What type of stone is made from volcanic ash?

Tufa is indeed the correct answer because it is a type of stone that is created from volcanic ash, particularly in the presence of water. When volcanic ash accumulates in a moist environment, it can combine with minerals to form tufa, which often has a porous and lightweight texture. This process typically occurs in areas near volcanic activity, contributing to the formation of this unique stone.

Pumice is also related to volcanic activity but is composed of lava that has cooled and depressurized rapidly, forming a light and airy texture due to trapped gas bubbles. Obsidian, on the other hand, is a naturally occurring volcanic glass formed from rapidly cooled lava, which does not contain ash in the same way tufa does. Lava refers to the molten rock expelled during a volcanic eruption, rather than a stone formed from the ash itself.

Thus, understanding the specific processes that lead to the formation of different types of volcanic stones highlights why tufa is characterized as being made from volcanic ash.