With our ideas of volcanoes we always associate the grand and the terrible ; and a volcanic eruption—a huge piece of artillery, with a mouth perhaps miles in circumference, shooting up rocks and burning lava—is truly a terrific sight. Volcanoes are exceedingly plentiful on our planet, there being no less than sixty-three principal ones; still, they are confined to certain localities, which occupy but limited portions of our globe. The question has often been asked, “ what is the cause of volcanoes 1” And truly, when we consider how disastrous some of these eruptions have been, no wonder the question of their cause has been forced upon the attention of almost ev61:y reflecting mind. It is one well worthy of some speculation, and requires a considerable amount of scientific knowledge to investigate. and this may be usefully employed either in pointing out errors or presenting new facts. Various opinions have been expressed respecting their origin and activity. One thing is certain, they are in no way connected with solar influence, for they exist under the tropics of South America, and are found in the frosty regions of Iceland. It was the opinion of Darwin, that the volcanic districts of the world had earthy crusts resting on lakes of igneous melted matter. Humboldt believes that the volcanic region of Quito, in South America— the whole of that vast Plateau—is a single volcanic surface, composed of a solid crust covering a lake of molten matter. Such opinions, however, have nothing to do with a general theory, of which there are two—one is astronomical, and asserts that this earth was originally a fiery molten mass, and that we live on its crust, beneath which all is molten fiery matter; the other theory is chemical, and asserts that they are caused by explosive materials deposited in huge quantities in the volcanic localities, and which, when saturated by some means with oxygen, and ignited, act exactly like any explosion of artillery. Leibnitz first suggested that this earth was originally in a fiery fluid state ; Sir Wm. Herschell afterwards suggested the hypothesis of matter being originally in a nebulous state, which, by condensation, developed great heat, and our earth became a fiery ball, the surface of which we now live upon being a mere crust, the rest not being cooled yet which, when reached by water, causes an explosion like a steam boiler. This is the nebular igneous theory. The author of “ The World Without” states how easy it is to account for volcanoes by this theory, by spying—” according to the fiery nebulous theory, the earth, at a depth of sixty- five miles, is 7000 degrees temperature, and if water percolates through fissures of the earth, we have a sufficient explanation of earthquakes and volcanoes." This theory is unsound, and will not stand the test of scrutiny. The arguments adduced to prove that the interior of the earth is a fiery molten mass, is, the increase of temperature found to exist as we descend in some mines, which is about 1 degree for every 45 feet. According to this rate, at 25 miles depth, the melting point of iron would be obtained ; but we have no facts to prove that the heat of the earth increases regularly to the centre; after a certain depth, it is perhaps uniform. What signify the experiments made in a few mines not over 2,000 feet, deep. From observations made by Kotzebue, Beechy, and Sir James Ross, the fact seems to be established that the waters of the ocean (it is also matter) are uniform in heat, at the depth of 7,200 feet. At the depth of 100 fathoms, as stated in Maury's Wind and Current Charts, the temperature of the water in “ the cruise of the Taney,” was 64°, while at 50 fathoms, one half, it was 70°. In the soundings by the sloop-of-war Albany, at 680 fathams, the temperature was 81°, while that of the air was 83°, and at 995 (5970 feet) fathoms it was only 80°, while the temperature of the air was 79°. Now if it were true that the heat increased downwards, at the rate of one degree for every 45 feet, as asserted by some, then with a temperature of air at 79°, the water of the sea at 5985 feet of depth, should be at the boiling point—212°. Instead of this it was only 80° at 5970 feet, only 15 feet less. How does this accord with a uniform increase of heat as one descends into the matter composing the earth ? Dr. Daubeny, and Sir Charles I.yell are ad vocates of the chemical theory, and the latter is a decided opponent of the central theory of heat. It is well known that when potassium is dropped upon water, it causes an explosion; if, in certain places of the earth, there were large deposits of this metal, and water percolate to or come in contact with it, a terrific explosion would ensue. It appears to us that volcanoes are local, and generally preceded by earthquakes. If the centre of the earth were fluid, according to the well-known laws of fluids those earthquakes, caused by volcanoes would affect equally every part of the earth's surface, a thing which we know they do not. Our attention was directed to this subject by reading some accounts of the recent eruption of Mount Etna. There is no positive certainty respecting the real cause of volcanoes ; but the general, yea, almost universal opinion expressed by writers on the subject, is that water in some way is an active agent in all eruptions. Water, however, in all likelihood, exerts no agency whatever; and a strong argument in proof of this, is, that in the moon there is neither atmosphere nor water, and yet the volcanoes of the earth are mere dwarfs compared with those on our satellite. Our views, then, are distinctly opposed to the prevailing igneous theory, and we choose, rather, to plead ignorance of the causes of volcanoes than adopt any theory which cannot stand the test of scientific analysis.
This article was originally published with the title "Volcanoes, their Causes—Igueous Theory" in Scientific American 8, 12, 93 (December 1852)