CHAPTER XX

SOME FORMS OF LAVA

In describing the wonders of volcanoes, we must not fail to say something of the many remarkable forms that lava is capable of assuming.

All volcanic lavas contain large quantities of an acid substance known as _silica_, or what is known better as _quartz sand_. This material exists in lava combined chemically with various substances called bases, the principal of which are alumina, magnesia, lime, iron, potash, and soda.

Although there are many kinds of lava, yet all lavas can be arranged under three great classes according to the quantity of silica they contain.

_Acid lavas_ are those in which the quantity of silica is greatest. In these lavas the silica, which varies from 66 to 80%, is combined with small quantities of lime or magnesia, and comparatively large quantities of potash or soda. Some of the most important varieties of acid lavas are known as _trachytes_, _andesites_, _rhyolites_, and _obsidians_.

_Basic lavas_ are those containing from 45 to 55% of silica. They are rich in lime and magnesia, but poor in soda or potash. Some of the most important of basic lavas are the _dolerites_ and _basalts_. Generally speaking, basic lavas are of a darker color than acid lavas, and fuse at much lower temperatures.

_Intermediate lavas_ are those containing silica in the proportion of from 55 to 66%.

While the temperature of liquid lava has not been very accurately determined, yet, since we know that molten lava is able to melt silver or copper, its temperature must be somewhere between 2,500° F. and 3,000° F., the melting point varying with the chemical composition.

According to Dana lavas can be divided into the following classes according to their fusibility; i. e., _lavas of easy fusibility_, such as _basalts_; these lavas fuse at about 2,250° F.; _lavas of medium fusibility_, including andesites; these lavas fuse at about 2,520° F.; _lavas of difficult fusibility_, such as trachytes; these lavas fuse at about 2,700° F.

But what is, perhaps, most curious about lavas is that when the surface of a freshly broken piece of cold lava is carefully examined, it is found to contain a number of small crystals of such mineral substances as quartz, feldspar, hornblende, mica, magnetite, etc.

The best way to study the different forms of lava crystals is to prepare a thin transparent slice of hardened lava and then examine it with a good magnifying glass. It will be found that the slice consists of a mass of a glass-like material through which the crystals are irregularly distributed, not unlike the raisins and currants in a slice of not over rich plumcake.

When examined by a more powerful glass, such as a microscope, cloudy patches can be seen distributed irregularly through the glass-like mass. When these patches are examined by a higher power of the microscope they are seen to consist of small solid particles of definite forms known as _microliths_ and _crystallites_. It has been shown by a careful study of these minute objects that they form the exceedingly small particles of which crystals are built up.

If we fuse a small quantity of lava and then let it slowly cool, the glassy mass will be found to contain numerous crystallites. On the other hand, when fused lava is permitted to cool quickly, it takes on the form of a black, glass-like mass known as _obsidian_ or _volcanic_ glass, a very common form of lava in some parts of the world.

In some lavas there are found larger crystals that appear to have been separated from the glassy mass, under the great pressure that exists in the subterranean reservoirs at great depths below the volcanic crater, and then floated to the surface surrounded by the glass-like material. Now when we examine these crystals with a higher power of the microscope, we frequently find in them minute cavities containing a small quantity of liquid and a bubble of gas, thus causing them to resemble small spirit levels. The liquid in such cavities has been examined chemically and in most cases has been found to consist of water containing several salts in solution. Sometimes, however, the liquid consists of liquefied carbonic acid gas. These wonderful things will be discussed at greater length in the Wonder Book of Light.

When the mass of molten rock or lava that comes out of the crater of a volcano is thrown upwards in the air the condition it assumes by the time it falls back again to the earth depends on the height it reaches. If this height is great the lava chills or hardens before reaching the earth, and assumes various forms according to the size of the fragments. The largest of these fragments are called _cinders_; the finer particles _volcanic dust_; while most of those of intermediate particles are known among other things as _volcanic ashes_.

