The Gilchrist Lectures.

Dr. Martin Duncan on Volcanoes and their Causes.

On Thursday evening, Dr, P. Martin Duncan delivered the last of the series of Lectures in Doncaster in connection with the Gilchrist Educational Trust, his subject being “Energies within the earth – Volcanoes and their causes.”
The Rev, A. H. Faber, vicar of Sprotbro’, presided. The audience can scarcely be said to be large, though nearly twice as many were present as was the case the previous week.

The Chairman, who was received with applause, remarked that he would not pretend to introduce Dr, Duncan – he introduced himself the previous week more powerfully than he could do. (Applause)

The Lecturer observed that at the last lecture he endeavoured to explain how mountains were formed as parts of continents by the energy of heat within which was in the earth on olden times. He would now direct their attention to some totally different mountains, which were also due to the same energy of heat, which was acting at the present time. They differed in shape, substance, position, and method of formation. These mountains were called eruptive, or volcanic mountains; sometimes they were called volcanoes. Volcanoes were known occasionally, and sometimes continually to be in eruption, that was to say, noise, vapour, and substance came from the upper part, and molten rock flowed over the flanks. They increased in dimension with age, and the more eruption the greater was the height and general size.

Their shape, always that of a cone, differed, because some might have a broad base and a gentle slope, while others had a narrow base and a great slope. The substance which formed them were not gravels, clays, limestones, those mineral substances which constituted the layers that formed ordinary mountains, but they were substances which had been brought up from unknown depths by high pressure steam – substances that had been dissolved out of deeply-seated rocks under the influence of heat and pressure of water. Those substances were more or less crystalline in their appearance, for to the naked eye, or under the influence of a lens, they showed pretty geometrical figures. Fortunately, there were two instances of volcanoes which had originated since men had written histories, so that the origin of these interesting hills could be studied. The first one he had to notice was figured on the diagrams before them.

It was called Monte Nuovo, and was situate near the Bay of Naples, its height being only 440 feet. That mountain was not in existence before the year 1538. In that year the inhabitants of the district were alarmed by hearing subterranean noises., with occasional slight shocks of earthquake. Mount Vesuvius was at no great distance, and the people knew that such underground noises in that region were always followed by an eruption. One morning they were greatly surprised to find that the sea shore was broader than it had been before. They collected a large quantity of fish and believed that the sea had retired so may feet. On enquiry a few miles distant, however, they found that the level of the sea was exactly the same, and they became more alarmed than ever.

It was then perfectly evident that the land had risen, some energy beneath having produced a force which was acting upwards. Before night set in there was a great explosion, and the inhabitants, who had flocked to the neighbouring hills, heard the roaring of steam, the splashing of water, and the falling of rocks. This went on throughout the night, and just before dawn it ceased. At break of day there lay before them a mountain, 440 feet high, and a mile and a half round, which had been erected in the night. The action, as he had said, ceased at daybreak, and the volcano had been quiescent ever since. When the hill was examined, it was found to be made up of a curious jumble of stones looking like powdered clinkers, and of mud. The top of the hill had a cavity, a funnel-shaped hole, which led nearly to the level of the sea, The substance forming this mountain had come up the hole, then gone out into the air, and fallen all round, and of course the force which had ejected them kept the space clear. The substances did not in any way resemble the rock which was near the surface in the district, and must therefore have come from a considerable depth below.

Two centuries after this a remarkable eruption took place in Mexico. About 1750 similar sounds to those previously noticed in Italy were heard in Mexico, and occasionally there was a greater amount of working of the ground. People began to leave the neighbourhood, but fortunately for science some few Europeans remained and witnessed a great explosion, which split up the earth, leaving a great crack for nearly a mile in length. From that crack came mud in the first instance, in great quantity, then hot water, and then an uprush of steam, evidently at extreme pressure, carrying with it rock, mud, and so forth. The Europeans retired from the district, and it was known by common observation and common rumour that these noises were continued month after month. Some nine months afterwards, however, they were tempted to return. They found things pretty quiet, but the aspect of the county was totally altered.

