Popular Science Monthly/Volume 20/January 1882/Volcanic Products
THE most abundant of the substances ejected from volcanoes is steam, or the vapor of water, which issues in prodigious quantities during every eruption. With it frequently appear numerous other volatile matters—the acid gases, hydrochloric, sulphurous, carbonic, and boracic acids; sulphuretted hydrogen, hydrogen, nitrogen, ammonia; the volatile metals, arsenic, antimony; and some other substances not usually volatile, but which are nevertheless easily carried away in fine particles when a current of steam is passed over them. These various substances react upon each other, and give rise to the formation of many new compounds. Deposits of sulphur result from the action of sulphuretted hydrogen and sulphuric acid on each other; hydrochloric acid forms, with the iron in the rocks, the yellow ferric-chloride which coats the vents, and is often mistaken for sulphur. The iron, lime, and alkaline materials of the rocks are converted by the acids into soluble salts, which, being washed away by the rains, leave a white powdery deposit of silica that so much resembles chalk that travelers have been led to describe islands in which it appears very abundantly as being composed entirely of that substance. Some of the gases, as hydrogen and sulphuretted hydrogen, are inflammable, and may sometimes be seen at night playing over the fissures in faint, lambent flames, occasionally brightly colored with metallic salts. These transient flames are not, however, to be confounded with the red, glowing light which is commonly called "volcanic flames," but which is not really a flame, only the reflection in the vapors of the glowing mass in the crater beneath. Some of the volatile products of volcanoes are of economical value, like the sal-ammoniac, sulphur, and boracic acid of Vulcano, which have given rise to extensive chemical works.
Solid substances are ejected and accumulate around the orifices, where they frequently form large mountains. They are fragments of rock torn from the formations through which the eruptive stream passes—crystallized minerals, or matters which, derived from sources far below the earth's surface, issue in an incandescent or molten condition, and to which the name of lavas is properly applied. The lavas are composed of the silicates of aluminium, magnesium, calcium, iron, sodium, and potassium in different degrees of combination or mixture. Oxygen, in silicic acid or in a metallic oxide, makes up nearly half the weight of all of them, the metalloid silicon one fourth, and aluminium one tenth of the most of them. Silica or silicic acid—rock-crystal or flint—is present in proportions varying from one half to four fifths of the whole mass. Those lavas in which silica is present in larger proportion are called "acid lavas"; those in which the base is in greater proportion, "basic lavas." Between these are the "intermediate lavas," in which the proportion of silica is lower than in the acid lavas, and the proportion of bases is lower than in the basic lavas. Of the five great groups into which geologists have divided the lavas, the rhyolites are acid, the basalts are basic, and the trachytes, andesites, and phonolites are intermediate.
The structure of lavas can be more clearly ascertained by studying them in the condition of transparent or semi-transparent slices under the microscope. Most of them are made up of crystals of different minerals, varying in size from those which are hardly visible to the naked eye to those of an inch or more in length. Others appear glassy in structure. Under the microscope, they are shown to be made up of two kinds of materials, a base or ground-mass of a glassy character, and distinct crystals irregularly distributed through this glassy base, like the raisins in a cake. In some cases the vitreous part makes up the whole mass of the rock; in others smaller or larger numbers of crystals are seen to be scattered through a glassy base; while in others again the crystals are so numerous that the presence of an intervening vitreous ground-mass can be detected only by the aid of the microscope. When slices of the glassy materials are examined with high powers, cloudy patches are seen diffused through the substance, which under still higher powers resolve themselves into innumerable particles having very definite outlines, some transparent and some opaque. At the same time fresh cloudy patches are brought into view, requiring still higher powers for their resolution; and so the process may go on, as it does in examining the nebulæ with the telescope. The minute particles thus brought into view are called microliths or crystallites. Sometimes, instead of being indiscriminately diffused, they are collected in groups of very definite form, resembling the frost-work on window-panes. In other cases they unite in radial groups about certain centers, and thus build up globular masses to which the name of "spherulites" has been given.
In Figure 1 No. 1 represents a glass through which microliths or crystallites of different dimensions and character are diffused. In
Fig. 1.—Sections of Igneous Rocks, illustrating the Passage from the Glassy to the Crystalline Structure. 1. Vitreous rock. 2. Semi-vitreous rock. S. Vitreous rock with spherulites. 4. Rock with cryptocrystalline base. 5. Rock with microcrystalline base. 6. Rock of granitic structure built up entirely of crystals.
Nos. 2 and 3 the crystallites have united to form regular groups. No. 4 represents a glassy ground-mass containing microliths (a crypto-crystalline base), through which distinct crystals are distributed, and is typical of the features presented by most lavas. Nos. 5 and G illustrate the characters presented by lavas which have consolidated at considerable depths below the surface.
