Any process by which a group of individual particles is clumped together into a single mass. The term has a number of specialized uses. Some types of rocks are formed by the agglomeration of particles of sand, clay, or some other material. In geology, an agglomerate is a rock composed of volcanic fragments. One technique for dealing with air pollution is ultrasonic agglomeration. A source of very high frequency sound is attached to a smokestack, and the ultrasound produced by this source causes tiny particulate matter in waste gases to agglomerate into particles large enough to be collected.