Fig. 1

Photographs of the activities and salt fallouts. a The crater bottom on May 19, 2011. The crater lake is almost dried out, presenting a brownish plume emitted from a high-temperature vent at the center of the crater bottom. Several other vents also emitted gases intensively. Small amounts of crater lake water remained around the vents. b The salt flakes settled on a backpack on May 19, 2011, at the leeward side of the plume toward the northern rim of the crater. c Ash eruption on January 13, 2015. d A salt shell observed on the ground on January 13, 2015. e The ash eruption on March 17, 2015. f A salt shell fallen from the gas plume 1 h after the temporal cessation of the ash eruption on March 17, 2015. g The salt shells collected on March 17, 2015, and stored in a box. The salts seem to be partly dissolved due to atmospheric moisture and partly recrystallized. h The salt lumps collected on March 26, 2015. i A bomb covered in fine white salts, found on December 27, 2014. j A close-up of the bomb in the laboratory. Most of the salts were fell off during transport because the salts are very fragile