On the connexion of Volcanic Action with Changes of Level

Abstract

The purpose of this paper was to show that “volcanic action is not the cause, but the effect, of secular changes of level; and secular changes of level are due to the subsidence of the surface of the interior, as the interior contracts in cooling.” Change of level is a differential action, and consequently cannot be due to the cooling of a sphere by radiation into space. Volcanic action cannot be due to a spontaneous outburst of the expansive force of the earth’s internal heat; for this could not burst through a crust once formed by cooling. Changes of level and volcanic action were explained as follows:—The interior of the earth is constantly cooling, and as it cools must contract; but the cold surface-strata cannot contract with it; and as their weight keeps them in contact with the core, they are compelled to form ridges like those on the skin of an apple which shrinks in drying. When such a ridge rises into an arch, the hot matter below rises and fills the arch, forming the igneous core of a mountain-chain. Volcanoes are formed when in these foldings the surface is broken through, so as to liberate the expansive force of the internal heat. Darwin has shown, in his work on Volcanic Islands, that volcanoes are formed only in regions which are rising or have recently risen. The reason of this is, that an upward bend in a stratum is resisted only by the weight of the stratum itself, while a downward bend is resisted by the pressure on the strata below; so that an upward bend is more likely than a downward one to become great enough to produce fracture.