A mountain stood in the midst of their old country 1,650 ft. in height. It was one of several, and it had a conical shape. There was a space at the top leading down to an unknown depth, and the whole country which had been noted for the richness of its vegetation, was covered with a curious glassy, clinker-looking rock. That substance was called lava, constituents of which the lecturer described, and stated as a peculiarity of it, that whilst it soon cooled externally the internal mass remained liquid long after the portion exposed to the air became solidified. Like Monte Nuovo, the volcano, which was known as the volcano of Jorullo, had not since evinced any energy whatever, These, then, were two instances of volcanic mountains belonging to the peculiar kind which were said to be extinct.

The next kind of volcano to be considered was that which occasionally was in action, and was called intermittent. The most interesting was Mount Vesuvius, in the Bay of Naples. Seventy-two years before Christ the country around Naples and Vesuvius was the seat of great luxury, and several important towns were built there, including Herculaneum, Pompeii, and Stabiae. They were situated on the sides of a beautiful hill, which the inhabitants had always looked upon as an ordinary hill.

Thirty years before Christ, a Grecian geographer and mineralogist Strabo, visited the district, and on the top of this hill he noticed a basin-shaped cavity, which struck him as resembling that of active volcanoes which he had seen at Etna, in Scilly. In his book he described this mountain as a worn out volcano, the substances composing it resembling those of the volcanic mountains of his own land. About the year 63 after Christ, the towns situated on the flanks of the hill began to suffer from slight shocks of earthquake, one shock being so decided that it brought down a temple in Herculaneum. We knew this because many centuries afterwards an inscription was dug up giving the date of the earthquake, and stating that the municipality of the town had to rebuild the temple.

That shock was followed by underground noises, and the springs which came from the hills brought down water which was occasionally warm. Then suddenly that fearful eruption took place, which Bulwer, who obtained his account from Pliny, so accurately described in his novel, “The last days of Pompeii. The eruption was accompanied by a great noise and the destruction of 1,000 feet of the top of the mountain, The steam which blew it off carried it into the air, the rock being broken up into excessively minute particles, which gradually covered up the town of Pompeii. The town of Herculaneum was also destroyed, and the whole country around ruined.

For three days and three nights the terrific uproar continued, and then the mountain become quiet. Year after year nature gradually began to re-assume its beauty, vegetation came on the hill, and no eruptions were recorded for several years. From 63 or 69 to 1036 there were only five or six great eruptions, and these took place at a period when little notice was taken of such occurrences. The eruption of 1063 lasted for three years and was the first attended by an ejection of larva,

In 1631 there was another great eruption, which broke out at a different part of the hill, and again in 1822, when more than 600 feet of the summit fell in bodily, thus reducing the height of the mountain from 4,000 feet to 3,400 feet. Since then, the mountain had been more or less in eruption, the principal features being the pouring of larva down the hills, and the splitting of the mountain by the uprush of steam. It was this uprush of steam which struck everyone who had seen a volcanic eruption as a most extraordinary thing. It gave a notion of the tremendous power in action. The steam appeared to come from a natural pipe, which passed through the body of the mountain to unknown depths. Blocks of stone were sent up by it and fell on the flanks of the hill. Enormous quantities of volcanic ash were likewise cast up, covering the country for 20 or 30 miles round like so much snow, only it was dark in colour. Various gasses also came up. There was a hydrogen gas, which gave the luminous appearance presented by volcanic mountains when in eruption; carbonic acid gas, which generally came down when the eruption was approaching its end; hydrochloric acid, a remarkable gas; and sulphuretted hydrogen gas, which was exceedingly offensive and would also burn. All these substances came up with the larva from volcanoes, the solids falling around and building the cone, the steepness of the flanks depending upon the solidity of the materials ejected.

They must now consider how volcanoes were formed, and in doing so they must find out how the steam was formed, how it escaped from the tremendous pressure where the rock was turned into larva. There was no difficulty in accounting for the heat, with increasing depth into the earth there was increasing heat, and at a considerable distance below the soil there would be sufficient heat to melt any substance on the surface of the earth if there were no pressure. Pressure increased the fusing point of rocks. If that pressure could be removed fusion would take place. The next thing was the water – how did it get down there. They knew that water was found in all rocks, and that it was got from the lowest borings. They would say that heat would force up the water, but experiments had proved that water would get anywhere. The pressure of the rocks would prevent if flushing into stream, and keep it as hot water in a critical condition. Hot water, under pressure and in the face of great heat, would dissolve any crystalline rock; it would do so in the laboratory, and therefore they were justified in saying it would in the depths of the earth. How this took place below the volcano must be explained by the movement of the earth.