The microliths have been proved to be the minute elements from which common crystals are built up; and cases occur in which a group of them may be seen gradually assuming the outward form and internal structure of a crystal, and in other cases crystals may be found which are undergoing a disintegrating action, and are then seen to be made up of minute elements similar to the microliths.
The same materials which go to form a lava may assume a glassy condition, or that of a rock built up entirely of crystals. Geologists have given distinct names to the glassy and crystalline forms of lavas, which correspond with the five great classes into which lavas have been divided as follows:
The obsidians do not exhibit enough differences to demand a distinction.
When the large crystals imbedded in granitic rocks and in some lavas are examined with the microscope, they are often found to contain numerous minute cavities, each of which resembles a small spirit level, having a quantity of liquid and a bubble of gas within it.
In No. 1 of Figure 2 a group of such cavities is represented, one of which is full of liquid, while two others are quite empty; the remaining cavities all contain a liquid with a moving bubble of gas. In No. 2 two larger cavities are shown, containing a liquid and a bubble of gas. In Nos. 3, 4, and 5 the liquid in the cavities contains, besides the bubbles, several minute crystals; and in No. 6 we have a cavity containing two liquids and a bubble.
In the largest of such cavities the bubble may be observed to change its place when the position of the cavity is altered, so as always to lie at the upper side, just as in a spirit-level; while in the smallest cavities the bubbles appear to be endowed with a power of spontaneous movement, and are seen continually oscillating from side to side and from end to end of the hollow, as in Figure 3, where the dark line shows the path pursued by the bubble. These cavities are exceedingly minute, and so numerous that there must be millions of them in some crystals; indeed, in certain cases, as we increase the magnifying power of the microscope, new and smaller ones continually become visible; and it has been estimated that in some instances the number of them amounts to from one thousand million to ten thousand million in a cubic inch of space. In many cases the included liquid is water containing saline matters in solution, and often so saturated that free crystals of the salt appear floating in it; in other cases it is some hydrocarbon; and sometimes it is liquefied
Fig. 2.—Minute Cavities, containing Liquids in the Crystals of Rocks.
carbonic acid. The presence of these liquids under such circumstances shows that the crystals have been formed under an enormous pressure.
The surface of fluid and semi-fluid lavas is covered with vast quantities of froth or foam which has been generated by the action of the escaping steam. If the lava consists of a mass of crystals floating in a liquid magma, this froth cools into the rough, cindery-looking material which is called scoria. If the lava is glassy it becomes pumice, a mass of minute glass bubbles drawn out in one direction by the movement of the mass while it was still in a plastic state. Fragments of scoria and pumice are often thrown by a violent escape of steam to a height of hundreds or thousands of feet into the atmosphere. While going up and coming down, they encounter each other and wear each other away by their frequent rubbing, with a noise which is one of
Fig. 3.—Minute Liquid-Cavity in a Crystal, with a moving Bubble. (The path of the bubble is indicated by the dark line.)
the most noteworthy accompaniments of volcanic eruptions. Mr. Poulett Scrope, who watched the Vesuvian eruption of 1822 for nearly a month, remarks that at first fragments of enormous size were thrown out, but that they were gradually reduced by constant re-ejections, till at last only the most impalpable dust issued from the vent—a dust which was so excessively finely divided that it went everywhere, even into the most closely fastened boxes. Mr. Whymper estimates that no less than two million tons of dust must have been ejected during a single slight outburst of Cotopaxi which he witnessed; and Professor Bonney calculates from actual examination that it would take from four to twenty-five thousand particles of this same dust to make up a grain in weight!
The temperature and consistency of lava-streams vary greatly, and the variations give rise to differences in the appearance of the cooled mass. The surface of the stream cools rapidly in the air, so that it appears dull-red at night and black by day—like a great mass of rough cinders—while all is of a white heat beneath, and may be so seen at night shining through the rough, cindery masses. Some streams are very liquid, resembling rivers and filling every channel in their course; while others, cooler and stiffer, might be more fitly compared to glaciers, creeping along so slowly that the fact of their movement can be established only by the most careful observation. The stiff lavas leave a crust wrinkled and folded like coils of rope, and are then frequently called "ropy lavas." The very liquid, fast-flowing lavas leave a surface covered with rough, cindery masses presenting jagged projections. Admirable examples of the ropy lava are afforded by the Vesuvian lava-stream of 1868 (Fig. 4), and by the lava-cascade of the Island of Bourbon, where the stream, flowing over a precipice, has let t heavy pendent masses resembling the drippings of a guttering candle. Another illustration of the forms produced by very viscid lavas is afforded by the so-called "mamelons" of the island of Bourbon. The flow of rapid
Fig. 4.—Vesuvian Lava-Stream of 1858, exhibiting the Peculiar "Ropy" Surfaces of Slowly-moving Currents. (From a Photograph.)
