Final answer:
Calcareous rocks like limestone feature openings such as joints, microscopic openings, and solution cavities, which are formed by the dissolution of calcium carbonate when it comes into contact with acidic water.
Step-by-step explanation:
The openings in rocks that are associated with calcareous rocks, such as limestone, include joints, microscopic openings, and solution cavities. Limestone is particularly susceptible to erosion when in contact with acidic water, which creates solution cavities through a process where carbonic acid in groundwater dissolves the calcium carbonate in limestone over thousands of years. This dissolution process can lead to the formation of caves with their associated features like stalactites and stalagmites, which are forms of calcite deposits.