The life history of a volcano: a lecture from 1891 22 February 2008Posted by admin in history of volcanology, volcanological works.
LIFE HISTORY OF A VOLCANO.
A Geological Study.
[From The Leeds Mercury, 30 May 1891, p. 5]
At the meeting of the Leeds Geological Association on April 21st, in the absence of Mr. S. Chadwick, F.G.S., who was announced to read a paper on “Flint from the Yorkshire Chalk,” a lecture on “Volcanoes” was given by the Hon. Secretary (Mr. W. L. Carter, M.A., F.G.S.) illustrated by a number of beautiful lantern slides.
What is a volcano? The old answer was, “A burning mountain, from the summit of which issue flames and smoke.” This, the lecturer said, was a misconception from beginning to end. A volcano is essentially an opening in the earth’s crust communicating with the heated interior. The volcanic mountain is the mass of materials which has been ejected from this aperture. A volcano does not burn, “i.e.,” there is no combustion at the summit, but it emits large quantities of steam and fine ash, which together form the so-called smoke, and streams of white hot lava, the flashing light from which reflected by the overhanging clouds looks like flame. The name “volcano” comes from the Island of Vulcano, near Sicily, which was styled by the ancients “The Forge of Vulcan.” This volcano is now fairly quiescent, and a chemical works was established in the crater some years ago, by an enterprising Scotch firm, for the condensation of the sulphurous and other chemical gases which it gave off. One night, however, an eruption took place, blowing the manufactory into the air, and effectually stopping that branch of industry. One naturally turns to Italy for examples of volcanoes, because it is easily accessible, and illustrations of all stages of volcanic action are found there. There are three Italian volcano districts — (1) the Roman, in which there is no active vent, but perfect craters and great crater lakes; (2) the Sicilian; and (3) the Neapolitan, both of which are partly terrestrial and partly marine. With regard to the supposed connection of the sea with volcanic action, as a source of water-supply, it is interesting to note that the two regions which still touch the sea are active, whilst the Roman region, which has been separated from it by the advance of the coast-line, is extinct.
Formation of the volcano.
One especially good instance of the birth of a volcano has been recorded by four eye-witnesses. In September, 1858 [sic: clearly an error, the correct date is 1538], Monte Nuovo, a hill 430 feet high, and 8,000 feet in circumference, was raised in three days on the shore of the Bay of Naples. There had been earthquakes for two years, which increased in frequency, when a fissure was formed which emitted springs, first of cold and then of boiling water. On Sunday night, September 29th, the earth suddenly burst open, and great quantities of ashes mixed with water were ejected, covering the country for miles round, and the sea retreated for some distance. No lava issued from the cone, which was entirely composed of ashes; and there has been no eruption on the same spot since. Thus is formed the simple volcanic cone. Passing on to more mature stages of volcanic existence, we will take Vesuvius as an example. In prehistoric times, this mountain would have formed a perfect cone, the top of which, being blown away, produced the wide crater-plain which existed up to A.D. 79. Previously there is a no record of volcanic activity, but in that year the western side of the old volcano was blown away, and a new cone formed. By this eruption Pompeii and several other towns in the district were destroyed. Since then the volcano has been in constant activity. Vesuvius is divided into several zones. The lower cultivated slope, merging into the plain, is composed of disintegrated volcanic rock, and is very fertile. This extends to 1,500ft. above the sea-level. The Desert Platform, a barren and desolate waste continually invaded by fresh lava streams, reaches from 1,500ft. to 2,500ft. On the eastern side is the ridge of Monte Somma, which half encircles the active vent, and is part of the original cone destroyed by the eruption of A.D. 79. The present cone is three miles in circumference, and 1,500ft. in height above the Desert Platform. The summit alters with every eruption, and smaller secondary cones are formed inside the chief crater. Etna is a more complex example of the colossal volcano. It is 10,840ft. above sea-level, and there are several zones of vegetation between the base and the desert region. There you see nothing but scoriae, lava, and snow. The great height of this cone causes the hydrostatic pressure in the central pipe to be too great to allow of the lava now reaching the central crater; and by the yielding of the sides, parasitic or daughter cones are formed. At subsequent stages in the existence of a volcanic cone the ground underneath becomes fissured, and an easier vent for the volcanic forces being thus provided, small cones or puys are formed in the plain around the base of the volcano.
As an example of a great eruption, we may take that of Vesuvius in 1872, in which several adventurous tourists lost their lives. The flow of lava was so profuse that Professor Palmieri wrote — “The cone seemed completely perforated, and lava oozed, as it were, through its whole surface. I cannot better express it than by saying — Vesuvius sweated fire.” Terrific discharges took place from the summit, ejecting enormous quantities of steam and ashes, reaching 5,000ft. above the top of the mountain, which were carried slowly along by upper air currents. Darkness was produced over a considerable area, and the rain of ashes devastated the crops and caused great alarm. One lava stream on the western side carried away large portions of two villages and so rapid was its flow that the villagers were barely able to save their portable possessions, and some lost everything. The decadence of the eruption was accompanies by a storm of thunder and rain, which brought down the hurtful gases and salts from the great cloud, shrivelling up grass, vines, and trees. Eruptions are not, however, always so violent as this, and in the case of Stromboli the sequence of events can be watched at close quarters without danger. Stromboli is a volcanic cone near Sicily, which rises out of the Mediterranean to a height of 3,090ft. and has been in constant activity for at least 2,000 years. The crater is in the side of the mountain, and from it clouds of vapour issue continually. The outbursts occur at intervals of from one to twenty minutes, and are unequal in intensity. By care an observer can climb to a point above the crater and watch the eruption. The black slaggy bottom of the crater is traversed by fissures, from which many jets of vapour curl quietly up. From larger apertures bursts of steam take place at intervals and molten rock wells out. In other openings a viscid substance is seen slowly heaving up and down, until a gigantic bubble is formed, which, bursting violently, sets free a great mass of steam, which carries great fragments of the molten rock high into the atmosphere. Thus in this working model, as it were, of a volcano, we see the essentials of all volcanic eruptions: — 1. The existence of apertures communicating with the interior of the globe. 2. The presence of lightly heated matter beneath the surface. 3. Great quantities of subterranean water, which by contact with this molten rock becomes suddenly converted into steam. These three conditions explain eruptions alike on the smallest and the largest scale.
