The landfill offers an opportunity to study these mechanisms. Actively migrating low-density waste components include tires and other hollow products. Tires tend to float to the surface of the waste material from the flexing action of the tire after compaction. Vibrations induced by the movement of construction equipment supply some of the energy that causes subsidence around the flexing tire. The flex action and vertical migration of the tire can be compared to the inferred remanie of some bivalves and brachiopods.
Passive mechanisms apply to very low-massed items such as plastic drinking bottles.
Large, low-density gastropod tests can experience similar physical processes. The accumulation of gasses from decomposition of the organism in the shell can cause the shell to float. It is possible, then, to model the movement of shell material in a taphocoenosis by observing the migration of waste in the landfill. The landfill consists of two distinct chemical units depending upon the moisture content of the lower saturated and upper unsaturated units. Incomplete reduction and oxidation occur in the unsaturated zone.