noun
tuff
A type of rock formed from consolidated volcanic ash. Used in geology and related fields.
この地域の地層には凝灰岩が多く含まれている。
The strata in this area contain a lot of tuff.
火山灰 is volcanic ash, the loose material; 凝灰岩 is the consolidated rock formed from it.
From 凝 (ぎょう, 'to solidify') + 灰 (かい, 'ash') + 岩 (がん, 'rock'). The reading ぎょう is the kan'on reading of 凝.