All active volcanoes were found in districts where the earth had risen up within a short period before the first eruption. The land had bodily risen up a certain amount above sea level, and it was that rise, which had taken off a certain amount of pressure, that allowed the heat and water to erode the rock at a particular depth, and into that weak space went steam. Then it took up larva, which cooled as it went up, forming a peculiar substance, and it was usually some years before the force was developed enough to send up steam carrying the molten rock. As the mountain rose in some instances 20.000feet, they might imagine the tremendous amount of energy required to force it up. How could the peculiar nature of the eruption be accounted for? Sometimes the mountain was quiet for many years, and then suddenly sent forth steam perhaps every minute. It was because a great deal of the eruption took place in the “pipe” of the volcano, as had been demonstrated by experiments at the hot water springs in Iceland.

The lecturer went on to say he had told them that one theory of the causes of volcanoes was that they were the result of the former great heat of the earth, a relic of that heat which enabled the earth to grow like the sun, then to become fluid, and then solid. But there was another remarkable theory which was influencing men’s minds at the present time, and was probably correct. They would have grasped at the last lecture that if the layers forming ordinary mountains had been crushed up from side to side, at great depths as well as near the surface, an enormous amount of friction must have occurred. They knew that near the surface rocks had been altered by a slight amount of heat, the result of friction; deeper down they were bound to believe that the friction would be greater. It was quite possible that heat would develop during mountain formation, and that supplied what was requisite to act with the water in dissolving rock.

Therefore it was true, the common mountain ranges produced the volcanoes, and not, as people generally believed, that the volcanoes produced the mountain. One must look at the position of active volcanoes. They were invariably found on the flanks of great mountains. They were never in the centre of continents, bur were always tolerably near the sea, 200 0r 300 miles being the greatest limit. The great crush which produced the mountains out of the continent would take place at the edge of it where the sea was, and it was just that place down which more water would get than anywhere else. He had mentioned hydrochloric acid gas. That could be got out of sea water. It was probably the inrush or getting in of sea water which produced this gas which was invariably found in volcanic eruption. This second theory, that during the crushing and formation of mountains heat was developed sufficient to fuse rocks and produce volcanic action, was gaining ground every day. He trusted that he had given evidence to show them that the earth was at work, and that there was movement of material, and therefore they were ready to say that there was still energy within the globe. (Loud Applause).

The chairman said before they separated, they had a debt to pay to two or three people. First of all, they must express their feelings of gratitude to Dr. Duncan for the double trouble he had been at on their behalf. Dr. Duncan was there the previous week as well as that night. That was one dept they owed. They ought also to feel grateful to the Gilchrist Educational Trust. It was a great trust and found the funds which enabled them to send down to towns like Doncaster representatives such as Dr. Duncan, to spread valuable information in towns when there was an expression of need for it. Doncaster said they wanted the Gilchrist lecture and down they came, and he said in the gratitude they paid to Dr. Duncan they ought also to be very grateful to the trust. (Applause).

And thirdly there was a small society – that was small in point of number of members, but still not insignificant – which was called the Doncaster Microscopical and Literary Society, and it was owing to that society that they had got down those lectures at all. And while he was saying that, he would go a little further and say that if the Corporation, who had given a hundred proofs of its regard for the education of the people of the town, had only known about the Gilchrist Trust, and its great value they would never have left it to a little body like the microscopical Society to undertake that venture, but they would have taken it upon their own shoulders, or at all events they would have let them have the pleasure of sitting in that hall without the melancholy reflection of having to pay for it. While he was on his feet, perhaps they would allow him to offer to Dr, Duncan, a very grateful vote of thanks in their name, not only to himself but to the Gilchrist Trust; of whom he was a representative (Applause).

Dr. Duncan in reply, said he would be very glad to tell Dr. Carpenter, who was secretary of the trust, of their kind reception of the proposal that evening. He was sure the trustees would be gratified if the people were thankful. He sincerely sympathised with the Microscopical Society. He thought they had done a great thing to bring so many working men together, as he saw present that night, He was the father and president of the National Microscopical Society which took all other Microscopical Societies under its care. At present the Doncaster Society was independent, but before long he hoped they would become affiliated with them, and continue in their good work. (Applause).

The audience then dispersed.

From The Doncaster Reporter, Wednesday, March 21, 1883.