We have already seen that when an explosive volcanic eruption occurs there is suddenly thrown out of the crater of the volcano a huge column of various substances that rises sometimes as high as 30,000 feet or even more. The smaller fragments of lava are quickly cooled and form volcanic ashes, sand, cinders, or dust. These are rapidly spread out by the wind in the form of a black cloud, that not only covers the mountain but reaches out over the surrounding country, completely shutting off the light of the sun. From this cloud particles of red hot ashes, cinders, sand, etc., begin to fall, the largest particles near the crater of the volcano, and the smaller particles at much greater distances. In very powerful explosive volcanic eruptions such as Krakatoa, the finer dust may be carried to practically all parts of the world.

Volcanic ashes consist of a fine, light, gray powder. These particles take the name ashes from their resemblance to the ashes left after the burning of pieces of wood or coal in an open fire. The name, however, as Geicke points out, is unfortunate, since it is apt to lead one to suppose that volcanic ashes consist of some burned material. Such an idea is erroneous, however, since ashes do not consist of anything that is left after burning, but merely of fine particles of molten rock that have hardened by cooling. When in the shape of what is known as volcanic dust these particles are so exceedingly small that they can readily make their way through the smallest openings in a closed room just as does the finest dust in the rooms of our houses when they are shut up. There are cases on record where people have been suffocated by the entrance of volcanic dust in closed rooms to which they have fled for safety during volcanic eruptions.

_Volcanic sand_ consists of the coarser particles of chilled lava that are partly round and partly angular. They are of various sizes up to that of an ordinary pea. Volcanic sand is formed by the breaking up of the lava by the explosion of the vapors as they escape from the lava on relief from pressure. Volcanic dust when examined by the microscope is found to consist of very small particles that are more or less crystalline.

But besides the above there are larger fragments known as _lapilli_, consisting of rounded or angular bits of lava varying in size from that of a pea to an ordinary black walnut. These sometimes consist of solid fragments, but are usually porous, sometimes so much so that they readily float on water.

A curious form sometimes assumed by lava consists of what are called _volcanic bombs_. These are formed during explosive eruptions, when masses of liquid lava are hurled high up into the air. During their flight they take on a rotary motion, which tends to make them globular, so that cooling, while still revolving, they assume the form of a more or less spherical mass. At times, however, they are still sufficiently soft when they strike the earth to be flattened out in the form of flat cakes. When of a spherical form these are very properly called volcanic bombs.

That volcanic bombs have actually been subjected to a spinning motion while in the air can sometimes be shown by the fact that masses of scoriæ are frequently found in the interior with air cells largest at the centre of the bomb.

Volcanic bombs are sometimes thrown from the crater to great distances. During one of its recent eruptions, Cotopaxi threw out bombs that fell at a distance of nine miles from the crater.

According to Dana another form of lava bombs is sometimes found on the slopes of the active volcanoes of Hawaii, where masses of lava acquire a ball-like shape while rolling down an inclination.

What are sometimes called volcanic bombs, but which are more properly _volcanic vesicles_, are produced by small fragments of lava which are thrown up in the air for only a moderate height and, on cooling, assume pear-like forms. Fig. 25 represents the appearance of volcanic vesicles. The direction in which these vesicles moved through the air while in a molten state is indicated by their shape, the blunt end being the end towards which the particles were projected.

But by far the greater portion of the hardened lava; i. e., the coarser, heavier particles, fall back on the mountain, and collecting around the crater build up volcanic cones, as already described in the case of mountains of the Vesuvian type.

There are two different ways in which the melted lava is broken up into fine particles when it is thrown upwards from the crater of the volcano. Nearly all lava contains large quantities of steam that are shut up, or occluded in the mass, being prevented from escaping by reason of the pressure to which the lava is subjected. The lava is released from this pressure as it is thrown out of the crater. The steam or gases escape explosively and thus break the lava into fine liquid spray, which rapidly hardens.

There is another way in which small particles of lava are formed. Sometimes large pieces of hardened lava are shot upwards into the air with a velocity as great as that with which a heavy projectile leaves the muzzle of a large gun. These heavy particles striking against one another, either while rising or falling, are broken into smaller fragments. Sometimes, indeed, these fragments fall back again into the crater from which they are again violently thrown out, and are again broken into smaller fragments either while rising or falling.

