Note 98 — Chapter V

Mr. Scrope (‘Volcanoes,’ Longman and Co., 1862) clearly explains how these fissures, injected by gas, steam, and lava from beneath, burst open at the weak points and thus form the circular or elliptical volcanic vent. The expansion and explosion of the pent-up steam, when coming in contact with the external air, is like the ignition of powder in a gun-barrel. The rocks, which are the walls of the fissure, are broken up and yield by the equal pressure on every side, and the character of the surface thus becomes approximately circular and elliptical where the sides of the fissure offer more resistance than its length.