Trumao is not a city, but rather a specific type of soil belonging to the Andosol order found in central and southern Chile. These soils are formed from young volcanic ash that has been redeposited by aeolian processes or mobilized as alluvium.
The characteristic mineral composition of trumaos includes allophane, imogolite, and a series of paracrystalline and non-crystalline clays. Due to these properties, the soil is distinguished by its high porosity and low bulk density. Two distinct varieties exist based on moisture content: the dry variety is known simply as trumao, while the humid variety is referred to as trumao húmedo.