Most volcanoes, loud and pushing, build their cinder cones openly of fiery ash and lava. But a few volcanoes work under cover. Their molten lava never reaches the surface, but quietly pushes up the earth's rock layers as water from a burst pipe raises a blister in an asphalt pavement. Last week scientists were studying a report by Professor Hidezo Tanakadate, geographer at Tokyo's Hosei University, on the only undercover volcano whose birth and growth have been observed by scientific men.
Late in 1943, Masao Mimatsu, postmaster and amateur volcanologist of Sobetsu, a small town in southwestern Hokkaido, was working on routine papers. Once in a while he looked out the window at his pet volcano, intermittently active Mount Usu, two miles away. On Dec. 31 he heard a mighty rumbling and the ground began to tremble. Shouting "Ji-shin!" (earthquake), he rushed outdoors and looked again at Mount Usu. The tall black volcano showed no signs of life.
Rising Land. Mount Usu stayed quiet, but for six months the ground around it shook every day. A square-mile area of terraced grain fields to the east rose slowly until the land could no longer be cultivated. The villagers of Fukaba (pop. 153) came to Postmaster Mimatsu for advice. Since there had been no actual eruption, he assured them that the rising would stop soon.
It did not stop. By June 1944, a rounded mountain 450 feet high had risen quietly. Upbraided by Fukaba's villagers for his bad advice, the postmaster bought their ruined fields for 28,000 yen. "With the money I paid them, and their earthquake insurance and the extra jobs made by the disturbance," he says, they are ' funka narikin" (volcanic new-rich).
The mountain went on growing, but not so quietly as at first. Steam burst from its top, digging a small crater which filled with mud and water. Steadily the explosions grew more violent; the steam smelled of sulphur and broke out strongly enough to toss rocks high in the air. But still there was no hot lava or other volcanic matter. The rocks and sand thrown out were just local material torn loose by the steam.
Source of Wonder. As the ground rose, a railroad track had to be moved and the bed of a nearby stream had to be deepened. The postmaster kept copious records of every move the mountain made. Geologists decided that it was caused by a "laccolith," a mass of molten material that had forced its way toward the surface, raising local rock strata instead of breaking through them.
Before the shy volcano stopped growing late in 1945, it reached a height of about 1,000 feet. Said Professor Tanakadate, of the rare phenomenon that had been observed with such care: "It may be a source of fear and destruction to the ordinary inhabitants of the area, but to scientists it is a source of wonder and delight. Actually, we scientists know so little."