It was on August 7, 1977 - the 32nd summer from the emergence of Showashinzan (Showa New Mountain) - when Mount Usu initiated large-scale pumice eruption at the mountain top.

Since its awakening from a long sleep of thousands of years in 1663, Mount Usu has erupted for nine times at an interval of 20 to 50 years. Each eruption was preceded by a series of noticeable earthquakes, and Mount Usu came to be called "the mountain that does not tell lies". However, eruptions can occur at any part of the mountain, and eruption sites cannot be foreseen. Another characteristic of Mount Usu's volcanic activities is massive ground deformations accompanying eruptions.

The eruptions in 1977 were explosions at the summit, which people dreaded most as eruptions at the top used to damage massively until the 19th century.