Products of Eruption.
Many vapours are given off by a volcano, including acid gases, such as chlorine, hydrochloric acid, and sulphurated hydrogen. In less active stages carbonic acid is emitted in large quantities. Steam, however, is the most important vaporous product of volcanic action, forming 990-1000ths of the whole cloud. It has been calculated that during an eruption at Etna, which lasted one hundred days, that the steam emitted each day would, if condensed, have produced four and a half million gallons of water. Thus we cannot be surprised that floods of water play a considerable part in the devastation which accompanies a volcanic eruption. These water floods are produced either by the condensation of steam, or by the melting of masses of snow, owing to the rapid rise in temperature, or by the disruption of subterranean reservoirs. An instance of the last was seen in Java in 1817, when a lake of hot, acid water, filling a large crater, was suddenly discharged, with frightful destruction. Water rushing down the cone collects the volcanic dust, and forms a mud lava, which afterwards consolidates into tuff. It was by such a mud lava that Herculaneum was engulfed in A.D. 79. Various fragmentary materials are ejected by the force of an eruption. The finest of these form a light grey powder called ashes, though they are not products of combustion. So fine is this ash that Mr. Whymper estimated that though two millions of tons were ejected in one eruption of Cotopaxi, there would be from 4,000 to 25,000 particles in each grain weight. During an eruption in Nicaragua in 1835, there was darkness over an area of thirty-five miles in radius; the ground twenty-four miles from the mountain was covered ten feet deep, and the ash fell 700 miles from the centre of eruption. Larger fragments, from the size of a pea to that of a walnut, and great blocks, are often ejected from volcanoes. In Java, in 1772, a vally nine miles long was filled with angular blocks to a depth of 50 feet. Volcanic bombs, which are lumps of lava, rounded by rapid rotation as they move through the air, are found in almost all sizes. Lava is molten rock resembling slag. It is white hot at its exit from the crater, but soon becomes a dull red. Lavas differ in liquidity according to the amount of the included steam, and also according to their chemical composition and temperature. Viscous lavas give off very little steam and take on ropy structure owing to the crinkling of the cooling surface by the continued movement of the lower layers. Liquid lavas give off much steam, are rapid in flow, and form a sharp, cindery surface. Trachytic (basic) lavas from thin, widely extended sheets. The enormous size of some lava streams may be estimated from the fact that the flow from one eruption in Iceland would form a mountain greater than Mont Blanc.
Having thus dealt with the mature and vigorous volcano, it remains to enumerate the characteristic of its declining strength. These consist largely of vaporous emanations. A notable instance is that of the Solfatara, a crater in the Phlegraean (Burning) Fields to the north of Naples. It evolves gases and sulphurous fumes, depositing pure sulphur. Its last eruption was in 1198. Geysers (gushers), which are eruptive fountains of water and steam, exist in areas of decaying volcanic activity. Sinter cones and terraces are also formed by the emission of hot springs holding silica in solution. At a further stage carbonic acid gas alone is given off, as in the case of the Dog’s Grotto near Naples. Extinct craters are found in many parts of the world, and there are many examples in the Phlegraean Fields. The largest is Astroni, which is one mile in diameter, having in the centre a boss of trachytic rock, which probably is the plug of the old vent. Now the crater is overgrown with oaks and undergrowth, and is used as a Royal preserve for hunting wild boar and other big game.
The action of atmospheric agencies on extinct volcanic cones gradually lays bare their innermost parts, and reveals many interesting details of structure. There have been in times past several volcanoes in Britain larger than Etna. The Mull volcano, which was once upwards of 12,000ft. in height, is now reduced to a group of hills about 3,000ft. high. In Skye there is the wreck of a similar colossal cone, and Ben Nevis is carved from the inner masses of another such mountain. Arthur’s Seat, Snowdon, and Cader Idris are also formed of volcanic rocks. That volcanic action is not yet entirely spent in the British area is evidenced by hot springs and earthquakes. The hot spring at Bath, for instance, pours forth daily 180,000 gallons of water at 120 degrees Fahrenheit, and it is said that the materials which it has brought to the surface in solution during the last 2,000 years would be sufficient to form a cone as large as Monte Nuovo. Earthquake shocks are also occasionally felt in our country, such as that which did so much damage in Essex and Suffolk a few years ago, showing the existence of pent-up forces beneath the surface. The successive stages in the existence of a volcano have thus been traced from its earliest commencement in a mere fissure, around which a simple cone was built up, to a colossal volcano such as Etna, with numerous daughter cones. From this period of full maturity we have followed the fiery giant into a state of decrepitude, when he is able to fume and nothing else. Then comes the period of extinction, and as time goes on the cone is broken down by atmospheric denudation until its hidden framework is brought to light, and entering nature’s dissecting-room we are able to study the anatomy of the volcano, and thus many of the mysteries of its life history are made clear to us.
An interesting discussion followed, in which Messrs. Thrippleton, Jefferson, Bedford, and the President took part, and the thanks of the meeting were unanimously given to the lecturer, and to Mr. Bedford for his management of the lantern. A hearty vote of thanks also was accorded to those who had lent lantern slides, and especially to Mr. Branson for a splended series of Italian views.