Most solutional caves form in what rock type?

Most solutional caves form in limestone.

Limestone is especially vulnerable to dissolution because it is largely made of calcium carbonate. Rainwater and groundwater absorb carbon dioxide from the air and soil, producing weak carbonic acid. As this water moves through cracks, joints, bedding planes, and faults, it dissolves the rock and gradually enlarges underground passages. The resulting landscapes are called karst and often include sinkholes, sinking streams, and springs.

Limestone is the standard answer, but it is not the only possible host rock. Solutional caves can also form in chalk, dolomite, marble, gypsum, salt, and other soluble materials. Those examples are less abundant or occur under more specialized chemical conditions, so they do not displace limestone as the canonical quiz answer.

Some famous caves formed through sulfuric acid rather than ordinary carbonic-acid seepage. Lechuguilla Cave and Carlsbad Caverns in New Mexico are associated with groundwater affected by hydrogen sulfide from below, yet their surrounding host rock is still limestone.