Fig. 73.—Peak of Teneriffe in the Canary Islands (12,182 ft.), surrounded by great crater-rings.
Sometimes, as in the case of the Lago di Bracciano, the eruptive forces appear to have entirely exhausted themselves in the prodigious outburst by which the great crater was produced. But in other cases, as in that of the Lago di Bolsena, the eruptive action was resumed at a later date, and small tuff-cones were thrown up upon the floor of the crater; these now rise as islands above the surface of the lake. In other cases, again, the eruptive action was resumed after the formation of the great crater-ring, with such effect that bulky volcanic cones were built up in the midst of the crater-ring which surrounds them like a vast wall; examples of this are exhibited in the extinct volcanoes of Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the globe, such as Teneriffe ([fig. 73]), the volcanoes of Mauritius and Bourbon (figs. [74] and [75]), and many others that might be cited, are thus found to be surrounded by vast crater-rings. Vesuvius itself is surrounded by the crater-ring of Somma ([fig. 76]).
Fig. 74.—The volcano of Bourbon, rising in the midst of a crater-ring four miles in diameter.
Fig. 75.—The volcano of Bourbon, as seen from another point of view, with three concentric crater-rings encircling its base.
BASALTIC CONES IN TRACHYTIC CRATER-RINGS.
This formation of cone within crater, often many times repeated, is very characteristic of volcanoes. The craters mark sudden and violent paroxysmal outbursts, the cones are the result of more moderate but long-continued ejection. Sometimes, as at Vesuvius in 1767 ([fig. 15], p. 85), we find a nest of craters and cones which very strikingly exemplifies this kind of action.
