My experiments with volcanoes

by Thomas Augustus Jaggar

Meantime trajectories like those of a hose sent falls of gravel for miles, to Ottajano on the opposite side of the mountain. These also came from the central crater. On the west flank, at the observatory, the house was rocking, and heavy stones forced its occupants to retreat. Matteucci and his staff went halfway down the cone, to return next day. Explosions dwindled during the next fortnight, though one day an adverse wind from the crater carried carbon dioxide and hydrogen sulfide almost asphyxiating some persons. Thereafter cauliflower clouds of white steam arose and the noise of big avalanches was heard.

The clinker field that invaded Boscatrecase was 16 feet thick, and houses were cut in two by a slaggy torrent. In Ottajano, on the opposite side of the mountain, flat tile roofs collapsed, buried under three feet of heavy gravel, some of it the size of an apple. Nearer the crater, boulders five feet in diameter were thrown a mile. The volcano was probably blocked inside by welling lava on the Boscotrecase side, which caused it to vomit steam and earthy avalanche material obliquely outward on the opposite, Ottajano, side.

The Italians have a word, sprofondimento, which means to make profound by insucking, that expresses what happened. This plexus of uprush of slag and inrush of avalanche, against a water-steam geyser, both happening at once, was very different from the quiet outpouring of lava during the preceding years. It definitely meant rupture of earth blocks, deep escape of that lava probably at the underocean part of the radial cracks, and deep entrance of spring water into incandescent vacated chambers. It meant a rupture crisis, collapsing the peak, and a new geyser quite unrecognized. The eruption ended when the slag pressure was relieved, the mountain blocks had settled, and the frozen slag had shut off groundwater. The remaining lava entered into decades of deep accumulation and gas bubbling, the solfataric phase. That which ended the thirty-year upbuilding was probably downward pressure due to weight of surface heaping of the cone. Cracking released water inward.

The next thirty-eight years were to culminate in a similar crisis for Vesuvius which lasted ten days, and again its peak collapsed. This was in March 1944, when our American troops entered Naples. It is interesting that these culminations have been from a third to a half century apart, but the meaning of intervals can only be really understood when volcanoes like Etna, Stromboli, and Vesuvius are grouped together. The same thing is true of Kilauea and Mauna Loa, and of Pelée and St. Vincent. Ponte reports the eruptions of Etna as ten years apart, similar to the sunspot interval; and Perret notes a ten-year interval for the smaller eruptions of Vesuvius. We measured an eleven-year interval for Hawaii, with culminations close to the minimum of sunspots. A culmination is when lava goes down and keeps quiet, or when sunspot numbers go down and remain few. No one knows why, or of any connecting cause.

Three eleven-year culminations make a third of a century, when at Kilauea and Vesuvius, something bigger happens. Sunspots have numbered a suspiciously similar curve at corresponding dates.

Photographs of Vesuvius taken just before the 1944 collapse showed the 1906 crater hole completely filled and overflowing. There was an inner flat floor, a conelet standing in the middle. The 1944 eruption collapsed the conelet, split the big outer cone, and sent flows to destroy San Sebastiano and several villages. The torrents of ash killed people and the electric station of the funicular railroad was destroyed, as usual. The mountain split in several directions.

Just as in 1906, the stages of the 1944 outbreak were lava flows, mixed lava gushing intensely liquid, crateral caving in, tremendous gas emission, black ash changing to vapor and white ash as the emission increased, and ultimately white steam. The black ash was the contemporaneous lava with dark augite; the snowlike white ash was ground up old lavas, containing the white crystals, leucite.

The liquid phase took an unusual fountain form, resembling that of Mauna Loa in Hawaii, and nine spells of bright incandescent explosive fountaining occurred. The collapse began on March 13; the fountaining occurred during March 20 to 22, with jets of bright liquid lava and flames, 1,000 to 3,000 feet high; and the crater became a lava lake. The flames were occasioned by hydrogen within the lava itself, and perhaps some carbon gases. This liquid fountaining phase was the culmination of explosions, making pumice, with water vapor the gaseous product. Ash fell four feet deep three miles away, and some fell on the Adriatic coast. Both white steam clouds and black ash clouds arose with the fountains, white and black side by side.

The net effect was to leave a bowl 1,500 feet in diameter and 800 feet deep, floored with avalanche gravel. This reconstructed the funnel of 1906, and as in 1906, the height of the rim was 4,100 feet after eruption. In other words, the thirty-eight years had filled the vast crater, only to have 1944 engulf and eject the contents, and strew them down the slopes, adding an immense weight to the outer shell of the cone.

A hundred million cubic yards of lava was poured out, and 50 million cubic yards of ash now lie on the volcano. Three times as much was carried far away, and the volume of gases was ten times as great. The rock fragments, probably 200 times as great, were engulfed by avalanches.