You will, probably, remember several instances of volcanic eruptions where masses of rock were thrown violently up into the air out of the crater. These larger masses are known as _volcanic blocks_. They probably consist of masses of hardened lava that have collected in the tube of the volcano during some of its periods of inactivity. Sometimes, however, they consist of fragments of rocks that are not of volcanic origin. Cases are on record where volcanic blocks have been thrown out of the craters in so great quantities as to cover the surface of many square miles of land with fragments hundreds of feet deep.

There is sometimes formed on the surface of a pool of lava as it collects in the craters of such volcanoes as Mt. Loa or Kilauea, when the volcanoes are not in eruption, a material resembling froth or scum. The same thing sometimes occurs on the surface of some kinds of lava as it runs down the side of the mountain. In this way a very light variety of highly cellular lava known as _pumice stone_ is produced. The action which thus takes place is not unlike that which occurs during the raising of a lot of the dough from which bread is made, where the carbonic acid gas which is formed during the raising of the dough expands, and produces the well-known open cellular structure of well-raised bread. In the case of pumice stone, however, this raising goes on to such an extent that the mass consists often of less than 2% of solid matter, the remainder being a tangled mass of air.

Fragments of lava that possess a cellular structure form what are known as _scoriæ_. The lightest of all kinds of scoriæ is what is known as _thread-lace scoriæ_. Here the thin walls consist of mere threads. Figs. 26 and 27 represent the appearance of thread-lace scoriæ from Kilauea. The separate threads are very fine, being only from one-thirtieth to one-fortieth of an inch in thickness. As can be seen, this form of scoriæ have six-sided or hexagonal shapes. You can form some idea of the great lightness of such scoriæ when you learn that they contain only 1.7% of rocky material. Indeed, they contain so little solid material that a layer of volcanic glass only one inch thick, if blown out into scoriæ, would be able to produce a layer sixty inches thick.

Another curious form sometimes assumed by lava, especially in the case of Kilauea, is where the lava is spun out in the form of long silk-like hairs. This is called by the natives _Pele's hair_, after the name of their goddess. Inasmuch as the origin of this form of lava was at one time generally attributed to the action of the wind in drawing out thread-like pieces from the jets of lava thrown upwards from the pool, it will be interesting if its true cause is explained.

Dutton, in his report on the Hawaiian volcanoes, refers to the formation of Pele's hair as follows:

"The phenomenon of Pele's hair is often spoken of in the school books, and receives its name from this locality. It has generally been explained as the result of the action of the wind upon minute threads of lava drawn out by the spurting up of boiling lava. Nothing of the sort was seen here, and yet Pele's hair was seen forming in great abundance. Whenever the surface of the liquid lava was exposed during the break-up the air above the lake was filled with these cobwebs, but there was no spurting or apparent boiling on the exposed surface. The explanation of the phenomenon which I would offer is as follows: Liquid lava coming up from the depths always contains more or less water, which it gives off slowly and by degrees, in much the same way as champagne gives off carbonic acid when the bottle is uncorked. Water-vapor is held in the liquid lava by some affinity similar to chemical affinity, and though it escapes ultimately, yet it is surrendered by the lava with reluctance so long as the lava remains liquid. But when the lava solidifies the water is expelled much more energetically, and the water-vapor separates in the form of minute vesicles. Since the congelation of all siliceous compounds is a passage free from a liquid condition through an intermediate state of viscosity to final solidity, the walls of these vesicles are capable of being drawn out as in the case of glass. The commotion set up by the descending crust produces eddies and numberless currents in the surface of the lava. These vesicles are drawn out on the surface of the current with exceeding tenuity, producing myriads of minute filaments, and the air, agitated by the intense heat at the surface of the pool, readily lifts them and wafts them away. It forms almost wholly at the time of the break-up. The air is then full of it. Yet I saw no spouting or sputtering, but only the eddying of the lava like water in the wake of a ship. The country to the leeward of Kilauea shows an abundance of Pele's hair, and it may be gathered by the barrelful. A bunch of it is much like finely shredded asbestos."

