On the morning of April 4, a preliminary stage of the eruption was inaugurated by the opening of a new radial fissure about 500 feet below the summit of the cone ([Fig. 132 a]), and by early afternoon the cone-destroying stage began with the rise of a dark “cauliflower cloud” or pino to replace the lighter colored steam cloud. The cone was beginning to fall into the crater, and old lava débris was mingled in the ejections with the lava clots blown from the still fluid material within the chimney. From now on short and snappy lightning flashes played about the black cloud, giving out a sharp staccato “tack-a-tack.” The volume and density of the cloud and the intensity of the crater explosions continued to increase until the culmination on April 7. On April 5 at midnight a new lava mouth appeared upon the same fissure which had opened near the summit, but now some 300 feet lower ([Fig. 132 b]). The lava now welled out in larger volume corresponding to its greater head, and the stream which for ten months had been flowing from the highest outlet upon the cone now ceased to flow. The next morning, April 6, at about 8 o’clock, lava broke out at several points some distance east of the opening b, and evidently upon another fissure transverse to the first ([Fig. 132 c]). The lava surface within the chimney must still have remained near its old level,—effective draining had not yet begun,—since early upon the following morning a small outflow began nearly at the top of the cone upon the opposite side and at least a thousand feet higher.
Fig. 131.—Scoriaceous lava encroaching upon the tracks of the Vesuvian railway (after a photograph by Sommer).
Fig. 132.—Map of Vesuvius, showing the position and order of formation of the lava mouths upon its flanks during the eruption of 1906 (after Johnston-Lavis).
The culmination of the eruption came in the evening of April 7, when, to the accompaniment of light earthquakes felt as far as Naples, lava issued for the first time in great volume from a mouth more than halfway down the mountain side ([Fig. 132 f]), and thus began the drainage of the chimney. At about the same time with loud detonations a huge black cloud rose above the crater in connection with heavy explosions, and a rain of cinder was general in the region about the mountain but especially within the northeast quadrant. Those who were so fortunate as to be in Pompeii had a clear view of the mountain’s summit where red hot masses of lava were thrown far into the air. The direction of these projections was reported to have been not directly upward, but inclined toward the northeast quadrant of the mountain; but since with a northeast surface wind the heaviest deposit of ash and dust should have been upon the southwestern quadrant of the mountain, it is evident that the material was carried upward until it reached the contrary upper currents of the atmosphere, to be by them distributed.
Fig. 133.—The ash curtain which had overhung Vesuvius lifting and disclosing the outlines of the mountain on April 10, 1911 (after De Lorenzo).