Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 24.—Concentric Folds on mass of cooled Lava.

In other cases the lava, if somewhat more liquid, may in issuing quietly without great outbursts of steam, accumulate in great bottle-shaped masses, which have been compared to 'petrified fountains.' Cases of this kind have been described by Professor Dana as occurring on the slopes of Hawaii (see [fig. 25]).

Fig. 25.—Mass of cooled Lava formed over a spiracle on the slopes of Hawaii.

MINIATURE CONES ON LAVA-STREAMS.

When the steam escapes with explosive violence from a spiracle ('bocca') on the surface of a lava-stream, minute cinder cones, like those described as being formed in 1872, are the result. [Fig. 26] represents a group of miniature cones thrown up on the Vesuvian lava-stream of 1855: it is taken from a drawing by Schmidt.

Fig. 26.—Group of small Cones thrown up on the Vesuvian Lava-current of 1856.

Some of these appear like burst blisters or bubbles, while others are built up of scoriaceous masses which have been ejected from the aperture and have become united while in a semi-fluid condition. Other examples of these spiracles or bocche on the surfaces of lava-currents may be seen in the figs. [22] and [23], which are copied from photographs.