You have probably often seen the beautiful frost pictures that collect on the panes of glass in a room where the ventilation has been neglected. These pictures consist of groupings of ice crystals that collect on the surface of the windows, when the moist vapor-laden air in the room is chilled by contact with their cold surfaces. Now the crystals formed in cooling lavas are sometimes grouped in forms closely resembling frost pictures. A few of such forms are represented in Figs. 28 and 29 in lava from Mt. Loa and Mt. Kea.

Certain varieties of lava, especially that which is found in dikes, form cool, beautiful columns called basaltic columns. They are due to the contraction that occurs on the cooling of the material. Instances of basaltic columns are seen in the Giant's Causeway, on the northern coast of Ireland, as well as in the Isle of Cyclops on the coast of Italy. The general appearance of the latter is represented in Fig. 30.

It is a curious fact that the entire mass of basalt does not generally take the columnous form but only certain layers which terminate suddenly above and below at structureless masses of basalt, as shown in Fig. 31. These columns, however, are always found at right angles to the cooling surfaces as seen in the figures. They may, therefore, be inclined at all angles to the horizon.

When molten lava is only thrown up a short distance into the air from a crater it is still partially molten when on falling it again reaches the earth, and therefore clings to any surface on which it falls. There are thus built up curious cones known as _driblet cones_, in which the separate drops covering the sides of the cone can be distinctly traced. Driblet cones are represented in Figs. 32 and 33. Here, as can be seen, the separate drops can be readily traced as they run down a short distance before cooling.

We have already referred briefly to the _lava caves_ or _grottoes_, that are formed in some of the lava streams issuing from Vesuvius, Etna, or Hawaii. These caves consist either of a number of communicating huge bubbles, or of the tunnels that are formed in the lava by the hardening of the outside of the lava streams as they flow down the sides of the mountain, and towards the close of the eruption are afterwards emptied by the molten lava within continuing to flow to a lower level before solidifying. Now, in the interior of these caves, there are often found on the walls, as well as on the portions of the floors of the caves, immediately below them, curious pendants, like icicles, or, more correctly, like the _stalactites of limestone_ that are seen hanging to the walls of caves in limestone districts, where they are formed as follows: as the rain water sinks through limestone strata it dissolves some of the lime, when, slowly falling, drop after drop, from the roofs of the caverns, small particles of lime are deposited on the roof, and in this manner a pendant of limestone is formed. The water that falls to the floor of the causeway immediately below, also builds up a dome-like hillock called a stalagmite. In due time the pillar reaches downwards, and the opposite hillock upwards until the two meet, thus forming great natural pillars that appear to hold up the roof of the vast cave in which they have been slowly formed. A number of _lava stalactites_ are represented in Fig. 34.

Now, in a similar manner these lava stalactites, formed in the lava caves or grottoes, are caused by the stream as it escapes from the walls of the caves depositing on them stalactites of various lava minerals it has dissolved as it slowly passed through them.

But the most important of all volcanic products is _volcanic dust_. This, as we have seen, is so light that it remains longest in the air, and is often carried by the winds to great distances from the volcano from which it escaped. It may interest you to know that some of the most fruitful of the great wheat fields of the western parts of the United States owe their extraordinary fertility to immense deposits of volcanic dust that have been thrown out from some of the great volcanoes of the geological past, now found in an extinct condition in these parts of the United States.

According to Russell, immense deposits of volcanic dust are spread over vast areas in Montana, Southern Dakota, Nebraska, and Kansas, as well as over parts of Oregon, and Washington, and, indeed, over large areas of southwestern Canada and Alaska.

It is practically certain that many of the eruptions producing this dust occurred within historic times. There must, therefore, have been many times in these parts of our country when the dense ash clouds hiding the sun turned the day into night and destroyed the forests and other vegetation by showers of red hot ashes. There were produced, too, the same great dread, and possibly loss of life as common during historical eruptions. It is pleasing, however, to think that while these great catastrophes brought suffering and dread to the people who then lived on the earth, they were, nevertheless, but the forerunners of those fruitful fields that at a much later age were to bless the people who afterwards lived on them.