currents is generally accompanied with the disengagement of vast quantities of steam, and this, doubtless, has much to do with the formation of the cindery projections which characterize the cooled surfaces of such streams. Little parasitic volcanoes are often formed on the top of the lava by the action of the escaping steam. Some such miniature volcanoes, formed on Vesuvius in 1872, were so small that they were carried away on boards to be employed as illustrations in the lecture-rooms
Fig. 5.—Concentric Folds on Mass of Cooled Lava.
of the University of Naples. When very viscous lavas are forced through fissures, they arrange themselves in concentric masses, like that represented in the illustration (Fig. 5), which is from New Zealand.
Fig. 6.—Mass of Cooled Lava formed over a Spiracle on the Slopes of Hawaii.
More liquid masses give rise to variously shaped structures, the bottle-shaped heaps of the "petrified fountains" (Fig. 6), or forms illustrated by the groups (Fig. 7) of small cones from Vesuvius. The contraction
Fig. 7.—Group of Small Cones thrown up on the Vesuvian Lava-Current of 1855.
of lava in cooling tends to produce fissures through the mass, breaking it up into prisms. Hence we have the columnar structure of basalt. A similar structure is exhibited in all kinds of lavas, and in other rock-masses which have been heated by contact with igneous rocks and gradually cooled. Columns of a dissimilar character arc produced by the unequal cooling of different parts of the stream, so that, if the stream be thick, the lower parts will form stout, vertical columns of great regularity; while the upper part, cooling less regularly, will produce smaller and less regular columns (Fig. 8). Fingal's
Fig. 8.—Section of a Lava-Stream exposed on the Side of the River Ardèche, in the Southwest of France.
Cave, in the Island of Staffa, has been formed in the midst of a lava-stream which has been cooled in this manner. The thick, vertical columns, which rise from beneath the level of the sea, are divided by joints and have been broken away by the action of the sea, and a great cavern has been produced, the sides of which are formed by vertical columns, while the roof is made up of smaller and interlacing ones; and the whole structure bears some resemblance to a Gothic cathedral. The columns formed in cooling vary in size from those of the Shiant Islands, near Skye, which are eight or ten feet in diameter and five hundred feet long, to the minute columns, an inch or two in length and hardly thicker than a needle, of the volcanic glasses. The larger columns are formed in slowly cooling masses. The quantity of matter that is ejected from volcanoes in the form of lava is truly enormous. Lava-streams have been described which have flowed for a distance of from fifty to a hundred miles from their source, and which have had a breadth varying from ten to twenty miles; some are five hundred feet thick, or even thicker. A mass estimated to be equal in bulk to Mont Blanc flowed out in a single eruption of Reykjanes in Iceland, in 1783. In many parts of the earth's surface, among which are tracts in our Rocky Mountain regions, successive lava-sheets have been piled upon one another to the height of several thousand feet, and cover areas of many hundred or even thousand square miles. The marks of the effects of the passage of the hot volcanic matters, with the powerful chemical agents with which they are charged, are left upon the adjoining rocks, where, for a considerable distance from the vent, limestones are converted into statuary marble, sandstones into quartzite, clays become hardened and lustrous, and coals assume a form like coke and graphite. Crystalline minerals and gems are formed, the rarest ones under a combination of powerful forces of heat and pressure which has been imitated by man only in the feeblest degree, most notably in the production of minute artificial diamonds by Mr. Hannay. The steam-jets which issue from volcanic fissures carry up fragments of rock torn from the sides of the vent, in the cavities of which beautiful crystallized products are often found. The various metallic minerals have nearly all been brought from away down in the earth's crust and deposited upon the sides of rock-fissures, the same volcanic forces opening the cracks through the solid rock, and then bringing up the metallic compounds and causing them to crystallize on the sides of the fissures. The cavities of the igneous rocks, when filled with water, constitute laboratories in which real chemical reactions take place—where the materials of the lava are gradually dissolved and recrystallized in new combinations, and the agates, the onyxes, the rock-crystals, the Iceland spars, and the class of zeolites have been formed. "No one can visit a large collection of crystalline minerals without being struck with the great number of beautiful substances which have thus been formed as secondary products from volcanic materials."
