My experiments with volcanoes

by Thomas Augustus Jaggar

After the evening earthquake, however, when many buildings had shaken down, all except public officials were ordered to leave for the back country. Young men’s clubs organized to receive the refugees along the roads which led into the interior of Satsuma province, while temples and schoolhouses were impressed into service to house them. The migration of more than 50,000 people with packs on their backs and with handcarts bearing household goods, demonstrated how easily the Japanese people took to a nomad existence. This hegira came to an end on Wednesday, when Dr. Omori arrived from Tokyo, sized up the seismic record and the fiery crisis of Tuesday night, and took the grave responsibility of announcing that the population of Kagoshima might safely return. This was done, he was right, and no further damage beset the city.

Through all of this eruption, so different from Pelée in administrative control, no one was killed by the volcano, though one or two old people died of shock. One old lady who refused to leave her home on the island survived. Village roofs were bent down, crushed, and half buried under a heavy snowfall of ash, and it was notable that flat-roofed cottages were crushed, whereas those with steeper roofs were less damaged. Orange orchards were hopelessly destroyed.

At the west shore of Sakurajima in a place called Hakamagoshi, a fiery blast rushed down to the sea from the rift. Trees were stripped of limbs and bark, saplings were bent away from the volcano, and wood fiber on stumps was shredded by flying rocks. This blast was very short lived and never reached across to the city. It bore the marks of being similar to the downblasts of Mount Pelée. The lava flows kept on for a year and built new shore islands.

I had the remarkable experience of being rowed in a skiff over the submerged tongue of an eastern flow, trailing a thermometer in the increasingly boiling water. When the steaming water about us reached scalding temperature, we had the unpleasant thought that if we should capsize we would be cooked. We found boiled horses and cattle along the beaches, and thousands of dead fish. A climb near the eastern flank vent showed a portion of the moving lava flow pouring down the slope into a glowing cavern under a shell of its own bouldery texture.

The thousands of dollars of relief which came to Japan from America and elsewhere were handled with scrupulous honesty, and the inhabitants of the island were rehabilitated on Tanegashima, another island of the Ryukyu Archipelago.

Scientific investigations showed by leveling that the mountain had been lifted a few feet by the internal penetration of the lava, and reexamination of the benchmarks along roads extending out radially indicated that the north end of the bay bottom and shore had definitely sunk, as though underground lava had been withdrawn from that region, to push up, swell, and overflow the mountain. This effect of subsidence outside was traced and shown to gradually lessen for a hundred miles from the place of greatest sinking. Investigation carried out by the geologist colleagues of Omori culminated in a monumental publication which demonstrates the solidarity of the Japanese methods of science. And both Omori and Professor Koto published books on Sakurajima in English, with maps, photographs, curves, and seismograms.

Omori, in 1910, had anticipated movement of the earth about a volcanic center as swelling up one place and sinking down in another while eruption was going on. At that time, he described Usu Volcano at the opposite end of Japan, where leveling instruments showed graded changes in height made by the Usu eruption. A remarkable physiographic character of Usu Mountain, and of the adjacent basin of Lake Toya, is that basin and dome appear complementary, just as Kagoshima Bay was compensated by Sakurajima. This same pairing of lake with volcano has been noted in other parts of Japan, as though tumefaction by lava penetration and lava eruption had robbed the underpinning of an adjacent piece of ground, which lowered and became a lake by filling up with groundwater.

From Sakurajima I went to Bandaisan, or Kobandai, a famous volcano in central Japan northwest of Tokyo and on the shore of a beautiful lake. It looks like an ordinary rocky peak, but its fame was made by a steam explosion from its flank which blew out the side of the mountain and left avast sulfurous quarry with numerous solfataras and hot springs. Bandai was known by geologists to be one of a chain of volcanoes, but prior to 1885 its activity was in question. One morning the sky was darkened by the overwhelming explosion, and vast volumes of rock from the outbreak poured down as a landslide and completely dammed a river system. It left extraordinary little heaps in the new dammed up lake. These appeared to be individual blocks of rock against which heaps of debris were piled so as to leave pyramidal humps scattered over the surface of the impounded water near the volcano. An excellent report in English on this eruption was published at the time, and the eruption became the type of what geologists call a phreatic explosion, meaning pure steam. There was doubt as to whether any fragments of new lava were thrown up.

I took with me to Bandaisan a photographer-guide. We camped in a mountain inn with thatched roof, visited a hot spring resort, and hiked to the crater where we measured temperatures and took photographs. It was a vast flat-floored shelf, dug out of the side of the mountain, with steam jets and puddles of boiling water at the back. Looking out at the new water-filled valley with its many islands at the base of the slope below the crater, we could see shoreline levels higher than the present beach, where the damming had produced the highest stand of the water. The eruption and landslide overwhelmed villages and killed many people, though it lasted only a few days. It was on the side of the mountain remote from the older lake. In clambering over the broken debris, which looked more like glacial deposits than volcanic agglomerate, I picked up some pieces of vesicular basalt that were definitely lava. Wada, a Japanese geologist, had found the same thing, and we both concluded that these were an internal live basalt blown to fragments in the Bandaisan eruption, but that most of the material was from the shattered old mountain.

My interpretation of Bandaisan is that it is an old volcano in the line of Asama and other volcanoes of central Japan, and that the line is a deep crack always full of lava in the depths, which is selective of outlet, depending upon what part of the crack opens as the path of least resistance. Eruption may be occasioned by lava wedging upward at one volcano, or by lava sinking downward at another volcano, according to the way the medial rift of continental Honshu is warped and stressed by the earthquake forces. One part of a volcano chain is always sinking, with lava withdrawn. Another part is always swelling up, with lava penetrating the cracks under active crater pits, like that of Asama.