Fig. 81.—Map of the volcanic group of the Lipari Islands, illustrating the position of the lines of fissure on which the volcanoes have been built up.
SHIFTING OF VENTS ALONG FISSURES.
We have described the action going on at Stromboli as typical of that which occurs at all volcanic vents. Stromboli is, however, one among a group of islands all of which are entirely of volcanic origin. The volcanoes of this group of islands, the Æolian or Lipari Islands, are arranged along a series of lines which doubtless mark fissures in the earth's crust. These fissures, as will be seen by the accompanying map ([fig. 81]), radiate from a centre at which we have proofs of the former existence of a volcano of enormous dimensions. It is a very interesting fact, which the studies of Prof. Suess have established, that the earthquakes which have so often desolated Calabria appear to have originated immediately beneath this great centre of volcanic activity.
Fig. 82.—The Puy de Pariou in the Auvergne, illustrating the shifting of the centre of eruption along a line of fissures.
When two volcanic cones are thrown up on the same line of fissure, their full development is interfered with, and irregularities in their form and characters are the consequence. In the plan ([fig. 82]) and the section ([fig. 83]) an example is given of the results of such a shifting of the centre of eruption along a line of fissure. By the second outburst, one-half of the first-formed cone has been removed, and the second-formed overlaps the first.
Fig. 83.—Ideal section of the Puy de Pariou.
Sometimes a number of scoria- or tuff-cones are thrown up in such close proximity to one another along a line of fissure, that they merge into a long irregular heap on the summit of which a number of distinct craters can be traced. An example of this kind was furnished by the line of scoria-cones formed above the fissure which opened on the flanks of Etna in 1865 (see [fig. 84]).