凝灰岩是一种火山碎屑岩，其组成的火山碎屑物质有50%以上的颗粒直径小于2毫米的火山尘和火山灰压实固结而成，成分主要是火山灰，外貌疏松多孔，粗糙，有层理，颜色多样，有黑色、紫色、红色、白色、淡绿色等，根据其含有的火山碎屑成分，可以分为：

凝灰岩是常用的建筑材料，也可以作为制造水泥的原料和提取钾肥的原料。

凝灰岩向熔岩过渡的类型，称为凝灰溶岩，其中火山碎屑物质含量少于50%。

火山灰凝固成岩为凝灰岩（tuff）。**公馆、三张犂与六张犁一带之山坡地，出现有凝灰岩露头，富灰烬状气孔，风化者相当松软，故多坟墓。

Tuff (from the Italian tufo) is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered tuffaceous.