III.—The Fissure Theory of Volcanoes

Extract

Dr. Hans Beck has adduced an example of a volcano which, according to him, has been formed independently of a fissure. The volcano pierces the centre of a faulted block, the Herdubreid, in Iceland, and on the vertical fault-faces there is no sign of any fissure. The example is probably unique in the world, and seems at first sight to negative the hypothesis that the escape of gases which tear through the earth's crust and form the chimneys of volcanoes is in the first place initiated by a fracture; on closer examination, however, the fact that the volcano stands in close relation to the faults which bound the horst, and the many cases which are known to occur where a fracture in the earth's crust may be healed at the surface so that the rocks about the fracture are subsequently more resistant than before, seem to point to the Herdubreid volcano being a normal fissure-formed volcano, only that it stands in the same relation to the fracture as a parasitic cone stands to the central crater. In other words, the chimney is an escape vent leading below the surface to one of the bounding faults of the horst.