Kuju volcano is a group of active stratovolcanoes and lava domes in the northeastern corner of the large Aso caldera in central Kyushu, Japan.
Its historic eruptions recorded in the 17th and 18th century and as recently as 1995 were phreatic or hydrothermal explosions, caused by overheated ground water. The latest eruption where magma arrived at the surface was about 1600 years ago and formed Kuro-dake lava dome at the eastern end of the complex.
Kuju contains many hot springs and hydrothermal fields and there are 2 geothermal power plants. A particularly active hydrothermal area is Iwoyama on the NE flank of Hossho-zan lava dome, where a fumarole field was mined for sulfur during over 500 years.
Recent studies suggest that Kuju should be considered one of the more hazardous volcanoes in Japan because of its history of producing pyroclastic flows from lava dome collapse.