as five feet a day; and it reached altogether a height of 840 feet above the crater's lip. It was calculated to be about 300 feet in diameter at its base. It continued to push itself up for some months, sometimes sinking a little, sometimes rising like a colossal piston above a steam boiler. Its greatest height was 1100 feet above the height of Mount Pelée, and therefore at a height of 5143 feet above the sea-level.