Soils and soft rocks which tend to swell or shrink due to changes in moisture content are commonly known as expansive soils. Two major groups of rocks serve as parent materials of expansive soils. The first group consists of ash, glass, and rocks from volcanic eruptions. The aluminium silicate minerals in these volcanic materials often decompose to form expansive clay minerals of the smectite group, the best known of which is montmorillonite. The second group consists of sedimentary rocks containing clay minerals. Expansive soil causes many millions of dollars worth of damage a year in damage to homes, commercial buildings, roads and highways, buried utilities, and other structures.