Hot springs, geysers, and carbonic-acid springs, which are also volcanic in their origin, afford another variety of curious products. Hot springs often contain in solution large quantities of silica, which has been taken up at the moment of its separation from the alkalies or alkaline earths with which it has been combined, and which is deposited when the water, having reached the surface, is relieved from pressure and cools down. Thus are formed the basins of the geysers of Iceland,
Fig. 9.—Sinter-Cones, surrounding the Orifices of Geysers. 1. Basin of the Great Geyser, Iceland. 2. Hot-Spring cone. 3. Old Faithful. 4. The Giant Geyser. 5. Liberty-Cap. (2, 8, 4, and 5 are in the Yellowstone Park district of the Rocky Mountains.
and the curious structures with fanciful names which distinguish the geysers of the Yellowstone. The outlines of some of these structures are given in the figure (Fig. 9). The deposited silica is known to geologists as "sinter." Carbonic-acid water, in a similar manner, losing its acid on exposure to the air, leaves a deposit known as "travertine," sometimes in great masses. In the Auvergne, the travertine exists in large rocks which take the form of natural aqueducts and bridges; in Carlsbad it has filled the whole bottom of the valley, and lies under the foundations of the town; and in Rome it has furnished the stone for St. Peter's and all the principal buildings. When springs charged with silica or carbonate of lime appear upon the slope of a hill composed of loose volcanic materials, they give rise to the remarkable structures known as sinter and travertine-terraces. The water flowing downward from the vent forms a hard deposit upon the lower slope of the hill, while the continual deposition of solid materials within the vent tends to choke it up. As a new vent can not be forced by the waters through the hard rock formed below, it is opened a little higher up (Fig. 10). Thus the site of the spring is gradually
Fig. 10.—Diagram illustrating the Mode of Formation of Travertine and Sinter-Terraces on the Sides of a Hill of Tuff.
shifted farther and farther back into the hills. As deposition takes place along the surfaces over which this water flows, terraces are built up inclosing basins. Of structures of this kind we have remarkable examples in the sinter-terraces of Rotomahana in New Zealand, and the travertine-terraces of Gardiner's River in the Yellowstone Park.
We sometimes find examples of volcanoes which by the action of denuding forces have had their very foundations exposed to view. Such examples occur in the Western Islands of Scotland, where we are able to trace the ground-plan of the volcanic pile, and study the materials which have consolidated deep beneath the surface in the very heart of the mountain. An admirable specimen of them is given by the "dissected volcano" forming the Island of Mull, which was probably originally nearly thirty miles in diameter at its base, and ten or twelve thousand feet high, but is now reduced to a group of hills few of which are more than three thousand feet high. Here, as shown in the figure representing the ground-plan (Figs. 11 and 12), great masses of granite, syenite, and diorite—the crystalline representatives of the first extruded lavas—are penetrated by intrusions of gabbro, the granitic form of the later ejected lavas. From these great intrusive masses of highly crystalline rocks proceed in every direction
Fig. 11.—Plan of the Dissected Volcano of Mull, in the Inner Hebrides.
spurs or dikes, which are evidently the radiating fissures formed during the outwelling of igneous materials from below, injected by fluid substances. Besides the vertical or oblique dikes we also find horizontal sheets, which, passing from the central masses, have penetrated between the surrounding strata, often to enormous distances. The hard crystalline materials and dikes remain now as mountains; the remnants of lava-streams stand as isolated plateaus; while the softer materials have been carried away. The matters ejected from volcanoes are often carried by winds or currents to very great distances. Pumice floats out upon the ocean, and has been found so thick near tin; Lipari Islands that a boat could hardly make progress through it, and so abundant near the Solomon Islands that it took ships three days to force their way through the floating masses. Volcanic dust has been blown all over the ocean and across it, and has been found by deep-sea soundings to cover the bottom of the deepest parts and those farthest from the land.
The results of volcanic action, whether viewed singly or collectively, appear immense, and seem to indicate that the earth is or has been the prey to tremendous and terrible forces. Yet the action passes, and probably always has passed, without inflicting any permanent disturbance upon the condition of the earth's surface over more than the most limited areas. Clear proofs exist that the volcanoes of the Hebrides, of the Auvergne, and of Hungary, were clothed in Miocene times with luxuriant forests. The Island of Java, near the heart of the present most active volcanic center on the globe, is at the same time one of the richest and most fertile spots in respect to vegetable and animal life. The slopes of Vesuvius afford the best field for vineyards, and are in constant demand for that purpose, notwithstanding the danger of spasmodic outbreaks, for the eruptions of the volcano are short and its periods of repose are long. Volcanic action is only one of the ordinary forces of nature, probably quite as beneficial as destructive in the long run.