Guatemala

by William Tufts Brigham

As is usual, the earthquakes were accompanied by the discharge of sulphuretted hydrogen, now in such quantities as to be very unpleasant at the city of San Salvador. On the 9th of January there appeared floating on the surface numerous flakes of a black foam composed of ferric sulphide, which in contact with flame burned with a slight explosion. On the 20th, at eleven o’clock in the evening, a great disturbance was noticed in the midst of the lake, and the next morning a pile of rocks was seen, from whose midst arose a column of vapor. For more than a month this vapor column was visible, and the pile of rocks near the centre of the lake increased, while the water was heated and the sulphurous vapors extended over all the neighborhood. Beyond this no permanent volcano was formed above the level of the lake (1,600 feet above the sea).

It is dangerous to form conclusions as to the general course of volcanic action anywhere, for science is very much in the dark as to the causes of eruptions and earthquakes, as to the condition of the interior of our globe, whether fluid or solid, and also as to whether the lavas poured out during an eruption have been fluid since the earth was formed, or have been suddenly melted either as cause or effect of what we call an eruption. In the Central American volcanic region, as was stated at the beginning of this chapter, little has been done in the way of scientific exploration, and the facts recorded, beyond popular accounts of some especial disturbance, are so meagre that no large space would be required to present them to the reader. This is not, however, the place to enter into a scientific discussion, and I must content myself with a few bare statements.

In the first place, the volcanoes of the country discharge both ashes and lava, the latter being most frequently trachytic. Basaltic lavas occur, though less frequently than in Mexico and farther northward; and the columnar structure seen so well at Regla in Mexico is very rare in Guatemala. On the other hand, pumice and obsidian, which are classed with the acid or trachytic lavas, are abundant, the latter furnishing material for knives, while the former has many applications in the arts of the present day. I have seen both basalt and basaltic rapilli in eastern Guatemala near the boundary of San Salvador, and basaltic sand is common on the southern coast.

Another feature of the Central American volcanoes is their remarkable regularity of form. This is due to the fact that the emissions consist of ash and lava of slight fluidity. In the Hawaiian Islands, where the basaltic lava is more fluid than in any other volcanic region, the lava-streams often flow for months, and extend fifty or sixty miles from the crater, building by successive eruptions a cone of great diameter in proportion to their height; Mauna Loa having a diameter of ninety miles at the sea-level, with a height of less than fourteen thousand feet and a slope of about seven degrees. The eruptions of the American volcanoes are mainly of masses of rock which are piled regularly about the base, in this way increasing the height, and great quantities of sand which fills the interstices, and finally of lava in a thick, viscid state which clings to the slopes of the growing cone and cements together the sand and larger fragments. No lava-stream, at least of modern times, has been found at any considerable distance from its source.

From the specimens I collected in some of the ravines which traverse the older deposits, I saw that in former ages the outflow was not only different from that of modern times, but of great variety of form in contemporaneous streams, although the chemical composition did not vary essentially.

Earthquakes are mainly due to the injection of intensely heated lava into strata of cold rock in the process of forming dikes. When a volcano pours its lava out of its summit-crater, the eruption may be wholly free from earth tremors, as is often the case on the Hawaiian Islands; and this gives rise to the popular belief that active volcanoes are in some way a safety-valve for the subterranean forces. When, however, the shrinkage of the earth’s crust or the explosive force of pent-up vapors cracks the solid rock, thus giving passage to the molten mass which must be supposed to underlie this volcanic region, the sudden contact of two bodies of very different temperatures (perhaps two thousand degrees) must cause vibrations entirely sufficient to account for the worst earthquake recorded. That the supply of molten rock is ample beneath the crust of this region, we have proof in the constant activity of Izalco, which for more than a century has poured out lava with the other ejections.

This theory of earthquake action is so simple that it must commend itself to any one who has observed the powerful vibrations excited by placing a cold kettle upon a hot stove, or by admitting with force a stream of hot water into a bath-tub partly filled with cold water. It may be stated also that lava is a remarkably poor conductor of heat (I have been able to walk over a crust that bent beneath my weight, and again where I left footprints in the half-hardened lava), and solid lava might retain a temperature of less than two hundred within a few feet of a molten mass ranging among the thousands of degrees. The secular refrigeration of the subterranean molten masses due to the slight conductivity of solid lava is well illustrated in the temperature of hot-springs, that remains unchanged for centuries.

Eruptions are usually of an explosive nature in the Central American region (as described in the outbreak of Coseguina), and the ejected ash is scattered often to a great distance to form by its decomposition layers of soil especially fitted for the cultivation of coffee, sugar, and the vine. Sulphur is not so abundantly deposited as at Ætna, Hekla, or even the Mexican volcanoes.