Asama is the Vesuvius of central Japan near the village of Karuizawa, famous as the resort of American missionaries. Bandaisan is one of a line of volcanic peaks north of Asama, all of which have hot springs and solfataras. The explosion of Bandaisan, where the big natural lake represents the groundwater level of abundant rainfall, occurred when the underground lava column suddenly sank rapidly by the gaping open of the deep rift. The water poured into red hot cavities, while the lava was rising and erupting by frothing up in the depths of one of the other volcanoes. The results of Bandai’s explosion were first, earthquake collapse, which was assisted by vast outjets of boiling steam from groundwater, and then the blowing out of the mountainside.
Of special interest is the spacing, twenty to forty miles, between volcanoes along such a system as Asama-Bandai. The underlying cracks must be in echelon arrangement, and the spacing is a function of the thickness of the upper earth crust and its capacity through the ages of producing spaced-out widenings or bends in the crack, above whatever shell confines the lava. The same spacing of the new and old volcanoes is true in the Caribbees and in the Costa Rica-Mexico line. There an old peak might make a Bandaisan by unforeseen breakage and steam development.
This applies also to the Ryukyu-Sakurajima line. I visited Kaimon at the extreme south end of Kyushu, a steep dome blocked on top by a lava plug. South of here to Suwanose Island, an active volcano, the spacing of islands is similar to the northward spacing of Sakurajima, Kirishima, and Asosan, following the same law of selected vents and offset cracks. Kirishima thirty miles north of Sakurajima is a treacherous and dangerous volcano that made a bad explosion just prior to the Sakurajima eruption. I saw on the rim of its summit cavity a breadcrust bomb, a triangular block of rock eight feet long, with its surface beautifully tessellated with gaping cracks. This breadcrust fracture indicates that the fragment of glowing andesite was thrown up while pasty, then congealed on its surface to smooth glass and continued to swell evenly with internal gas, so as to rupture the glassy surface as expanding dough.
At Aso Volcano farther north I entered a natural gateway into a cauldron nine miles across, surrounded by a wall, and with a hilly country inside, from which a river escaped through the gateway. The summit peak in this landscape proved to contain an active pit on top. The pit was steaming and the source of the steam was boiling puddles of mud at the bottom. This was the “Halemaumau” of Asosan, which has had a record of many eruptions near the city of Kumamoto. The chain of Kyushu volcanoes ends, after the usual spacing, with a volcano at Nagasaki.
From Shimonoseki Strait, going northeastward, new belts of volcanic fissures have built the mountains of central Japan, cut across northwest of Tokyo by what Naumann called the fossa magna or big trench. This is famous in the history of Japanese geology, for which this German geologist laid the foundations. The fossa magna extends northwest and southeast, through Fujiyama and Oshima Volcanoes to the Ogasawara Islands and the Bonin Islands, scene of volcanoes making and disappearing, from craters under the ocean.
Omori had discovered historical similarities between the eruptions of this chain and those of the Ryukyu chain. This is significant, because as we go from the small spacing of the individual volcanoes, we come to some deeper and larger fracturing of the whole crust of the earth that determines a spacing of hundreds of miles between such larger arcs of rupture as those of Kyushu and the Bonin Islands. As all are volcanic and have been so since the birth of the globe, it is unthinkable to me that they are anything but deep fractures which go down to the earth’s core. The surface geology of marine strata is a mere veneer compared with the deep and ancient igneous rocks.
I went to New Zealand in 1920, taking with me in manuscript form the Hawaiian Observatory results of the past decade. Notable among geologists there was Dr. Allan Thomson, director of the Dominion Museum in Wellington. Dr. Thomson and his distinguished father, the Honorable William Thomson, guided Mrs. Jaggar and me all the way from Auckland to Dunedin. It was my task to give lectures on volcano research, to show lantern slides of Mount Pelée and Kilauea, to tell about seismographs and cycles, and to urge upon New Zealand science the importance of establishing a volcano observatory system in the Taupo Belt of volcanoes.
Here, in 1886, had occurred the terrific eruption of Tarawera. Here are spaced out volcanoes extending north into the islands of Tonga. Here, possibly, along the Cook Channel between the North and the South Islands, is a transition from volcanoes to earthquakes, and quite possibly another fossa magna worthy of comparison with Japan. Off to the east lies the profound linear Tonga Deep, compensating the New Zealand volcanic uplift. This is analogous to the Tuscarora Deep east of Japan.
We were fortunate to procure accommodations in Rotorua, the boiling geyser district, at the time of the visit of the Prince of Wales, later King Edward VIII, and to see the hakas, or dances, of an encampment of 5,000 Maoris, gathered to honor British royalty.
I was interested in the relics of liquid basalt collected on the lip of the great rift through Tarawera Mountain. The rupture extends the length of Rotomahana Lake, which sank away as a groundwater phenomenon in 1886. This, like Bandaisan, was one of the great steamblast eruptions of history. It was right on the line of volcano spacing extending from White Island in the Bay of Plenty, to Ngauruhoe and Ruapehu Volcanoes, beyond Lake Taupo at the south. Here was a land of echelons of deep cracks, building up along scores of miles from submarine eruptions such as Falcon Island in the Tonga group. Farther south is the dangerous looking White Island close to the New Zealand shoreline, resembling Bogoslof, and so on to the lava volcanoes at the south. Big earthquakes have been characteristic, along with uplift, of both shorelines of Cook Strait.