noun
solfataric clay
Technical geology term for clay formed in solfataric (volcanic fumarole) environments. Rare in everyday conversation.
この地域の温泉余土は、かつての火山活動で形成された。
The solfataric clay in this area was formed by past volcanic activity.
Compound of 温泉 (onsen, 'hot spring') and 余土 (yodo, 'residual soil/clay'), referring to clay associated with hot spring or fumarolic activity. The exact historical derivation is uncertain.