Urban expansion is a fundamental ecosystem function of cities. All cities expand, and some as Latin American cities, at very fast speeds that can reach 40 m2/minute.
This expansion can be understood in two main ways: as an ecosystem behavior, where cities like any other ecosystem, are accumulating matter. In fact it is this accumulated matter, as physical structure which allows energy and information to flow. The second form to understand expansion, that derives from the first, is metabolic : the process of material accumulation is in both cases, natural and artificial ecosystems (urban), a metabolic process , where accumulated matter comes from out of the system.
This process of material accumulation has received very littl attention in urban studies. Is this paper I proposed a new indicator to measure this material accumulation process: technomass. As in natural ecosystems is biomass, technomass is the general sum of accumulated matter in different spatial compartments. From here it is possible to set ecosystem's and metabolic behavior of urban systems, which allows to understand in a better way how they work and how they will evolve. Finally, biomass and technomass are in constant competition for the same space. This is the expression of the metabolic process of land cover change.