Page:Popular Science Monthly Volume 50.djvu/262

248 is considered the primary source from which injections are derived.

It has been thought by some who have speculated on the condition of the earth's interior that isolated reservoirs of fused rock exist in the generally cool earth's crust, due to unequal cooling another origin for such lakes of lava may be postulated if we consider them as injected magmas not yet cooled.

A mental picture of the probable occurrences that give origin to subterranean intrusions and volcanoes, and account for many observed phenomena in this connection, may be sketched in outline as follows:

The earth is hot and potentially plastic within, and cold and rigid at the surface. Unequal cooling and the shifting of material on the surface are disturbing conditions that tend to change the shape of the plastic interior, and to crumple and break the crust. If a fissure forms in the lower surface of the crust, the potentially plastic material beneath will become plastic on account of the removal of resistance to pressure, and be forced into the break, and a dike be formed. Under certain conditions the plastic material rising in a fissure may expand between layers of stratified rock so as to form laccolites, subtuberant mountains, etc.

If a break extends entirely through the crust, molten material forced into it may reach the surface. As the molten lava rises in such a break, it passes through rocks that are more and more highly water-charged, the water is vaporized, or perhaps its elements are dissociated, and the vapors and gases formed are absorbed by the fluid rock. As the lava comes to the surface the steam and gases absorbed under great pressure escape and furnish some of the most striking phenomena of volcanic eruptions. Loss of heat as a magma nears the surface also favors the escape of occluded gases.

In this view of the nature of volcanoes it is evident that an arrest of pressure on the reservoirs from which they draw their lavas would stop their action. If a fissure extends through the earth's crust to the potentially plastic interior, it is difficult to see how an outflow of molten material would be checked unless the conduit should become closed. Under the vast pressure that exists at a depth of several miles it is impossible to comprehend how fissures can exist, but the plastic material beneath is under pressure of a similar order of magnitude, tending to force it out through any opening that may be formed. A near balance between the pressure tending to close a fissure and the pressure on the magma below tending to maintain a communication with the surface may therefore be conceived to exist; when the balance is in favor of extrusion volcanic eruptions follow, and when the reverse is the case the conduits become closed.