The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

Of course, you will understand that we have given only a few of the most notable of the eruptions of Mt. Vesuvius. Since the yea a. d. 1500 there have been no less than fifty-six recorded eruptions, that of the year 1857 being especially violent.

Omitting these eruptions we at last come to the great recent eruption of 1872.

Fortunately, the eruption of 1872, as well as still more recent eruptions that have occurred, have been more accurately described than have most volcanic eruptions, for the Italian Government, recognizing the value to the natives of Italy of a knowledge of what was going on at the crater of Vesuvius, has maintained for the past thirty years an observatory on the western part of the mountain. This observatory has been placed in charge of Prof. Luigi Palmieri, a well-known student of volcanoes and earthquakes. At this place records are kept of the behavior of the volcano, of all earthquake disturbances, as well as other phenomena. At the same time, by the use of photography, excellent pictures have been obtained showing the appearance of the sky during an eruption.

Vesuvius had been in a quiet state from November, 1848, to the year 1871, when small quantities of lava flowed continuously for several months. Again, early in 1872, other quiet eruptions of lava continued for weeks at a time. Finally, on April 26th, of that year, a violent explosive eruption occurred. The following account has been taken from Palmieri's report, entitled, "The Eruption of Vesuvius in 1872."

On April 23d the recording earthquake instruments, the seismographs, were greatly affected. On the evening of the 24th lava streams flowed down the cone in various directions. These streams were continued on the 25th and the 26th, so that on the night of the 26th the observatory lay between two streams of molten lava that threw out so much heat that the glass windows in the observatory were cracked, and a scorching smell was quite perceptible in the rooms. The cone of the mountain was deeply fissured, lava escaping freely from all the fissures, so that the molten rock appeared to ooze from over its entire surface, or as Palmieri expressed it, "Vesuvius sweated fire."

This great cracking or fissuring of the cone was accompanied by the opening of two large craters at the summit, that discharged, with a great noise, immense clouds of steam, dust, lapilli, and volcanic bombs. These latter are very curious and consist of masses of soft lava that are thrown high into the air by the outrushing columns of steam. Being rotated or spun, as they rise in the air, they assume a spherical shape. Some of these volcanic bombs were thrown to a height estimated by Palmieri to have been nearly 4,000 feet above the top of the mountain. When the height of a projectile is known, the velocity with which it left the opening from which it was projected or thrown can be estimated, so that the volcanic bombs must have left the crater at a velocity of about 600 feet per second.

On the 27th, in the evening, the lava streams ceased flowing, but the dust and lapilli continued to fall during the 28th and the 29th. On the 30th the detonations decreased and by the 1st of May the eruption was entirely over.

Palmieri calculated that the quantity of molten rock thrown out during this eruption was sufficient to cover an area of about 1.8 square miles to an average depth of about thirteen feet.

As we can see from the above descriptions, the volcanic activity of Vesuvius is characterized by long periods of rest followed by periods of activity. The periods of rest are measured by years, and often by centuries; the periods of activity by days or hours.